U.S. General Services Administration

PBS Guideline for

RAISED FLOOR SYSTEMSWith and Without Underfloor Air Distribution

PBS Guideline for

RAISED FLOOR SYSTEMSWith and Without Underfloor Air Distribution

A Supplement to:PBS P-100: Facilities Standardsfor the Public Buildings Service

Developed through:National Institute ofBuilding SciencesWashington, D. C.

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PBS Guideline for

RAISED FLOOR SYSTEMSWith and Without Underfloor Air Distribution

U.S. General Services AdministrationPublic Buildings ServiceOffice of the Chief Architect1800 F Street, N.W.Washington, D. C. 20405

www.gsa.gov

A Supplement to:PBS P-100: Facilities Standardsfor the Public Building Service

Developed through:National Institute of Building SciencesWashington, D. C.

FEDERAL BUILDING, SAN FRANCISCO, CALIFORNIA

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This PBS Guideline for Raised Floor Systemswith and without Underfloor Air Distributionhas been developed as a supplement to theFacilities Standards for the Public BuildingService (PBS P-100), which provides designstandards and criteria for new buildings,major and minor alterations, and work inhistoric structures. To realize space flexibil-ity and cable management benefits, the PBSP-100 requires incorporation of RaisedAccess Flooring (RAF) in all new construc-tion where “office functions will take place,”and in “appropriate areas of courthouses.”The PBS P-100 also recognizes that addi-tional benefits might be realized if the RAFin those areas is adapted to incorporateunderfloor air distribution (UFAD) forheating, ventilating, and air-conditioning(HVAC). To address the special issuesinvolved in RAF with or without UFAD, thePBS P-100 refers to this Guideline fordesign and construction details.

Over the past decade, GSA has been incor-porating RAF/UFAD systems into thedesign of U.S. Courthouses, Federal OfficeBuildings, and special applications facilitiesthroughout the country. Extending the useof UFAD systems from special applications,such as single-zone computer rooms, tomulti-zone applications in general buildingspaces is a reasonably new concept when

Executive Summary

compared to the more “conventional airdistribution” (CAD) methods of overheaddistribution, fan-coil systems, etc. There-fore, this Guideline has been published toassist architects and engineers in the selec-tion and design of these systems that providethe functional requirements in GSA facili-ties, in accordance with PBS P-100. Inaddition to the design professionals, theGuideline is intended for use by the con-struction industry (e.g., constructionmanagers, contractors and tradesmen) andby building managers and other manage-ment personnel. This Guideline is based onlessons learned from review of actual designsand operations of GSA and private sectorfacilities that incorporated RAF, with orwithout UFAD; from published research,and from manufacturers’ information onRAF and UFAD components.

A fundamental lesson learned is that theprimary purpose for raised access flooring innew or renovated GSA facilities is to providehorizontal pathways that enhance theflexibility of power, data, and communica-tions systems, and that this purpose can berealized with or without the potentialsecondary benefits of UFAD. However, theselection of RAF as an alternative to conven-tional flooring and suspended ceilingsystems demands supplemental design

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resolution regarding structural, acoustic andvibration, fire and smoke management,safety, and security issues.

Another fundamental lesson learned is thatadapting RAF for UFAD systems to achievethe HVAC performance requirements in thePBS P-100 also demands supplementaldesign and construction coordinationregarding multi-discipline and multi-traderesolution of incremental effects on the RAFrequirements, as well as air leakage, thermalloads, moisture detection, fire and smokemanagement, and indoor environmentalcontrol issues. Moreover, if a pressurizedplenum design is to be used rather than acavity design with ducted supply air, theplenum must be designed, constructed andmaintained (throughout the life of thebuilding) as an airtight enclosure that alsoprovides for horizontal cable management;occupant satisfaction with thermal, airquality, and acoustic conditions; structuralstability; fire and smoke management;security and physical safety, and energyefficiency. The pressurized plenum designalso involves many general constructionmaterials not normally considered to per-form in an airtight configuration. In fact,numerous trades and associated componentsand products have been identified as beinginstrumental in ensuring a quality pressur-

ized plenum design, including: concrete,masonry, gypsum drywall systems, jointsealants, curtain wall systems, raised accessflooring, insulation, vapor barriers, expan-sion joints, firestopping, waterproofing,millwork, carpet and other floor finishes,electrical conduits and power wiring,plumbing and ductwork. With this in mind,the need to integrate structural, mechanicaland electrical performance requirementswith the architectural features of the facilityduring the earliest stages of the design andconstruction processes becomes obvious.

Recognition of excessive air leakage inpressurized underfloor plenums as a mainobstacle to assuring excellence in UFADdesign has led to the stipulation within thisGuideline of designing and testing amockup of any UFAD system proposed foruse within a federal facility. Traditionally,conventional overhead air supply ducts areconstructed, insulated and tested (limitingair leakage rate to 1 to 3 % of supply airflowrate) according to strict industry standardssuch as those provided by the AmericanSociety of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and theSheet Metal and Air Conditioning Contrac-tors’ National Association (SMACNA).However, in the case of UFAD systems,where the plenum - which is essentially an

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assembly of architectural components - oftenfunctions as an air supply duct, no credibleindustry standards or testing techniquesexist to assure such plenums achieve andmaintain air-tight status. Thus, designingand testing of a mockup will assure thehighest available standard of performancefrom our UFAD systems.

One of the main reasons for installing RAFis the efficient re-location and managementof power, data, and telecommunicationcables beneath the floor: this inevitablyinvolves the moving of floor components,which has the potential for compromisingthe structural integrity and efficiency ofUFAD systems operating with pressurizedplenums. Thus, along with the constructionand testing of an initial mockup, use of RAFand UFAD systems involve a serious com-mitment to the continuous air-leakagetesting and maintenance of these systems’components over the life of the building. Itis also recommended that warrantees relatedto RAF/UFAD systems be increased fromthe usual one-year period to a period of fiveyears, to ensure the proper functioning ofthese systems over time.

By far the most important and over-arching lesson learned is that incorporation ofRAF, with or without UFAD, for the purpose ofachieving performance in accordance with PBSP-100 requires an early commitment tointegrated design, beginning at planning andpreliminary concept stages, and extendingthrough the construction and commissioningphases.

It should be noted that this Guidelineprovides the “what to do” regarding RAF/UFAD systems in federal buildings; the“how to do it “ is, in many instances, stillbeing “discovered” by the building industry.However, it is GSA’s hope that this Guide-line will be seen as a step forward in promot-ing excellence in the design, constructionand long-term maintenance of RAF/UFADsystems, leading ultimately to improvedoperations and maintenance, energy effi-ciency, thermal comfort, and customersatisfaction, not only within the federalgovernment, but within the buildingindustry as a whole.

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Summary of the Guideline

Part 1 presents design considerations andcriteria for the selection of Raised AccessFlooring products and how they mustinterface with the facility. In Sections 1.0and 4.2, RAF products are noted to consistof proprietary modular panels andunderstructure that vary widely in terms ofconstruction and cost. As supplementalinformation to the PBS P-100, Section 1lists areas that are most suitable for RAF;areas that are suitable but require specialdesign features, and areas that are generallyinappropriate for RAF.

Section 2 points out that economic consid-erations of RAF alternatives are needed, evenfor those areas where PBS P-100 specifies itsapplication. Therefore, Section 2 states thata benefit-cost method, which recognizes thevalue of RAF benefits as well as its costs,shall be used for two applications:

1) In the areas where RAF is required, thefeatures and characteristics shall beselected to optimize the benefits andcosts of RAF for cable management; and

2) In other areas being considered for RAF,the benefits and costs of RAF shall becompared to those for conventional

flooring and horizontal pathways forwiring and cabling distribution abovesuspended ceilings.

Some of the issues to be considered in thebenefit-cost analyses are discussed in Section3, Cable Management Technology and Plan-ning, and additional information is providedin the Appendix.

In Sections 4 – 7, architectural and electricaldesign conditions and criteria are addressedthat are supplemental to those in the PBS P-100:

Section 4.1 identifies additional soil andclimatic conditions and correspondingcriteria that must be addressed tominimize water and contaminantincursions into the RAF cavities forprotection of the wire and cable manage-ment systems, and to minimize the risksof fire and adverse health effects.Section 4.2 addresses structural condi-tions and specifies minimum criteria foruniform, point, impact and rolling loadsfor RAF products. In addition, guid-ance is provided for specifying stringerloading, pedestal axial loading, and

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overturning resistance, in response toanalyses of seismic, acoustic and vibra-tion requirements that are also addressedin this Section.Section 4.3 addresses supplementalissues related to the proximity in theRAF cavities of potential fire, smoke,and other hazards to the occupants, andspecifies minimum supplemental criteriato reduce risks to the occupants, firstresponders, and property assets.Sections 4.4 and 4.5 address supple-mental interfaces with adjacent finishesin new and existing buildings, and theissues regarding interfaces with thePlanning Grid in PBS P-100, paneledging, and cavity heights as theyimpact clear ceiling and buildingheights.Section 5 highlights and supplementsthe basic requirements in Chapter 6,Sections 10 –14 of PBS P-100 fornormal and emergency electrical powerwiring distribution, for communicationscabling beneath the RAF, and forgrounding.Section 6 addresses supplementalrequirements for documentation anddrawing details for RAF.Section 7 addresses supplementalinspection, testing and commissioning

issues associated with the design andconstruction of RAF systems.

Part 2 presents supplemental design consid-erations and criteria for the selection ofUFAD systems and how they must interfacewith the architectural, structural, mechani-cal and electrical systems. Section 8 ad-dresses the similarities and differencesbetween UFAD and CAD systems andincrements the listed areas in Section 1 thatare most suitable for UFAD; areas that aresuitable but require special design features,and areas that are generally inappropriate forUFAD.

Section 9 explains that economic consider-ation of UFAD alternatives is needed,although the PBS P-100 identifies UFAD asone of the four types of acceptable HVACBaseline systems. Therefore, a benefit-costmethod, which recognizes the value ofUFAD benefits as well as its costs, is re-quired for two applications:

1) In areas where RAF has been previouslyselected for cable management inaccordance with Part 1, benefits andcosts of providing UFAD with RAF,modified in accordance with Part 2, shallbe compared with CAD and RAF that is

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in accordance with Part 1 but notmodified in accordance with Part 2; and

2) In areas where RAF has not been previ-ously selected in accordance with Part 1,benefits and costs of providing UFADwith RAF, which is in accordance withParts 1 and 2, shall be compared toCAD with conventional flooring andhorizontal pathways for wiring andcabling distribution above the sus-pended ceiling.

Some of the issues to be considered in thebenefit-cost analyses are discussed in Section10, Interface between Cable Management andAir Distribution, and additional informationis provided in the Appendix.

Sections 11 – 15 focus on the architectural,structural, mechanical, and electrical designconditions and criteria that must be ad-dressed due to the introduction ofunderfloor air distribution through pressur-ized plenums (i.e., RAF with UFAD):

Sections 11.1, 12.1 and 12.2 addressadditional conditions to those in Section4.1 that must be controlled to:

Minimize heat transfer, and waterand contaminant incursions throughslabs and perimeter walls into thepressurized UFAD plenums, which

must be maintained at lower valuesof dry bulb and dew point tempera-tures than the unpressurized RAFcavities.Minimize the uncontrolled heat andcontaminant exchange between theUFAD plenums and the occupiedzones.Achieve acceptable values in Table5.1 of PBS P-100 in terms ofOperative Temperatures, defined byASHRAE Standard 55, in theoccupied zones to compensate forhigher air velocities at the floor, andincreased radiant heat exchange withthe floor panels of UFAD systems.

Section 11.2 identifies pathways of airleakage and water accumulation inUFAD plenums. Section 11.2.1 definesCategory 1 (i.e., general construction)and Category 2 (i.e., product) airleakage criteria. In order to reduceCategory 2 air leakage rates, RAF forUFAD shall be provided with stringers orother demonstrably equivalent air-sealingdevices, staggered carpet tiles, and floorpanel accessories with minimum leakagevalues. Criteria for air leakage requirethat, as a maximum, the total air leakagefrom all Category 1 and Category 2 leaksshall not exceed 0.1 cfm/ft2 floor area or10% of the design supply airflow rate,

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whichever value is smaller, when theplenum static pressure is maintained at itscontrol value (e.g., 0.1 in. w.g.). Section11.2.2 describes methods that must beconsidered for prevention of wateraccumulation in UFAD plenums.Sections 14.1 and 15.2 provide addi-tional information on testing proce-dures.Sections 11.3 and 11.4 address incre-mental structural, acoustics and vibra-tions issues that must be considered dueto RAF modifications required for theUFAD plenums.Sections 11.5 and 11.6 address addi-tional issues related to the effect that airmovement through the UFAD plenumshas on potential fire, smoke, and otherhazards to the occupants, and specifiesadditional minimum criteria to reducerisks to the occupants, first responders,and property assets in facilities withUFAD plenums.Section 11.7 addresses supplementalissues regarding the locations of floordiffusers and grilles in perimeter andinterior zones, specifies the maximumunducted distance (i.e., 50 ft) from thepoint of supply air entry into theplenum to the most distant air diffuseror grille in that zone, and reaffirms thatthe return air pathways from the return

air grilles of the UFAD systems to the airhandling units shall meet the samecriteria as specified for Plenum andDucted Return Air Distribution of CADsystems (Section 5.8 of PBS P-100).Section 13 describes additional electricalrequirements to those described inSection 5 that must be implemented forUFAD plenums: 1) compliance with theNational Electrical Code (NEC) forwiring and cabling in plenums, and 2) aminimum of 4 in. (100 mm) verticalclearance in the plenum to allow cablesto cross above the ducts.Section 14 addresses additional require-ments to those in Section 6 for docu-mentation and drawing details forUFAD.

Section 14.1 focuses on issues thatmust be addressed in the variousdivisions of Project Specifications toensure the air tightness and cleanli-ness of the UFAD plenum, includ-ing the construction of a full-sizemockup of the UFAD system, whichis to be tested in accordance withprocedures provided in Section15.2.1.Sections 14.2 – 14.6 describeadditions that are needed to draw-ings and to operations, maintenance,and housekeeping documents.

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Section 14.7 requires an extendedwarranty that must be coordinatedwith the Contracting Officer toensure continuous performance afterSubstantial Completion, includingtesting in accordance with proce-dures provided in Section 15.2.3.

Section 15 specifies methods of inspec-tion, testing and commissioning GSAfacilities that have UFAD. Threemethods of testing for plenum airleakage are listed:

In Section 15.2.1, methods ofdetermining Category 1 and 2 airleakage rates are specified. Prepara-tion of a mockup of the UFAD isalso specified.In Section 15.2.2, the lessonslearned from 15.2.1 shall be dis-seminated to: 1) all the tradesinvolved in the construction of theplenums as supplemental informa-tion, and 2) all inspection andapproval authorities on the project.After the permanent building floorplenums have been completed inaccordance with steps 1 – 4, Cat-egory 1 and 2 air leakage rates shallbe determined. If the results are notin compliance with Table 15.1 andSection 12.3, the causes of the leaks

shall be repaired or corrected andthe system retested until the ratesare in compliance.In Section 15.2.3, the tests con-ducted in 15.2.2 shall be repeatedwhenever an area exceeding 2,500ft2 is modified, and within 30 daysprior to completion of the warrantyperiod (See Section 14.7) for a1,000 ft2 area to be selected byGSA. If results are not in compli-ance with Table 15.1 and Section12.3, the causes of the leaks shall berepaired or corrected and the systemretested until the rates are in com-pliance, and two additional areasshall be selected by GSA and tested.If either of these areas is not incompliance with Table 15.1, allUFAD plenums within the facilityshall be tested and repaired asnecessary.

After air leakage testing is completed inaccordance with 15.2.2 and 15.2.3, theTesting, Adjusting, and Balancing (TAB)contractor or agency shall perform the TABwork in accordance with the project specifi-cations (see Section 15.3), and the commis-sioning process shall continue (see Section15.4).

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Executive Summary iSummary of the Guideline iii

Introduction 1Part 1: Raised Access Flooring (RAF) Systems 3

1.0 Features and Characteristics 32.0 Economic Considerations for Raised Access Flooring 63.0 Cable Management Technology and Planning 74.0 Architectural Design Considerations 9

4.1 Soil and Climatic Conditions and Criteria 94.2 Architectural, Structural, Seismic, Acoustic and

Vibration Design Conditions and Criteria 94.3 Fire, Smoke, Safety, and Security Zone Issues and Criteria 124.4 Interfaces with Architectural Materials 144.5 RAF Grids and Cavity Heights 15

5.0 Electrical Design Considerations and Criteria 166.0 Requirements for Documentation 16

6.1 Specifications for Raised Access Flooring 166.2 Drawing Details for Raised Access Flooring 176.3 Construction Documents before Release 176.4 Operations and Maintenance Documents 186.5 As-Built Drawings 186.6 Housekeeping 19

7.0 Inspection, Testing and Commissioning 197.1 Inspection 197.2 Testing and Commissioning 20

Part 2: Underfloor Air Distribution (UFAD) Systems 218.0 Features and Characteristics 219.0 Economic Considerations for Underfloor Air Distribution 2410.0 Interface between Cable Management and Air Distribution 2511.0 Architectural Design Considerations 2611.1 Soil and Climatic Conditions and Criteria 2611.2 Air Leakage and Water Accumulation Issues and

Criteria for UFAD Plenums 2711.3 Structural and Seismic Considerations 3111.4 Acoustics and Vibrations Considerations 3111.5 Fire, Smoke, Safety, and Security Zone Issues and Criteria 31

Table of Contents

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11.6 Detection and Drainage of Water and Moisture in UFAD Plenums 3311.7 Room Air Distribution and Return Air Pathway Coordination 3312.0 Mechanical Design Considerations 34

12.1 Thermal, Air Quality, and Acoustical Design Conditionsand Criteria 34

12.2 System Time Constant (Thermal Inertia) 3513.0 Electrical Design Considerations 37

13.1 Cables in Air Distribution Pathways (Plenums) 3713.2 Sharing Space with Mechanical Devices 37

14.0 Requirements for Documentation 3814.1 Specifications for UFAD, including a “Mockup” 3814.2 Drawing Details for UFAD 4114.3 Evaluate Construction Documents before Release 4114.4 Operations and Maintenance Documents 4114.5 As-Built Drawings 4214.6 Housekeeping 4314.7 Warranty Period 43

15.0 Inspection, Testing and Commissioning 4315.1 Inspection 4415.2 Testing 44

15.3 Testing, Adjusting and Balancing Air System 4715.4 Commissioning 47

Appendix 1Appendix A-1: Definitions 1Appendix A-2: Economic Considerations 5

A-2.1 Life-Cycle Cost and Benefit-Cost Methods 5A-2.2 Benefit Factors 8A-2.3 Risk Factors 11A-2.4 Cost Factors 14

Appendix A-3: UFAD Systems 17A-3.1 Functions and Characteristics of UFAD Systems 17A-3.2 Room Air Distribution System 19A-3.3 Capacity Control Options 19A-3.4 Ventilation and Humidity Control 22

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The Facilities Standards for the Public Build-ings Service (PBS P-100), published by theU.S. General Services Administration (GSA),provides design standards and criteria fornew buildings, major and minor alterations,and work in historic structures. Chapter 3,Section 1 of PBS P-100 states that RaisedAccess Flooring (RAF) “shall be incorporatedinto all new construction where officefunctions will take place,” and Chapter 9,Section 3 of PBS P-100 states that RAF“shall be used in appropriate areas in court-houses, which include courtrooms, cham-bers, offices, conference rooms, etc.” Theseapplications are primarily to realize spaceflexibility and cable management benefits.

PBS P-100 recognizes that, as the RAFapproach to cable management is required incertain areas, the opportunity to combinethe access flooring system with underfloorair distribution (UFAD) could be beneficialin many situations. Therefore, Chapter 6,Section 4 of PBS P-100 includes UFAD asone of the four allowable Perimeter andInterior Heating and Cooling Systems to beconsidered in the HVAC Baseline System.

Use of UFAD systems for heating, ventilat-ing, and air-conditioning of general buildingspaces is a reasonably new concept when

Introduction

compared to the more “conventional airdistribution” (CAD) methods of overheaddistribution, fan-coil systems, etc. There-fore, the GSA has published this Guidelineto assist architects and engineers in theselection and design of these systems. ThisGuideline is based on lessons learned fromreview of actual designs and operations ofGSA and private sector facilities that incor-porated RAF, with or without UFAD, frompublished research, and from manufacturer’sinformation on RAF and UFAD compo-nents.

Therefore, for projects where RAF, with orwithout UFAD, is to be incorporated into thedesign, this Guideline shall be used as asupplement to PBS P-100 in order tocomply with the functional and performancerequirements in the project’s design programand to achieve a total integration of allbuilding systems that will provide forcurrent operations as well as future changes.

In addition to the design professionals, thisGuideline is intended for guidance and useby the construction industry (e.g., construc-tion managers, contractors, and tradesmen)and by building managers and other man-agement personnel.

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Regarding the design and constructionindustries, UFAD systems require theintegration of numerous building systemsand components into HVAC systems in amanner that requires special attention todetails and issues that represent new chal-lenges to both designers and contractors.Thus, the major objectives of this Guidelineare to develop an awareness of these issuesand details, and to provide guidance thatwill assist the practitioners in the successfulexecution in both the design and construc-tion phases.

Regarding ownership responsibilities, thebenefits of both RAF and UFAD systems canbe realized for the many years of operatingthe buildings, if the management staff isattuned to the continuing maintenance ofthe cable management, underfloor house-keeping, and other issues addressed in thisGuideline.

This Guideline consists of two Parts: Part 1– Raised Access Flooring (RAF) Systems,and Part 2 – Underfloor Air Distribution(UFAD) Systems. Each Part is intended tostand alone, except for cross-references whereapplicable. An Appendix of supporting orsupplemental information follows Part 2.

ALFRED A. ARRAJ U.S. COURGHOUSE, DENVER, COLORADO

Photographer: Frank Ooms

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Part 1:Raised Access Flooring (RAF) Systems

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Modern use of raised flooring as modularunits is about fifty years old and was devel-oped for computer main frames mostly forthe ease of wire management and provisionof cooling to the equipment. More recently,using raised access flooring for many kinds ofwork spaces, both for wire management andair distribution, has gained popularity for itsflexibility and reduction of embeddedsystems in the building structure.

Overhead services have long been the mostcommon horizontal distribution means,typically in space above a suspended ceiling.Raised access flooring has considerablyfacilitated providing services to work stationsin open office planning. Having suchservices easily accessible potentially reducesthe need for skilled labor to change themand may reduce waste and disruption duringrenovations.

A modern raised access flooring systemconsists of proprietary modular panels andunderstructure that vary widely in terms ofconstruction and cost. The primary purposeof the RAF is to provide horizontal pathwaysthat enhance the flexibility of power, data,and communication systems. The Evalua-tion Sheets for Section 10270 in

Masterspec© provide excellent detailedinformation for the basis to select andspecify RAF types. Some of the basicfeatures and characteristics can be describedas follows:

Floor panels:Manufactured in English (typically 24"x 24") and metric (typically 600 mm x600 mm) versions.Constructed of die-cast aluminum orformed steel.Cores may be hollow or filled withlightweight concrete, cementitiousmaterial or wood.Coverings include high-pressure lami-nate, carpet, vinyl (conductive andstandard) and galvanized steel.Panels may be solid or perforated, orslotted for air distribution.

Understructure:Pedestals are adjustable and designed forthe loading required. They are typicallyplaced at all panel corners.The heights of the pedestals vary withthe need to accommodate systems underthe panels. They are available for floorsurface to slab as low as 2 ½ inches (60mm). The heights vary by manufac-

1.0 Features and Characteristics

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turer, by the type of specifiedunderstructure, and by the clearancesneeded for the horizontal distributionservices.Dependent upon the imposed loads, thepedestal pads are bolted or glued to thestructural slab. The panels are eitherbolted directly to the pedestals or laidon a stringer system.Stringers are bolted to pedestals forgreater stability and load-carryingcapacity (especially for rolling andseismic loads) or snap-connected to thepedestals for ease of removal.Stringers are available in various weightsfor strength, and may be formed toaccept pipe and conduit hangers.Stringers may nest with the panels or beindependent of the panels.Panels may rest on stringers with gasketsor pads for cushioning and some air-tightness, or may be placed on conduc-tive pads to provide continuity forgrounding.Finishes of understructure includegalvanized steel, aluminum and stainlesssteel.

Accessories vary widely by manufacturer, butsome common ones are:

Grommets to protect wiring throughopenings in panels.

Sound-deadening pads.Service outlets.Floor diffusers, registers and grilles.Ramps, stairs and railings.Air strip seals (non-structural stringers).

Functionally, a raised access floor is similarto a suspended ceiling. Many of the samecoordination needs apply to the former as tothe latter. Both types of space must accom-modate structure, cabling, ductwork andequipment and must resolve the demandeach system makes for space – especiallywhen “horizontal distribution systems” crossone another. However, suspended ceilingsaccommodate horizontal distributionsystems not usually found under raisedaccess floors: lighting, fire suppressionsystems, public address speakers, exhaustductwork, and plumbing drain traps andoffsets to risers from the floors above. Raisedaccess floors, on the other hand, bear theweight of the contents of the office area orother functional space including people,furnishings, equipment and some partition-ing. Spaces above suspended ceilings andbelow raised access floors have been used asplenums for air supply and return.

Both raised access floor and suspendedceilings are typically based on rectangulargrids, which are more or less visually evi-

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dent, but usually cannot be completelysuppressed without compromising theaccessibility required. It may be necessary toreconcile the grid architecturally to otherbuilding grids such as the GSA PlanningGrid (See Chapter 3, Section 1 PBS P-100),structural bays and other geometries.

The space under a raised access floor, like thespace over a suspended ceiling, can becomecluttered, dirty and contaminated. Like-wise, floor panels like lay-in ceiling panelscan become worn, distorted and damagedfrom removal and replacement, impact orother abuse.

The areas typically most suitable for raisedaccess include:

Computer rooms and other informationtechnology spaces.General open office areas.Training and conference areas.Exhibit spaces.Support spaces for offices, includingelectrical closets, fan rooms, etc.Clean rooms

Some areas suitable for RAF require specialdesign features:

Elevator lobbies: The sill of the elevatoropening must ensure edge support ofthe RAF – especially at freight elevators

– to support rolling loads as carts arerolled off the elevator onto the floor.Auditorium and courtroom seating areaswhere it is planned to move theseseating areas into different configura-tions. Stepped or sloped seating mayneed to be built on independent unitsabove the floor.Secure spaces: Credit unions, interviewrooms, private offices, Security ControlCenters, Secure Compartmented Infor-mation Facilities (SCIF), other secureareas as permitted by regulations of thegoverning agency. The secure construc-tion of the surrounding walls mustextend through the raised access floor tothe slab below, just as it must throughthe ceiling to the slab above.Prisoner spaces: such as prisoner holdingcells, secure corridors, etc. The U.S.Marshals (USMS Publication 64) mustapprove of the use of the RAF and thesystem’s tamper-resistant capabilities.Public lobbies, atria, main elevator andescalator lobbies, main public corridors,etc.: Care must be taken to insure thatwater cannot enter the underfloor spacefrom entrances where moisture is trackedin. Also, weights of maintenanceequipment such as lifts or rollingscaffold in high-ceiling public areasmust be determined and designed for.

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Other areas are inappropriate for raisedaccess floor:

Slab-on-grade locations: Long termprotection is difficult against heat,moisture, and contaminant transfer withthe soil, soil gases, ground water, andvermin through joints and cracks. RAFshall not be placed directly on concreteslab-on-grade surfaces.Toilets, showers, baths, janitors’ closets,dishwashing and other wet areas: Theseareas have plumbing fixtures whichshould not rest on a raised access floor.Potential leakage and high humidity arelikely to cause corrosion of panels andunderstructure, and growth of mold andbacteria on these surfaces.Kitchens and food preparation areas:For the same reasons as for the spaceslisted above, these areas should not beplaced on RAF. Although some breakrooms with kitchenettes have beenplaced on raised access floors when watersupply and drains have been possible byconnection to a wet column, the risk ofspillage of food and liquids and seepageinto the underfloor space makes theseareas a potential problem.Laboratories: Similar challenges inaccommodating plumbing and moisture

exist as for toilets, etc., depending uponwhether the laboratory has any wetprocesses. Moreover, the likelihood ofchemical, biological or other spills mustbe assessed.Fire stairs: Stair landings must interfacewith any adjacent raised access floors as aflush condition with the edge of theaccess floor well supported.Mechanical equipment rooms such asboiler, chiller, and air handling equip-ment rooms.Central storage rooms and loading areas,trash rooms, etc.UPS, emergency generator, and similarrooms.Child care centers where playthings,equipment and mats will often coveroutlets, and where floor outlets mayprovide tempting objects for children toplay with, drop things into, etc.

Wherever raised access floor is used, corespaces must have finish floors level with theraised access floor. This can be accomplishedby changing the structural slab elevation, bybuilding up a fixed filled floor with foaminsulation board, or by building a framedraised slab under such areas as stair landings,toilet rooms and elevator lobbies.

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2.0 Economic Considerations for Raised Access Flooring

As indicated in the Introduction of thisGuideline, Chapter 3, Section 1 and Chap-ter 9, Section 3 of PBS P-100, require theapplication of RAF in specific areas of newbuildings where office functions will takeplace and in appropriate courthouse areas.However, the features and characteristics ofRAF for these applications are not specified.Moreover, Section 1.0 of this Guidelineidentifies other areas where the applicationof RAF may be beneficial, and other areaswhere its application requires special designconsideration or is generally inappropriate.

Therefore, to ensure that the value of theRAF is maximized, a benefit-cost methodshall be utilized in which the life cycle costsand benefits of RAF features and characteris-tics are optimized for the applicationsspecified in Chapter 3, Section 1 andChapter 9, Section 3 of PBS P-100. Forareas where RAF is not required (see Section1.0 of this Guideline), the RAF life cyclecosts and benefits shall be compared withconventional flooring and horizontal path-ways for wiring and cabling distributionabove suspended ceilings. In this case, RAFis economically preferred if the benefit-costratio of RAF exceeds that of conventionalflooring and horizontal pathways for wiring

and cabling distribution above the sus-pended ceilings

The first cost of a raised floor, includingengineering, material cost, fabrication,installation, and inspection, can be signifi-cantly higher than that of a conventionalfloor. In addition, the maximum area thatmight be ultimately utilized is normallybuilt-out with a raised floor whether or notthe current, near term, or even the actualultimate function requires the use of RAF.Extra costs associated with power and datacabling can also be incurred using RAF.These can be considerable costs, which mustbe assessed in those areas where RAF isoptional. Stairs, elevator shafts, piping,standpipes, etc. may have to be lengthenedaccordingly, and shall be considered as“related costs.”

RAF is only one of many elements that mustbe assessed in determining value of a design.RAF may increase or decrease this value. Inaddition to first cost impacts on cablemanagement and flexibility, economicconsiderations shall include the impact thatutilization of RAF may have on health,safety, security, and sustainability (seeChapter 1, Sections 2 and 6 of PBS P-100).

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Appendix A-2 of this Guideline describesmethods for quantitative analysis and alsoprovides information relating to benefits andcosts that can be used in qualitative assess-ments. It includes discussion of:

Life-Cycle Cost and Benefit-CostMethods

Benefit FactorsRisk FactorsCost Factors

If a quantitative analysis is not possible for aspecific project, a qualitative assessment shallbe performed as outlined in Appendix A-2.

While accessible vertical distribution hastypically been accommodated in chases andshafts, horizontal distribution that can beaccessed for maintenance, repair and expan-sion is more difficult. Various systems havebeen - and still are - used. Probably themost common practice in modern buildingsis to use the space above a suspended or falseceiling for horizontal distribution, and tofeed the cabling and wiring down throughpartitions to outlets for power, communica-tions, and controls. In older commercialbuildings with masonry partitions, the dropswere typically cut into chases, then finishes(usually plaster) covered the chases andwiring. Renovations in these facilities oftenresulted in the abandonment of the existingwiring and the cutting of new chases. Withhollow partition construction, such asgypsum board on metal studs, these dropsare now more easily installed and can berelatively easily changed.

Above-ceiling distribution to offices, andother occupancies consisting of privaterooms, where all work furnishings adjoin apartition with its outlets, was - and remains- a viable means of providing services.Cabling and wiring for power, communica-tions, data and other needs can be distrib-uted over ceilings or under floors, but whenmodular work furnishings are used for anopen office environment, supplying from thefloor is visually more appealing than drop-ping the wiring from the ceiling in openspace areas.

In some instances, such as where flat plateconcrete slabs are exposed above and below,no space for concealment of wiring exists. Asa result, cellular floor ducts in metal floorpanel systems or embedded ducts may bebuilt into floor systems to feed up to “tomb-stone” outlets. The need for ever greater

3.0 Cable Management Technology and Planning

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flexibility of wire management and theincrease in sheer quantity of wires and fiberhas made a raised access floor (RAF) a moreappealing choice for many applications ofwire management (See Technology Infrastruc-ture, Chapter 3, Section 1, PBS P-100).Conversely, the large turning radii andseparation from power cables required byfiber optic cables have compromised theability of RAF to accommodate these newrequirements without increasing the floorheight, resulting in overhead cable trays(used in conjunction with bus ducts forpower) becoming a primary alternative toRAF (see Chapter 6, Section 11, PBS P-100). Also refer to Sections 1, 4 and 5 inthis Guideline for additional information.

Vertical alignment of electrical power, voice,and data closets is a critical function inplanning for RAF systems. Accessibleflexible horizontal pathways must be pro-vided from these closets (i.e., equipmentrooms) on each floor to the workstationoutlets. Horizontal pathways must provideat least three separate channels for separationof power and different communicationssystems. Moreover, the CAT 6 and fiberoptic cables must be separated from powercables. Independent channels are requiredin horizontal pathways for normal power;

emergency power; Building AutomationSystem (BAS) Sensor and control wiring; firealarm; security; and television and commu-nications, the latter which must includevoice and data wiring and cabling (seeChapter 6, Section 11 of PBS P-100). Therelative locations of these closets/equipmentrooms on each floor are critically importantto minimize the horizontal cross-overs of thevarious cabling and wiring, and the resultingclearance heights in the floor cavities.

As one of the purposes of modular worksta-tion furniture is its relative ease ofreconfiguration, disconnection andreconnection to floor-based systems isgenerally preferable and can use ordinaryplug-in devices.

Where raised access flooring is used, power,communications and data can be fed to theoccupied space from the underfloor spacethrough holes cut in the floor panels withhard-wiring run through grommeted holesto junction boxes or through proprietaryservice devices. The hard-wired connectionsmay be necessary for certain machinery andfor computer main frames, but are notusually needed for most office equipment.These service devices allow plug-in connec-tivity through devices that are flush with the

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floor or protrude into the occupied space.They often contain multiple services in asingle device.

Wiring and cable under a raised access flooris vulnerable to some of the same problemsas above the ceiling. Changes may result inabandoned wiring, conduit and devices.Eventually, an underfloor space can becomeas cluttered as a ceiling; it must be managedand kept clean. In the design phase, suffi-cient clearances must be provided to allowall underfloor systems to work together andto allow for maintenance, repair and replace-ment. With a raised access floor, an unan-ticipated conflict cannot be resolved duringconstruction by the equivalent of dropping aceiling in an area. So, providing adequateclearances from the earliest stages of design isessential, and includes the selection of the

raised access floor system. Some stringersystems drop below the bottom surface ofthe floor panel and thus reduce clearances.At least one manufacturer offers a 120 mm x120 mm (4 ¾ in x 4 ¾ in) under-structurestringer to span large items such as ductsand equipment under the floor, but thisadds significantly to the height needed floor-to-floor.

For renovations to existing buildings, raisedaccess flooring may provide a less intrusiveway to provide horizontal distribution thanby adding a suspended ceiling which maydestroy ornamental ceilings and the architec-ture of the interior. However, there aremany considerations, including interfacewith existing core elements such as stairs,elevators and toilets that can complicate thisapplication.

4.0 Architectural Design Considerations

Lessons learned from design reviews and on-site evaluations have revealed that successfulapplications of RAF require an early com-mitment to integrated design, with mem-bers of the design, construction, and prop-erty management actively participating indecision-making beginning at the prelimi-nary concept design phase (see Appendix A-3 of PBS P-100). This Section focuses on

architectural issues and criteria that must beaddressed by the design team throughoutthe design and construction phases.

4.1 Soil and Climatic Conditions andCriteria

Incursions from soil and climatic conditionsshall be controlled the same in facilities that

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have raised access floors as in those facilitieswith conventional flooring systems. There-fore, the design criteria for soil and climaticconditions pertain equally to facilities withunderfloor and overhead horizontal distribu-tion systems. However, RAF systems requirecontrol of three additional conditions: 1) thestructural slabs are the platforms for theRAF; 2) the slab and exterior wall surfaces inthe RAF cavities are generally not visiblewhen the floor panels are in place; and 3)the slab and wall surfaces in the RAF cavitiesare in continuous and proximate contactwith the cabling and wiring (see Section1.0). Thus, water and contaminant incur-sions into the RAF cavities from below andabove-grade sources shall be minimized toprotect the wire and cable managementsystems, and to minimize the risks of fireand adverse health effects. Criteria (i.e.,parameters and values) for the following soiland climatic conditions shall be defined,measured, and controlled during design,construction and operations:

For at-grade or below-grade floor cavi-ties:

Ground water pressure and tempera-ture at slab interface.Contaminants in ground water:unusual types or concentrations.Soil gases: unusual types or concen-trations.

Microbiological contaminants:unusual types or concentrations.Vermin: types.Pesticides in soil at slab interface:types and concentrations.

For at-grade or above-grade floor cavities:Summer and winter design dry-bulbtemperatures, and dew-pointtemperatures or water vapor pres-sures.Design wind and rain conditions:wind velocity and rainfall rates.Airborne contaminants: unusualtypes or concentrations.

4.2 Architectural, Structural, Seismic,Acoustic and Vibration DesignConditions and Criteria

As stated in Section 1.0, RAF systems areproprietary designs with many variations inthe specific features. Like curtain wallsystems, RAF systems are engineered bymanufacturers and are built from kits ofparts; they are usually not interchangeablefrom one manufacturer to another except forthe floor finishes, in some cases. Themanufacturers have established patentedpanel, stringer and connection designs fromwhich the architect/engineer must make aselection. Changes to the proprietary

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Overturning resistance of pedestalassembly.

As a minimum, or unless otherwise stated inthe building program, the selected RAFshall comply with the following structuralcriteria when all panels are installed:

Uniform load: 250 Lbs/ft2 (1210 kg/m2).Point load: 2,000 Lbs (4.5 kN).Impact load: 1,000 Lbs (kN).Rolling load (1 wheel): 800 Lbs (kN) @10,000 passes and 1,000 Lbs @ 10passes.

Criteria values for stringer loading, pedestalaxial loading, and overturning resistancemust be determined after analysis of seismic(see Section 4.2.2), acoustics and vibration(see Section 4.2.3), and other imposed liveloads. Initial selection of the RAF systemcan then be made based on compliance withthe set of criteria, as described in Section4.2.

4.2.2 Seismic Conditions andCriteria

Seismic and impact testing are not includedin the CISCA standard, but manufacturersassert seismic design capabilities. Generally,bolted stringer systems are used for seismic

designs are possible, but at a cost premium.Moreover, these changes will not havedemonstrated their functionality beforebeing built into the project.

So, in order to meet the specific GSArequirements and criteria for a project, theRAF system shall be selected carefully fromamong the choices offered by the industry.As sole-source designs and specifications aregenerally not allowed in GSA projects,selection of a product for a feature notavailable from other manufacturers may notbe sustainable.

4.2.1 Structural Conditions andCriteria

Raised access floor systems shall be selectedfrom those that are commercially availableand have been tested and rated under testmethods of the Ceilings and Interior Sys-tems Construction Association (CISCA)1 forthe following conditions:

Ultimate Load per panel, Lbs (kN).Concentrated Load @ 0.10" (2.5 mm)Deflection, Lbs (kN).Impact Load, Lbs (kN).Rolling Load 10 pass, Lbs. (kN).Rolling Load 10,000 pass, Lbs. (kN).Stringer loading.Pedestal axial loading.

1 In Europe, the comparable standard is EN 128252:2001 Raised Access Floors

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conditions with pedestal pads bolted ratherthan glued to the floor slab. For highpedestal conditions, additional seismicbracing is typically available, but the designof the system to resist a seismic force (Fp)shall be required of the manufacturer in thespecification. (This is the practice used inMasterspec© guide specification for Section10270).

RAF manufacturers generally list theirproducts with values given for performancein the above areas, and generally have lightduty and heavy duty product lines.

As a minimum, or unless otherwise stated inthe building program, the selected RAFshall comply with the following seismiccriteria when all panels are installed:

For seismic zones 3 and higher, stringersand pedestals, which are rated for thezone by the manufacturer, are requiredand the pedestal pads shall be bolted tothe slab.For pedestal heights of 12 in., or higher,in seismic zones 3 and higher, additionalbracing shall be provided, using meth-ods recommended by the manufacturer.Where this bracing is required, addi-tional coordination with horizontaldistribution services shall be imple-mented to assure adequate clearances.

4.2.3 Acoustics and VibrationConditions and Criteria

Three major areas of concern relate toacoustics and vibration control in RAFsystems:

The sound of impact on the floor panelsby walking and by rolling loads mayrequire damping of the panels withcushions on the pedestals or stringers, inaddition to adequate fill of the panelbody. Hollow metal panels do notperform well in this respect while panelswith cementitious, lightweight concreteand wood perform well. Die-castaluminum may need damping.The transmission of sound under thefloor from one space to another mayoccur. Where sound transmissionattenuation is required of a partitionsystem (i.e., see Chapter 3, Section 4:Special Design Considerations, and Table3.5 in PBS P-100), the same construc-tion shall be continued through theaccess floor to the slab where it shall besealed with an acoustic sealant. Wherethis partition system exists below thefloor panels, cable and wire managementshall assure that penetrations throughthe partition systems have been ad-equately sealed, acoustically.Vibration transmission is typically not

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addressed in RAF manufacturers’literature, nor are the criteria for designto attenuate vibration such as frommachinery, transformers or othersources. Where this is a sensitive issueor concern, a specialist consultantshould be engaged.

As a minimum, or unless otherwise stated inthe building program, the selected RAFsystem shall comply with the followingacoustic and vibration criteria when allpanels are installed:

The acoustic criteria in PBS P-100 applyequally to federal facilities with RAFsystems and conventional flooringsystems. It is the responsibility of thedesign team to meet these minimumstandards governing the acoustic perfor-mance of various spaces and usagecategories.The floor panels shall be concrete filledmetal or concrete.Carpet tile shall be used where acousticsis a concern, most notably in office workareas, so that maintenance of systemsunder the floor can be done withoutdestroying the carpet.The RAF system (i.e., panels, pedestals,and stringers) shall be designed so thatresonance frequencies of the RAF andslab are avoided. The vibration of the

combination of RAF system and the slabshall meet the Floor Vibration require-ments in Chapter 4, Section 4 of thePBS P-100.

4.3 Fire, Smoke, Safety, and SecurityZone Issues and Criteria

As indicated in the previous Sections,some features and characteristics of RAF aresimilar to overhead distribution systems.However, there are significant differences,including the proximity in the RAF cavitiesof potential fire, smoke and other hazards tothe occupants. This Section specifiesminimum criteria to reduce risks to occu-pants, first responders, and property assets.

4.3.1 Issues

Fire, smoke and life safety issues in facilitieswith RAF include:

NFPA 75: Protection of Electronic Com-puter/Data Processing Equipment ad-dresses a variety of requirements forraised access floors in “informationtechnology rooms.” For the purposes ofChapter 7, Section 16 in PBS P-100,“Essential Electronic Facilities” are“information technology equipmentrooms” as used in Chapter 8 of NFPA 75and related provisions.

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Detection and extinguishment systemsin the cavities under RAF systems arenot addressed in the model codes andare unevenly addressed by local jurisdic-tions except in areas covered by NFPA75. This lack of guidance could becomean issue if insulation or other materialswere to ignite in these cavities. Detec-tion by ceiling smoke or heat sensorscould be delayed and extinguishment byceiling sprinklers would be improbable.However, adding detection and extin-guishment under all raised access floorswould be a significant cost consider-ation.Detection and extinguishment systems,which are placed within the RAF cavity,must be coordinated with the above-floor systems and be included in the firecontrol center monitoring capabilities.If a fire were to occur in a RAF cavity,there could be a special hazard to firstresponders should there be failures inthe floor system or if sections of the floorpanels were missing at the time. In asmoke-filled, unlighted environment,the raised access floor could add anotherchallenge to the firefighters.Where fire and smoke-rated partitionsare required, they should pass throughthe raised access floor to the slab using

the same construction means andmethods as for such partitions aboveceilings, including firestopping, firedampers, etc. This procedure mustinclude the RAF cavity under doorways.

Security issues in facilities with RAF in-clude:

Where RAF is used within a securedarea, the secured wall or partitionenclosure must be maintained below thefloor as it is above a ceiling.If security includes acoustic isolation,the surrounding construction must havethe same acoustic properties below thefloor as above.Where openings occur in securedpartitions below the floor, they must beprotected in a similar manner as theopenings above the ceiling. Whereintrusion alarms are required above theceiling, they should similarly be pro-vided in the RAF cavity.The same special secure construction forSCIFs applies to RAF as to suspendedceilings.

Physical Safety issues in facilities with RAFinclude:

The full load capability of a RAF is onlyrealized when all of the floor panels arein place, as the buckling (lateral)

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strength of the floor depends on thepresence of the panels. The safety ofoccupants must be assured wheneverfloor panels and any associated stringersare damaged or removed.Procedures to assure safe performance ofthe RAF after stringers and panels havebeen replaced shall be specified (SeeSections 6 and 7 of this Guideline foradditional discussion).

4.3.2 Minimum Criteria

As a minimum, the selected RAF systemshall comply with the following fire-safety,smoke management, safety, and securitycriteria when all panels are installed:

The RAF zone size for fire-safety, smokemanagement and security shall notexceed 5,000 square feet of floor area.Where zones are established by code orprogram needs for the space served bythe RAF, the demising partitions shallextend into the space under the floor aswell as above ceilings.Fire extinguishment systems as requiredin Chapter 8 of NFPA 75 shall beprovided under RAF for “EssentialElectronic Facilities”, but not underRAF for other spaces unless so directedby the GSA Fire Protection Engineerthrough the Project Manager.

Fire detection systems, as described inParagraph 8.2 of NFPA 75, shall beprovided under all raised access floorsunless exempted by the GSA FireProtection Engineer through the ProjectManager.To minimize the detection time ofemissions from combustion precursors inRAF cavities under Essential ElectronicFacilities as provided in Chapter 7,Section 16 of PBS P-100, “early warningsmoke detectors” in accordance withNFPA 75 paragraph 8.2 shall be in-stalled under the RAF zones in thesefacilities (see Section 5.0 of this Guide-line for additional details). In consulta-tion with the GSA regional fire protec-tion engineer, the following shall bedetermined:

The area of coverage and responsetime of the detectors.The alarm function (at the FireCommand Center) and the controlfunctions (e.g., equipment andpower shutdown).

To minimize the risk of an electrical firebeneath the RAF caused by the inad-vertent release of a sprinkler head in azone, an interlock with the EmergencyPower Off (EPO) Station shall beinstalled (see Section 5.0 of this Guide-line for additional details). The control

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circuit shall be designed in consultationwith the GSA Regional Fire ProtectionEngineer.Egress requirements in Chapters 7 and 8of PBS P-100 do not address the specialcase of fires and toxic emissions in RAFcavities (i.e., fire, smoke, and chemicalsbeneath the breathing levels of theoccupants). Because of the increasedrisk as a function of wiring and cablingdensity and potential security breachesbeneath RAF, egress requirements forRAF zones shall be evaluated on a site-specific basis in consultation with theGSA Regional Fire Protection Engineer.The maximum number of contiguouspanels and associated stringers that canbe removed for access to the cabling,wiring or air distribution componentsshall be specified.To minimize safety risks to first respond-ers, GSA Project Managers and designA/Es shall review with local first re-sponders the locations and designs ofareas in the building where raised accessflooring is proposed, and take intoconsideration any recommendations toimprove safety.

4.4 Interfaces with ArchitecturalMaterials

The following supplemental interfaces withadjacent finishes in new construction mustbe considered during design:

In new construction, raised access floorsshould intersect partitions, columns andother vertical surfaces in the same way asconventional floor finishes, whichusually consists of a top-set base ofresilient wood or other material allowedby code. In the case of RAF, if thepanels abutting the vertical surface mustbe removable to access some equipmentor device there, the base must notprevent that. Panels on stringers areeasily removed without tools, so the nextpanel can be lifted out to allow the oneagainst the wall to be slid away withoutdamage. Panels bolted to pedestals maybe similarly removed, though tools areneeded to do so, and the edge panel’sfasteners to pedestals may be inaccessibleunder the baseboard or trim.Where stairs occur in raised floor areas,special detailing around the first riser,the stringers and other features will beneeded to ensure an acceptable condi-tion.

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Where raised access floor surfaces abutother floor surfaces, the detail of thejoint between the two must be designedto avoid damage to the access floorpanels from rolling loads as they crossthe joint. Generally, this will requirefirm support of the panel on that edge.In existing construction, the interfacewith vertical surfaces is more complex.

Where raised access flooring is used inexisting buildings, the following must beresolved:

Ramps to the raised floor from existingfloors such as stair landings, elevatorlobbies, toilet rooms and other spaces:ramps must comply with accessibilityrequirements (See Chapter 1, Section 10of PBS P-100).Existing door openings must havesufficient height to clear the raised floorunless the heads of the openings can beraised in architecturally acceptable ways.In historically and architecturallysignificant buildings, it may be inadvis-able or impossible to raise entire door-ways intact, and the raised floor mayinterfere with jamb plinths.Existing ornate wall paneling,baseboards, pilasters, etc. may not becompatible with raised access floor if thenew floor level cannot be accommodated

by raising the baseboard withoutspoiling the wall treatment.Existing window stools – especiallythose virtually at existing floor level –and under-stool radiators may not beeasily interfaced with new raised accessflooring.Existing ornate grilles such as for heatingand ventilating may not be easily raisedto clear the raised access floor.

Many of the above problems may be similarto those encountered when adding a sus-pended ceiling in an existing historically andarchitecturally significant buildings, whichmay have the added problem of obscuringornate ceilings.

4.5 RAF Grids and Cavity Heights

In general, raised access floor panels used inthe U.S. are 24" x 24" (600 mm x 600 mm)which is the same as the Planning Grid inPBS P-100. The panel edge should abut thefinished surface of a penetrating or demisingpartition.

The pedestal height is determined by theneed for wiring and cabling management orother systems below the floor, includinganticipated expansion. If ductwork, pipingand equipment must be accommodated

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along with wiring and cabling, the systemsmust be sized and laid out early in thedesign to ensure that adequate clearances areprovided between pedestals and from slab tolowest part of the understructure, such asthe stringers or service outlets.

Ceiling heights required in Chapter 3,Section 2: Space Planning of PBS P-100,

must be measured from the top of thefinished floor panel to the underside of theceiling, whether suspended or not. Theimportance of determining the pedestalheight very early in the design obviouslyaffects the entire building section and overallheights. Raised access flooring in itself mayor may not result in additional buildingheight, depending on ceiling conditions.

5.0 Electrical Design Considerations and Criteria

Basic requirements for normal and emer-gency electrical power wiring distributionand for communications cabling beneath theRAF, and for grounding of the RAF, aredescribed in Chapter 6, Sections 10 – 14 ofPBS P-100. The following items highlightand supplement the PBS P-100 require-ments:

Separate panelboards shall be providedfor the power circuits routed beneathRAF.All wiring beneath RAF shall be routedin rigid metal or flexible conduit tounderfloor distribution boxes; onedistribution box per bay is recom-mended.Flush-mounted access floor service boxesshall be attached to the underfloordistribution boxes by means of a modu-

lar, prewired system to facilitate easyrelocation.Where distribution of emergency andclean power is required in areas servedby RAF, special consideration shall begiven to assure the availability of thepower during emergency conditions(e.g., which, if any, emergency powercircuits should be located beneath theRAF).The standard option for deliveringcommunications services in Federalbuildings is by laying cable in a tray formain runs and then branching directlyon the floor slab below the RAF. Specialconsideration shall be given to thelocation of the cable trays in RAFcavities where plumbing or hydronicpiping is co-located.

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All RAF shall be grounded. A ground-ing conductor shall be bonded to everyother floor pedestal and shall be ex-tended to a common ground bus.Emergency lighting systems shall not bewired through the RAF.

For protection of emergency responders andoccupants in areas or zones with RAF (see

Section 4.3 of this Guideline), EmergencyPower Off (EPO) stations shall be located ateach major point of egress at that zone. Thefunction of the EPO stations shall be todisconnect all electrical power circuitsbeneath the RAF zone, before the responseteam enters the area. To avoid false alarms,the EPO stations shall be designed for useonly by authorized personnel.

6.0 Requirements for Documentation

6.1 Specifications for Raised AccessFlooring

If the Masterspec© Guide specificationSection 10270 is used, careful reference tothe Evaluations will provide guidance to theselection of raised access floor types inresponse to project-specific conditions.Special attention should be devoted to:

Requiring a pre-installation meeting toensure proper coordination of systemsintended to be placed under the floor,with the floor installation itself, and toemphasize the need to keep theunderfloor space clean.Requiring manufacturer certification insubmittals of performance requirementsspecified, especially seismic.

Clear detailing on design and shopdrawings of critical areas such as edgeconditions.Citation of all affected related sectionsincluding those in Divisions 15 and 16.Methods of cleaning the RAF cavities,including the installation of a centralvacuum cleaning system with multiplelocations to facilitate cleaning the RAFcavity.

If other guide specifications are used,attention to the same issues as above isstrongly recommended. Proprietary specifi-cations (and details) from individual manu-facturers are discouraged especially sincethey could result in exclusions of appropriatealternative brands, eliminate meaningfulcompetition, and result in higher costs.

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6.2 Drawing Details for Raised AccessFlooring

As proprietary products, raised access floorsshall be shown on the drawings generically,with the details focusing on the project-specific interfaces with other buildingcomponents and systems. Typically, detailsshall include:

Edge conditions at partitions, columns,walls, and built-up adjacent floors.Edges and thresholds under elevatorentrances, stair landings, ramps, etc.Partitions mounted on top of raisedaccess floors.Special acoustics and acoustic seals.Special trim and other details for raisedfloors in existing buildings, especiallywhere ornate wall, door and windowconditions occur.

Where more than one type of raised accessfloor, pedestal height or system occurs, plansshall clearly show the extent of each type orcondition.

Locations of underfloor equipment, if any,with respect to floor grid or above-floorfeatures shall be shown.

Locations of floor service outlets, registers,grilles and diffusers, if any, with respect to

modular furniture or other above-floorfeatures shall be shown.

6.3 Construction Documents beforeRelease

Because of the special details that are re-quired for RAF systems, additional qualitycontrol of the construction documents shallbe conducted. In addition to the Checklistsprovided in Appendix A.3 of the P-100, aSupplemental Review Checklist for RAFwithout UFAD is to be provided.

6.4 Operations and MaintenanceDocuments

The Property Manager shall be aware, byinstructions and documentation from theraised floor manufacturer and the designteam before, during and/or after installation,of the special issues involved in the operationof a building with raised access flooring,including:

Periodic cleaning of the underfloorspace, especially where panels have beenremoved for maintenance and repair,such as in computer rooms.Relocation of device outlets, floorregisters, grilles and diffusers, if any,when workstation furniture, partitionsor equipment are moved.

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Proper training of maintenance person-nel in:

Safety procedures in removing andreplacing panels and stringers.Cleaning products to be used onpanel surfaces and beneath the RAF.Handling and protecting panelswhen removing and replacing them.

Repair and/or replacement of floorpanels with damage, such as:

Excessive deformation from impactso that adjacent panels do not meettightly.Loss of panel edges allowing open-ings between panels.

Procedures for cutting openings in floorpanels, sealing edges, etc.

6.5 As-Built Drawings

Along with the transfer of responsibility tothe ownership team, and usually as part ofthe commissioning process, it is common toprovide the owner with key documents andrecords for reference in operating the facility.Benefits of raised access floors are the abilityto relocate or to provide electric power orvoice/data outlets and air supply outlets asthe need arises. To gain maximum benefitfrom the ease of these tasks mandates thatthe Property Manager maintain up-to-date

records on the location and routing of allcables, pipes and devices.

The As-built Drawings should be main-tained, preferably in electronic form, andeach time a modification is undertaken, thedrawings should be updated. It is highlyrecommended to remove abandoned cablesand wires, or indicate on the As-builtDrawings that they have been abandoned inplace.

When replacing cables, the color codingformat used in the construction phaseshould be adhered to. Any deviation fromthis format shall be indicated on the As-built Drawings.

The Property Manager should ensure thatfirst responders – especially fire fighters –know of the presence of the RAF so that inevent of an emergency, they are aware thatthey must proceed with caution in areaswith the raised floor (see Section 4.3 of thisGuideline).

6.6 Housekeeping

A visual inspection shall be made periodi-cally to assess the cleanliness of the floorcavity. At the completion of construction,

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the below-floor cavity should have beensealed and thoroughly cleaned. However,over time it may become contaminated withairborne dust, insect colonies, vermin,microbial growth, moisture, or other foreignmaterials. It is necessary that any suchinspection be planned in such a manner thatall areas can be visually inspected. The

cleaning of RAF cavities should be per-formed and documented by those whospecialize in this work, as there is always thepossibility of causing damage to cablelayouts or connections, mechanical devices,and, anyone working in the floor cavity maybe subject to personal harm because of theelectric power devices.

7.0 Inspection, Testing and Commissioning

Sections 3 – 6 of this Guideline includeconsiderable information concerning thecoordination of the multiple trades inassuring the structural, wire-management,fire safety, and security performance of RAFsystems. To achieve this required perfor-mance, diligence is required from multiplelayers of responsibility in inspecting thework as the construction activities proceed.

7.1 Inspection

In general terms, the levels of inspection forRAF without UFAD are the same as forconventional flooring systems, and are asfollows:

Level

1. Self inspection by tradesmen performingthe work on a specific task.

2. Inspection of a specific task by foreman.

3. Inspection of multiple tasks in a giventrade by foreman and responsibleexecutive.

4. Inspection of given trade of interfacewith other trades by foreman and/orresponsible executive.

5. Inspection of integrated trades and taskscomprising the whole by superintendentof construction.

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6. Inspection of integrated trades and taskscomprising the whole by responsibledesign team representative and commis-sioning agent (where applicable).

7. Inspection of integrated trades and taskscomprising the whole by owner’s agentor representative.

Some levels of inspection are understandablymore formal than others. In general, the topthree levels (5, 6 and 7) should require aformality of inspection and “approval” ofsome nature. This is particularly true whenthe successful achievement of numeroustrades is necessary for the performance of thecompleted RAF system.

7.2 Testing and Commissioning

Following the completion of the construc-tion and the various levels of inspection andapprovals, the system is ready to be tested,as part of the commissioning process.Structural and seismic performance tests (seeSections 4.2.1 and 4.2.2) may be requiredunder special conditions, otherwise inspec-tion for compliance with specifications maybe sufficient. However, performance testingfor cable and wire management (see Sections3 and 5), acoustics and vibration control (seeSection 4.2.3) and fire and security control(see Section 4.3) shall be conducted as partof the commissioning process. The commis-sioning agent shall serve an inspection anddocumentation role as defined in the level 6inspection step (see Section 7.1).

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U.S. COURTHOUSE, LOS ANGELES, CALIFORNIA

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Part 2:Underfloor Air Distribution (UFAD) Systems

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8.0 Features and Characteristics

During the closing years of the century,computer, data, and communication tech-nology had undergone extensive changesthat significantly impacted the workplaceenvironment. Area lighting was beingreplaced by a combination of environmentaland task lighting; and convenience poweroutlets were required for personal comput-ers, printers, and other electrical devices.And, there was the network of cables re-quired for communications, data transmis-sion, signaling, safety and environmentalcontrol systems.

As discussed in Section 1.0, a logical methodof distributing and managing this multitudeof wires was in an accessible underfloorcavity much like the computer rooms of fiftyyears earlier. Also, like the computer roomsof fifty years earlier, the presence of theunderfloor cavity provided the opportunityto use it as a supply air plenum to deliverconditioned air to the vicinity of the workstations for the purpose of air conditioningfor human comfort.

Thermodynamically, however, there is littlesimilarity between the earlier process ofserving single, reasonably high density and

When the raised access floor (RAF) cavityserves the secondary purpose of deliveringconditioned air to occupied zones (e.g.,office areas and courtrooms), the floorsystem becomes known as an Underfloor AirDistribution (UFAD) System. As indicated inSection 1.0 of this Guideline, the genesis ofcontemporary UFAD systems was from mainframe computer facilities in the mid twenti-eth century, in which the computer roomhad numerous types of computers and dataprocessors with relatively high and constantheat dissipation rates. The major objectiveof the cooling system was to remove the heatfrom the machinery while maintaining arelatively constant dry bulb temperature andrelative humidity in the computer room.These facilities generally consisted of oneroom conditioned as a single zone withconditioned air supplied into the floor cavityfrom a packaged air-conditioning unit orunits. Air supply outlets were generallyplaced in the floor panels near the heatsources in such locations as to minimizediscomfort to the individuals “operating” thecomputer machinery. The cooling loadswere usually high density and relativelyconstant.

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relatively constant machine cooling loadsand providing air conditioning for humancomfort from a single conditioning system tomultiple zones of control with widelyvarying loads.

Functionally, UFAD systems have manysimilarities to the more “conventional airdistribution” (CAD) systems, such asoverhead or fan-coil systems. Both UFADand CAD systems must provide the sameenvironmental conditions for human com-fort during load conditions that vary fromfull cooling loads, to no loads, to full heatingloads. At the same time, both systems areexpected to maintain the required ventila-tion rates at all occupied times in each roomor space being conditioned, satisfy differentpart load requirements in adjoining zones,maintain air-pressure differences betweenzones and, when required, provide zoneisolation for contaminant, acoustic andsecurity control. However, the methods forachieving this control are significantlydifferent for UFAD and CAD systems. CADsystems depend on induction of supply air(e.g., from ceiling diffusers or sidewallgrilles) with room air that is not in theoccupied zone to provide comfort conditionsin the occupied zone while ventilating andpressuring the zone. Conversely, UFAD

systems depend on mixing supply air (e.g.,from the floor plenum) with room air in theoccupied zone to provide thermal comfort,ventilation and pressurization. Moreover,the return air for both UFAD and CADsystems is typically through ceiling grillesinto return air plenums. Thus, thepsychrometrics for UFAD systems must bedesigned to include additional processes inorder to achieve the same conditions in theoccupied zones.

In addition to the psychrometric and airdistribution differences between UFAD andCAD systems, there are significant physicalissues that must be resolved concerning theunderfloor supply air plenum, includingwater incursion, alarm and drainage; airleakage into the conditioned space and intobuilding cavities; thermal inertia of plenumfloor slabs and walls; interferences betweenmechanical and electrical components anddevices; acoustical isolation; fire safety andalarms, etc.

The most fundamental step in the decision-making process of whether or not to use anunderfloor air distribution (UFAD) systemshould always be directed toward theobjective of providing an air-conditioningsystem that will meet all of the thermal

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elevator opening must ensure edgesupport for rolling loads as carts arerolled off an elevator onto the floor. Theplenum must be sealed at the sill of theelevator to ensure that the underfloorplenum does not leak into shaft spaces.Auditorium, jury box and other court-room seating areas with sloped orstepped floors. Special care must betaken to properly seal sloped areas andareas with steps. Scissor lifts to raisedplatforms for accessibility must bedetailed with bulkheads around themechanism to prevent air leakage fromthe plenum.Secure spaces such as secure corridors,credit unions, and the Security ControlCenter. If used in these areas, the secureconstruction of the surrounding wallsmust extend through the raised accessfloor to the slab below, and openings forUFAD airflow must be securely barredto prevent surreptitious exit.Response spaces, such as the Fire Com-mand Center, Engineering and BASCenter: These areas must function notonly at all normal periods, but alsoduring emergency conditions.

Some areas are inappropriate for UFAD:Slab-on-grade locations: Long termprotection is difficult against heat,

comfort, indoor air quality, acoustic andvibration (i.e., environmental quality)criteria without compromise. As in theselection of any such system, the designstudy should start with analyzing the zonesthat are to be occupied. The two majorissues to be resolved in this regard are first,room air distribution and, second, thecontrol techniques. (Additional informationis provided in Appendix A-3 of this Guide-line).

All building areas to be considered forUFAD should be carefully evaluated on acase-by-case basis, but the following aresome general guidelines for appropriateness.Similar to the applications for RAF discussedin Section 1.0, the areas that are mostsuitable for UFAD include:

Computer rooms and other informationtechnology or electronic equipmentspaces.General open office areas.Training and conference areas.Exhibit spaces.

Some areas require special precautions ifUFAD is used:

Elevator lobbies: If necessary, UFADmay be used, though the detail of theedge of the floor with the sill of the

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moisture, and contaminant transfer withthe soil, soil gases, ground water, andvermin through joints and cracks.UFAD shall not be placed directly onconcrete slab-on-grade surfaces.Slabs over garages, overhangs, andloading docks: Long term protection isdifficult against heat, moisture, andcontaminant transfer through joints andcracks in concrete slabs over theselocations. UFAD shall not be placeddirectly on concrete slabs over theselocations.Toilets, showers, baths, janitors’ closets,dishwashing and other wet areas haveplumbing fixtures which cannot rest ona raised access floor and could notsupport UFAD. Potential leakage andhigh humidity can cause corrosion ofpanels and understructure and encour-age growth of mold and bacteria. Therisk of overflow of water from fixtures orleakage from water piping is exacerbatedin UFAD because the plenum must beairtight, so the plenum could literallyfill with water and cause structuralfailure.Kitchens and food preparation areas areunsuitable for the same reasons as for thespaces listed above. In addition to thehazards of leakage and wet operations,the amount of equipment and heavy

traffic make floor diffusers undesirable.Some break rooms with kitchenetteshave been placed on raised access floorswith UFAD when water supply anddrains have been possible by connectionto a wet column. But the risk of spillageof food and liquids and seepage into theunderfloor space makes this a potentialproblem area. The risk of leakage ofwater into the plenum is as discussedabove for other wet spaces. The risk ofvermin below the floor is high.Laboratories – similar challenges inaccommodating plumbing and prevent-ing leakage into the plenum exist as fortoilets, etc., depending upon whetherthe laboratory has any wet processes,emergency showers, chemical spills, etc.Prisoner cells or holding cells.Fire stairs. Stair landings must interfacewith any adjacent raised access floors as aflush condition with the edge of theaccess floor well supported. UFADextended into the stair area must besealed to prevent flow of air into spaceswithin the stair, if hollow such as in steelconstruction, thence to other spaces;Mechanical equipment rooms such asboiler and chiller rooms, and rooms forprimary air-handling units (AHU).Central storage rooms and loading areas,trash rooms, etc.

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UPS, emergency generator, and similarrooms.Fitness centers where equipment, matsand other items could cover floordiffusers frequently.High traffic areas such as lobbies, atriaand major corridors where floor diffuserscould be easily damaged, become atripping hazard or collect dirt trackedover them.Dining areas where tables and chairswould regularly impede the diffuser airflow, and where spills of liquids and

solids are common. Floor air diffuserscould also be uncomfortable for diners.Child care centers where playthings,equipment and mats will often coverfloor diffusers, and where floor diffusersmay provide tempting objects forchildren to play with, drop things into,etc. Children on the floor may not becomfortable with the air distributionpattern, and could be at increased risk toexposures from contaminants in theplenum.

9.0 Economic Considerations for Underfloor Air Distribution

As indicated in the Introduction of thisGuideline, Section 4 in Chapter 5 of PBS P-100 identifies UFAD as one of the fourPerimeter and Interior Heating and CoolingSystems that may be considered as the HVACBaseline System. For the basic case wherethe application of UFAD is being consideredfor an area of the facility in which RAF haspreviously been selected in accordance withPart 1, the application of UFAD will requiremodifications to the design of the RAF inaccordance with Part 2. However, for thecase where an UFAD application is beingconsidered for an area in which an RAF hasnot been previously selected for cablemanagement, the RAF must perform in

accordance with Part 2 and the relevantsections of Part 1.

Therefore, to ensure that the value of thecontemplated UFAD system is maximized, abenefit-cost method shall be utilized inwhich the life cycle costs and benefits ofUFAD in areas with horizontal pathways forwiring and cabling distribution below theRAF, as modified for UFAD in accordancewith Part 2 (see Sections 10.0, 11.1 – 11.7and 12.1 – 12.3), are compared to the lifecycle costs and benefits of a facility with aCAD system and horizontal pathways forwiring and cabling distribution below theRAF, assuming it has been selected in

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accordance with Part 1 but not modified inaccordance with Part 2. For areas whereRAF has not been previously selected, thelife cycle costs and benefits of the contem-plated UFAD with RAF that is in accor-dance with Parts 1 and 2 shall be comparedwith a CAD system, conventional flooring,and horizontal pathways for wiring andcabling distribution above the suspendedceiling. UFAD is economically preferred ifthe benefit-cost ratio of UFAD exceeds thatof CAD.

UFAD is one of many options that must beassessed in determining value of a design.UFAD may increase or decrease this value.The benefit-cost analysis must include firstcost, long-term operations and maintenance,and continuous testing for acceptable levelsof air leakage from the UFAD plenum

throughout the life of the system. Economicconsiderations shall also include the impactthat utilization of UFAD may have onhealth, safety, security, and sustainability(see Chapter 1, Sections 2 and 6 of PBS P-100).

Appendix A-2 of this Guideline describesmethods for quantitative analysis and alsoprovides information relating to benefits andcosts that can be used in qualitative assess-ments. It includes discussion of:

Benefit-Cost and LCC MethodsIncremental Benefit FactorsIncremental Risk FactorsIncremental Cost Factors

If a quantitative analysis is not possible for aspecific project, a qualitative assessment shallbe performed as outlined in Appendix A-2.

10.0 Interface between Cable Management and AirDistribution

Many of the issues of cable management forUFAD are the same as for RAF (see Section3.0 of this Guideline). But the applicationof UFAD adds issues that must be addressedin cable management because the UFADplenum must be airtight.

All penetrations of wiring and cableentering or exiting the plenum andpassing through underfloor air barriersmust be made airtight. This may beaccomplished in some cases by gasketingor other methods of sealing. Methodsof achieving this air tightness shall be

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planned for and specified (see Sections11 – 15 for additional guidance).Even when the most rigorous care hasbeen taken to seal wiring and cable thatenters the plenum, even greater caremust be continued after occupancy tore-seal openings where cable, conduitand other wiring devices are removedand to be sure new wiring, cable andconduit are sealed when installed lateron. This latter level of care may be themost difficult to achieve since thereplacement of wiring will go on for thelife of the building, long after theoriginal occupants and managers haveleft. Methods of training contractorsand Property Managers on sustainingthe air tightness shall be planned for andimplemented (see Section 14 for addi-tional guidance).The potential hazard of fire and smoke isalways present with large aggregations ofelectric wiring, especially as time passesand wiring is changed, flexed andreattached. Because an underfloor airplenum of UFAD is where the primary

air supply is located, the propagation ofsmoke and other toxic emissions from anelectrical fire is far greater and immedi-ate than when the air supply is ducted.Additional methods of smoke and firedetection and protection shall beplanned for and implemented (seeSections 11.5 – 11.6 for additionalguidance).Methods of ensuring adequate clearancefor wiring, cabling, air ducts, and otherservices in the UFAD plenum shall beplanned for, detailed on drawings, andensured during construction, operationsand maintenance, and modifications (seeSections 11 – 15 for additional guid-ance).Where exiting from the UFAD plenum,power, communication and data wiringmust be placed in junction boxes thatare air-tight. Methods of ensuring airtightness during operations, mainte-nance and operations shall be plannedfor and implemented (see Sections 14and 15 for additional guidance).

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The introduction of plenum underfloor airdistribution (as differentiated from ductedunderfloor air distribution) through raisedaccess floors (i.e., RAF with UFAD) requiresspecial design attention of the architect tobring together a variety of building productsand systems to create an airtight plenum.The use of the underfloor space as an airplenum for cooling is as old as the use ofraised access floors in computer rooms. Fiftyyears of computer room experience and morerecent experience with UFAD have providedlessons that must be applied to futureUFAD installations if the systems are toperform according to desired expectations,including:

Maintenance of the plenum staticpressure to allow floor diffusers toeffectively supply clean and thermallytreated (e.g., cooled and dehumidified,heated and humidified, and filtered) airfor occupant health and comfort, and toachieve expected return air temperaturesand airflow rates.Prevention of contamination of supplyair with particulates, vapors and gasesthat may accumulate in the plenum.Coordination of the plenum space toaccommodate all systems – structural,architectural, mechanical and electrical –

that need to be in the space whileensuring that all will function properly.Operation and maintenance proceduresthat can ensure long-term health, safety,comfort, customer satisfaction, andeconomic performance.

11.1 Soil and Climatic Conditions andCriteria

Incursions from soil and climatic conditionsinto the supply air plenums of UFADsystems require special consideration andcontrol. These plenums must remainimpervious to air infiltration and water andcontaminant incursions from above-gradeand below-grade ambient sources through-out the life of the facility. The designcriteria for soil and climatic conditions thatare described in Section 4.1 of this Guide-line pertain equally to facilities with RAFwith and without UFAD. However, RAFwith UFAD requires control of four addi-tional conditions: 1) the incremental heattransfer through slabs and perimeter wallsurfaces to the plenums due to the require-ment to maintain the dry bulb and dewpoint temperatures of the supply air in theplenums at lower values than for RAFwithout UFAD; 2) the requirement to

11.0 Architectural Design Considerations

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maintain positive static pressures in theUFAD plenums; 3) the incremental require-ment to minimize leakage of air, water, soilgas, vermin and microbiological contami-nants through the joints, connections, andsubsequent cracks in the slab and perimeterwall surfaces exposed to the plenums (seeSection 8.0 of this Guideline); and 4) theincremental requirement to minimize thetransfer of soil or outdoor contaminants fromthe supply air plenums to the occupants orto the wire and cable management systems.Although the criteria are the same as thosegiven in Section 4.1, the methods of control,which are incrementally different for RAFwith UFAD, are described in the followingSections.

11.2 Air Leakage and WaterAccumulation Issues andCriteria for UFAD Plenums

11.2.1 Plenum Air Leakage andPerformance Criteria

The fundamental difference betweenRAF systems without UFAD andthose with UFAD is the need toconstruct the systems essentiallyairtight, if they are to serve as supplyair plenums. With ducted CADsystems, the need for less air leakagehas led to improved methods of

construction and the increasing useof various sealing methods as sys-tems have been designed with lessexcess capacity and for better energyefficiency. Specifications for airleakage rate limitations should bethe same for underfloor air plenumsas for supply ductwork operating inan equivalent pressure range(ASHRAE Handbook of Fundamen-tals 2001 Volume 1, Chapter 34).If this were the case, the expected airleakage as a percent of supply airflow would be 1.3 % (as determinedfrom Table 9, Chapter 34, for a ductwith Leakage Class 6 , a flow rate of3 cfm/100 ft2 and static pressure of0.5 in. w.g.). However, as discussedand categorized below (i.e., Catego-ries 1 and 2), UFAD plenums aremuch more complex than a ductsection, and air leakage through allpathways is expected to exceed thatfrom a duct section.

Therefore, as a maximum, the totalair leakage from all Category 1 andCategory 2 leaks shall not exceed0.1 cfm/ft2 floor area or10% of thedesign supply airflow rate, which-ever value is smaller, when theplenum static pressure is main-

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tained at its control value (e.g., 0.1in. w.g.) (See Section 15.2 foradditional performance criteriaand test methods).

From the standpoint of system performanceand these air leakage criteria, plenum leaksfall into two distinctly different categories:

Category 1 leaks (i.e., GeneralConstruction Leaks) are leaks of coolconditioned supply air from theunderfloor plenum into otherbuilding cavities thence either fromthe building or into return airpassages back to the air handlingunits or building exhaust air system.Such leaks have several negativeimpacts upon the system perfor-mance, the most immediate ofwhich is that, if the leakage rate issevere enough, there may not beenough air left to adequately coolthe space under high load condi-tions. Under that condition orconditions of even less such leakage,where the space load can still behandled, such leaks cause an energyburden of the worst kind – waste!All of the thermodynamic energyused to condition the air and theventilation (outdoor) air it contains

is wasted through loss. Addition-ally, the fan energy to move the airthrough the conditioning units andthe distribution system is wasted. Ifthe air leaks into plenums withsurface temperatures below theplenum air dew point temperature,condensation, with its consequences(e.g., mold growth, rust, deteriora-tion), can occur.

Category 2 leaks (i.e., ProductLeaks) are those that leak airthrough raised floor system compo-nents into the space to be condi-tioned. The major cause of trouble-some Category 2 leaks is specifyingthe wrong type products. Thecomponents through which suchleaks occur include:

Floor panel seams and edgeclosures.Electric power connection andoutlet devices.Communications & data cableoutlet devices.Air diffuser devices that do notclose tightly.

When specifying these devices, care shall betaken to specify ones that are designed to beairtight when subjected to pressure values

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that will be experienced with the respectivefloor system. There has been extensivediscussion in the industry regarding theeffects of the plenum air leakage into theoccupied space and only the mechanicalsystems designer can determine the leakagerate that would be detrimental to the systemperformance. It depends upon the loadanalysis for each individual space. Forexample, consider an interior room, say, aprivate office. When no one is occupyingthe office and the lights are out there is noload. If even a small quantity of air leakageprovides a constant flow of, say, 63º F airinto the room, the room air and its furnish-ings will ultimately reach 63ºF. Designersof early VAV systems called this phenom-enon the “ice box effect” which they solvedwith tight shutoff boxes, fan powered mixingor reheat – none of which are available toapply to air leakage from the floor. ThusCategory 2 air leakage should either beavoided or understood, and assurances takento offset the detrimental effects.

Another design option relating to Category2 leaks would be to provide zone bafflesaround each controlled zone and to ductsupply air into the zone through a VAVcontrol terminal – thus supplying theamount of air into the plenum that isrequired by the thermostat to handle the

room load. Under this scenario, the floorleakage would not be a thermal comfortfactor. The main disadvantage would be thenegative impact the duct and zone partitionswould have on cable routing and the loca-tion of machinery below the floor.

Because of the detrimental effect that leakagecan have on performance, the underfloorplenums shall be thoroughly leak tested andapproved by the general contractor, commis-sioning agent, architect/engineer of record andthe owner’s agent (see Section 15 for addi-tional guidance).

Except for computer room applications, fewmanufacturers offer products specificallydesigned for UFAD in non-computer roomenvironments, where leakage from theplenum and loss of static pressure is specifi-cally addressed. Section 15.2 providesadditional guidance and criteria that shall beused to minimizing air leakage, and Section11.6 provides additional guidance andcriteria that shall be used to ensure waterdetection and control.

Air leakage in RAF products and generalconstruction can occur through:

Joints or cracks between panels:In order that panels be readilyremoved and replaced for access

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purposes, the joints are usually notgasketed, so air can leak throughthese joints or cracks between thepanels. Even if gaskets are provided,removal of panels over time candamage the gaskets and edge trim.These cracks not only allow air toescape from the plenum, they alsoallow contaminants to enter theplenum by gravity or induction –especially during cleaning of thefloor surface.Where carpet tiles are staggered tocover the panel joints, leakage can bereduced, though field data on theeffectiveness under actual operationsare not available.At least one manufacturer offers anon-structural air-seal strip for useat panel joints where no stringers areused, though data on the effective-ness is not available.Stringer understructures may reducethe leakage between panels, thoughdata on the effectiveness is notavailable.

Panel accessories:The recessed units for plug-in ofpower, data and communications aretypically not designed to be airtight(i.e., they have been designed forRAF without UFAD). Experience

has shown these units to be asignificant source of air leakage (i.e.,approximately 10 – 15 cfm/device at0.1 in. w.g.). Control of this airleakage is a matter of product designthat the designer must resolve withproposed manufacturers by requir-ing maximum acceptable leakagerates in the specifications.Floor diffusers may leak significantlywhen supposedly closed. Experiencealso has shown these diffusers to be asignificant source of air leakage (i.e.,approximately 10 – 15 cfm/device at0.1 in. w.g.). Control of this airleakage is a matter of product designthat the designer must resolve withproposed manufacturers by requir-ing maximum acceptable leakagerates in the specifications (seeSections 14.1 and 15.2).

Intersection of the RAF with otherconstruction:

Most building trades are not accus-tomed to rigorous procedures toensure an airtight assembly, sodetailing must be very clear on theneed to seal the many potentialpoints where leakage might occur(see Sections 11.3 and 14 foradditional guidance):At all walls, partitions and columns,

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the floor panels must fit tightlyagainst all vertical surfaces surround-ing or penetrating the plenum.These joints must be sealed withsealant or gaskets.All wall finishes, such as gypsumboard, must extend to the slab ofthe plenum floor, be provided with aj-bead or other edge trim and besealed with sealant to preventleakage of plenum air into voidssuch as partition framing, furringand the like.At exterior walls where the plenumslab ends, airtight firestoppingmaterial must be used to preventleakage to the space below.At abutting built-up floor construc-tion, such as at toilet rooms, stairlandings and elevator shafts, solid airbarriers must be installed to preventleakage into shafts or voids in theadjacent floor construction.At expansion joints, the expansionjoint resilient filler constructionmust be airtight to prevent leakagefrom the plenum to the space belowor through the raised access flooritself.Where other equipment or construc-tion penetrates the plenum, such asplatform lifts, sliding fire doors,

stairs, etc., the penetrations must beisolated with airtight bulkheads.Wherever pipes, conduit, ducts orother items penetrate the wall of theplenum, they must be sealed fullyaround the perimeter.

Therefore, in order to minimize air leakagethrough joint and cracks between panels,through the interfaces with other construc-tion, and through the panels, themselves,raised access floors for UFAD applicationsshall be provided with stringers or otherdemonstrably equivalent air-sealing devices,staggered carpet tiles shall be provided, andfloor panel accessories with minimumleakage values shall be specified and in-stalled.

11.2.2 Water Accumulation andPerformance Criteria

Having sealed all joints of the air plenum tomake it airtight, it will then be effectivelywatertight as well, and vulnerable to indoorflooding. Should sprinklers discharge,hydronic or other devices leak, or condensa-tion build up on cold perimeter surfaces,water can accumulate:

In the event of a major leak such as asprinkler discharge, water could fill the

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plenum and, unless the structure isdesigned for the load, failure couldoccur. To prevent water accumulation inthe plenum, the following methods shallbe considered:

Moisture detectors in the plenumalarmed to a 24/7 monitoringlocation such as the security controlcenter would provide early warningso systems could be shut down.Some sensors are not very robustwhen accidentally stepped on orotherwise damaged.Floor drains designed to be airtightwhen inactive or self-priming floordrains may be more reliable (seeSections 11.6 for additional guid-ance and criteria).Design of the structure to support awater-filled plenum is possible, butmay be cost-prohibitive.

Other leaks or moisture accumulationmay not pose a flooding risk, but cancause significant damage to the wiringand cabling, corrode the RAF panels andunderstructure, and result in growth ofmold and bacteria on these surfaces. Tominimize the risk of water accumulationfrom other leaks or condensation, thefollowing methods must be considered:

Cold surfaces such as overhangs ofthe slab under the UFAD or contact

of the slab with the outdoor tem-peratures through spandrels shouldbe insulated and a vapor retardershould be installed on the warm sideof the surface (see Section 8.0 foradditional guidance on areas whereUFAD is not appropriate).Plumbing, sanitary, and hydronicpiping should not be installed inUFAD plenums.Provide supply air to the plenums atdew point temperatures at valuesnot to exceed 50oF (10oC).

11.3 Structural and SeismicConsiderations

The structural factors influencing the RAFdesign (see Sections 4.2.1 and 4.2.2) may beaffected by the UFAD requirements. RAFwithout UFAD may only require pedestalheights for wire management and be mosteasily built using stringerless construction(i.e., attachment of panels directly topedestals). UFAD may require greaterpedestal heights to allow for air transport,and for various system components to cleareach other.

Increased pedestal heights are morevulnerable to lateral instability,especially in seismic events, as hasbeen seen in computer rooms

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subjected to these forces. Theimportance of bracing increases withpedestal heights and the need tocoordinate lateral bracing withunder floor obstructions such asductwork is essential.Use of stringer understructure(bolted where appropriate forseismic conditions) improves thelateral stability of the floor system,especially when multiple panelsmust be removed, as may be the caseto maintain items under the floor.However, as stringer systems makeaccess to underfloor systems moredifficult and cost more thanstringerless installations, the tempta-tion to compromise the systemselection must be resisted.

11.4 Acoustics and VibrationsConsiderations

UFAD introduces to the RAF acousticsconsiderations (see Section 4.2.3) the need,in certain instances, to move air through anacoustical barrier under the floor. This maybe related to security if there is an eaves-dropping concern, so the means employedto maintain acoustical attenuation mayrequire approval of security authorities. The

types and locations of sound attenuators arelikely to have significant effects on thecontrol of pressurization and airflow in theUFAD plenums. However, the issues relatedto sound attenuation in the return airsystems are likely to be the same as in returnair plenums and ducts for other (e.g.,overhead) air distribution systems (seeSections 11.7, and 12.1 for additionalguidance).

UFAD also introduces additional vibrationissues. The location of the AHUs, terminaldevices in the plenum, and methods ofconnecting ductwork (i.e., perimeter reheatunits) to the floor diffusers can exacerbatethe vibration conditions (e.g., resonance)considered in Section 4.2.3. UFAD is notexpected to require a change in vibrationcriteria from 4.2.3, but additional methodsto achieve compliance may be required.

11.5 Fire, Smoke, Safety, andSecurity Zone Issues andCriteria

The introduction of air movement throughthe UFAD plenum introduces additionalissues and criteria to those in Section 4.3that must be addressed with regard to fire,smoke transmission, security, and egress.

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11.5.1 Issues

The fire, smoke, and safety considerationsare generally the same as those described forRAF without UFAD (Section 4.3), except:

With primary air supply movingthrough the UFAD plenum, the propa-gation of smoke, toxic fumes and fireinto the occupied zones may be acceler-ated.Fire extinguishment systems, whererequired, will add to the demand forspace under the floor and will requirestill more coordination through the GSAFire Protection Engineer and ProjectManager.Any sprinkler extinguishment systemcan further complicate the managementof the under floor plenum and poten-tially increase the needed pedestalheights.If sprinkler extinguishment systems areadded below the floor, the risk ofundetected accidental discharge rein-forces the need for moisture detectionand/or drainage of the plenum.Gaseous total flooding extinguishmentsystems as described in NFPA 75,paragraph 8.4 may avoid water damageand potential risk of overloading thebuilding structure from flooding theplenum.

UFAD introduces to the RAF securityconsiderations (see Section 4.3) the need tomove air through a secure barrier under thefloor. This additional concern can beavoided if the secured area is supplied by anindependent HVAC system. But if cost orother design factors dictate placement oftransfer openings for air through the securepartitions, adequate forced and surreptitiouspassage barriers, such as bars or grates (aswould be required for similar conditionsabove a ceiling), must be designed andapproved by security officials.

11.5.2 Minimum Criteria

In addition to the minimum criteria definedin Section 4.3.2, or unless otherwise specifi-cally excluded in the building program, theselected UFAD system shall comply with thefollowing fire-safety, smoke management,safety, and security criteria, evaluated withall RAF panels installed:

Fire extinguishment systems shall beprovided in plenums serving EssentialElectronic Facilities as provided inChapter 7, Section 16 of PBS P-100,and may be necessary elsewhere asdirected by the GSA Fire ProtectionEngineer through the Project Manager.To minimize the risk of an electrical firein a UFAD plenum caused by the

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inadvertent release of a sprinkler head ina zone, an interlock with the EPOStation shall be installed (see Section5.0). The control circuit shall bedesigned in consultation with the GSARegional Fire Protection Engineer.For physical and fire safety, the floormounted supply air diffusers and grillesshall be cast aluminum or steel, and theopenings shall be designed to preventpenetration by small heels (see Section11.7 for additional requirements).Egress requirements in Chapters 7 and 8of PBS P-100 do not address the specialcase of fires and toxic emissions inUFAD plenums (i.e., fire, smoke, andchemicals beneath the breathing levels ofthe occupants). Because of the increasedrisk as a function of wiring and cablingdensity, supply air transport through theUFAD plenum, and potential air leakageand security breaches in the UFADplenums, egress requirements for RAFzones shall be evaluated on a site-specificbasis in consultation with the GSARegional Fire Protection Engineer.

11.6 Detection and Drainage ofWater and Moisture in UFADPlenums

Protection from three major pathways ofwater accumulation shall be provided inUFAD plenums: 1) leaks from joints andcracks in slab and perimeter walls; 2)inadvertent floods from sprinkler dischargesor other accidents; and 3) condensation oncold surfaces in the plenum. Methods ofsealing and insulating surfaces in UFADplenums are described in Section 11.2 ofthis Guideline. However, to provide addi-tional protection, in case of water accumula-tion in the plenum, the following controlsshall be designed, specified and installed:

A moisture detection system for the slabsurfaces of the plenums with sufficientdetector sensitivity to provide warningthrough the Building AutomationSystem (BAS) so the area can be identi-fied and the source of moisture can belocated and repaired to prevent damageto the wiring and cabling. The types,numbers, and locations of the moisture

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detectors shall be selected based on thelikelihood of leaks from plumbing andhydronic piping in the plenum, and thelikelihood of condensation formation.The moisture detection system shall alsobe interlocked with the EPO Stations sothat the power in the plenum will beshut off after a specified time, if aresponse through the BAS has notoccurred.A drainage system in the slab of theplenum with sufficient self-primingtraps, or other comparable devices, tominimize damage to the wiring, cablingand UFAD devices in the plenum. Analarm shall be interlocked with drainagesystems and the EPO stations will alsobe interlocked with the drainage alarmso that the power in the plenum shall beshut off after a specified time, if aresponse through the BAS has notoccurred.

11.7 Room Air Distribution andReturn Air PathwayCoordination

Because the effectiveness of floor grilles anddiffusers in UFAD applications depends oncorrect vertical and horizontal throw of theair into the room, it is important to locatethese devices where they will not be covered

over or otherwise be impeded, or where theyare likely to cause occupant discomfort (seeSection 12.1 for additional details).

Many UFAD applications are associatedwith open space planning and using modu-lar furniture with compact work stationconfigurations. However, other applicationsinvolve private offices, conference rooms, andother enclosed zones as described in Section8.0. For both perimeter and interior UFADzones, the most common location for grillesand diffusers is in the raised floor panels.However, other locations often include low-sidewalls or partitions in which the grillesand diffusers primarily have horizontalthrows. Linear diffusers that are hard-ducted to fan-powered VAV boxes forheating and cooling are common for perim-eter zones, whereas ducted and unducteddiffusers and grilles in the raised floor panelsare common for cooling only in interiorzones. When linear diffusers are located inperimeter zones, care must be taken toassure that furniture (e.g., bookcases, filecabinets, credenzas, etc.) will not be placedon them. Similarly, care must be taken toassure that furniture, partitions, or otherdevices will not restrict the air distributionfrom interior floor diffusers:

Ensure that diffuser and grille locationsare shown on a drawing together with

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furniture placement, fixed and moveablepartitions, and other known items to beplaced on the RAF, so that physicalconflicts and occupant drafts areavoided, at least in the initial occupancy.Future conflicts can only be avoided byproper management of reconfigurationsof the space.When locating floor diffusers and grilles,conflicts with under floor features mustalso be avoided. Diffuser housings andassociated control devices project downinto the UFAD plenum and can conflictwith such items as ductwork, distribu-tion panels or air-handling units.(Additional air distribution consider-ations and performance criteria areprovided in Section 12).

UFAD plenum air distribution for a condi-tioned perimeter or interior zone shall notexceed unducted distances of 50 feet fromthe point of supply entry into the plenum tothe most distant air diffuser in that zone.

Multiple air-handling units per floor arerecommended, enabling precise/economicaloff-hour zone control. Connections (chilledwater/heating to air handling equipment)should enable “plug-n-play” change-out ofair-handling equipment if space thermalloads change significantly over the life of thebuilding.

The return air pathways from the return airgrilles of UFAD systems to the air handlingunits shall meet the same criteria as specifiedfor Plenum and Ducted Return Air Distribu-tion of CAD systems (see Chapter 5, Section8 of PBS P-100). The locations of thereturn air grilles to provide return air control(see Section 8.0 of this Guideline) shall bedetermined by the architect/engineer ofrecord, and shown on the constructiondrawings with sufficient details so that acomplete testing, adjusting and balancingprocedure can be achieved (see Section 15 ofthis Guideline).

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As a minimum, the conditions shown inTable 5-1 of PBS P-100 must be maintainedat all elevations between 150 mm (6 inches)and 1800 mm (6 feet) above the finishedfloor in all occupied zones. However, thetemperature values shown in Table 5-1 arefor dry bulb temperatures in occupied zoneswhere the air speed is less than 0.2 m/s (40ft/min) and net thermal radiant exchangebetween the occupants and surroundingsurfaces is negligible. Therefore, for UFADsystems, where occupied areas are in closeproximity to the floor grilles or diffusers thathave air velocities exceeding 0.2 m/s (40 ft/min), the acceptable values in Table 5-1shall be in terms of Operative Temperaturesas defined in ASHRAE Standard 55 (i.e., seeNote 13 in Table 5.1, Chapter 5, Section 3of PBS P-100).

To achieve these conditions, UFAD systemsrequire control of four additional factors notassociated with CAD systems: 1) the heattransfer through the structural slabs that arethe platforms for the RAF (i.e., the bottomsurfaces of the UFAD plenums); 2) theincremental heat, infiltration and watervapor transmission through exterior wallsurfaces that interface with the UFADplenums, which are approximately 10oF

The requirements for the design of UFADsystems, from a thermodynamic perspective,and from the perspective of design engineer-ing disciplines, are no different than those ofany other subsystem. The design engineermust thoroughly understand the fundamen-tal as well as the unique features and charac-teristics of the systems and components thatare being integrated into the building, andthen apply those features in such a way as toachieve the expected performance.

12.1 Thermal, Air Quality, andAcoustical Design Conditionsand Criteria

Design conditions must be equally con-trolled in facilities that have raised UFADsystems as in those facilities with CADsystems. Therefore, the design criteria forthermal, indoor air quality (IAQ) andacoustics and vibrations that are specified inChapter 5, Section 3: Design Criteria of PBSP-100 pertain equally to facilities withUFAD and CAD systems. However, withUFAD, special care is required to assure thatthe floor surface temperatures, and the airtemperatures and air velocities near the floor,will not result in thermal discomfort ordissatisfaction with the office environment.

12.0 Mechanical Design Considerations

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(5oC) lower than typical occupied zones; 3)the heat transfer through the floor panels tothe plenum; and 4) precise air temperatureand static pressure distributions throughoutthe UFAD plenums. These factors arefurther described in the following Sections.

12.2 System Time Constant (ThermalInertia)

A unique feature of UFAD systems (seeSection 8.0) is that the plenum throughwhich the supply air flows is the buildingstructure and the floor materials. If the non-active side of the structure (that not exposedto the supply air) is well insulated such thatit loses or absorbs a minimum amount ofheat from the surroundings, the entire masswill approach thermal equilibrium with thesupply air after several hours of operation.Thereafter, the supply air at the floordiffusers can be assumed to be essentiallyequal to the temperature of the air leavingthe air-handling unit and entering theplenum.

If, for capacity control purposes, an effort ismade to increase the supply air temperatureto reduce the capacity, the control responsecannot be predicted and thus an alternativeapproach to capacity reduction must beemployed.

Another design option that is not availableas a result of the thermal inertia phenom-enon is unoccupied cycle shutdown. Experi-ence reveals that it is very difficult, if notimpossible, with most UFAD systems toexercise an option of “night” or unoccupiedcycle shutdown for energy reduction. Theissue is, if a UFAD system is operating at,say, 63°F supply air temperature and thecooling system is turned off overnight or onweekends, the building and its structure willincrease in temperature, possibly evenexceeding the “occupied” indoor air tem-perature. Then, upon turning the systemback on, the supply air would gain heat,possibly even exceeding the room tempera-tures for several hours following startup.Thus, this Guideline does not encourage thepractice of unoccupied cycle shutdown

An alternative to unoccupied shutdownmight be an unoccupied control cycle thatwould make no effort to “pick up” the spaceload, but that would simply hold thestructure enclosing the air paths at thesupply air temperature.

Supply Air Reset Temperature:In VAV system design, if the roomhumidity is controlled by some meansother than the space cooling supply air,it is quite common to raise the tempera-

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ture of the air from an air-handling unitto that temperature required to serve thecooling need of the zone requiring themost cooling. This control option isdifficult to utilize successfully with aUFAD plenum because of the thermallag phenomenon. For systems in whichthis feature is otherwise deemed neces-sary, some engineers have installedducted systems in the floor cavity orinsulated the inside of the cavities withrigid thermal insulation. (Note: if thisinsulation option is being considered,refer to Chapter 5, Section 13: ThermalInsulation of PBS P-100 for additionalrequirements.)

Space Heating with UFAD:In areas of the country in which outdoortemperatures are such that buildingshave both heating and cooling loads, theheating loads cannot be served withwarm air through the air-handling unitsvia UFAD systems. Again, this isbecause of the inherent unpredictabilityof control when switching from coolingto heating and visa versa. When it isdesired to provide the heat from thefloor diffusers as is sometimes recom-mended for the floor outlets placedalong the outer or perimeter walls, thiscan be done by installing terminal

reheat coils working in conjunction withthe floor diffusers (see Section 11.7 inthis Guideline for related issues).

Heat Gain/Loss From PlenumIn most buildings, the UFAD plenumstructures are not in thermal equilib-rium with their surroundings. Suchcases are: 1) where a spandrel beamforms one wall of the plenum and issubjected to solar and transmission heatexchange with the outdoors; 2) thelower face (e.g., floor slab) of the ple-num is exposed to the “warm” return airplenum of the space below; 3) the faceof the plenum is exposed to outdoortemperatures in the space below (such asan outdoor soffit, unheated garage, orslab-on-grade); or 4) an interior verticalclosure is exposed to a non-conditionedspace. These situations can usually beanalyzed through well-accepted tran-sient heat transfer analyses and theimpact predicted. Once the dynamicsand magnitude of the heat transfercharacteristics have been determined,control algorithms can be developed toprevent these heat exchanges fromdetrimentally impacting the perfor-mance.

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13.0 Electrical Design Considerations

13.2 Sharing Space with MechanicalDevices

Whenever power wiring or cabling piercesthe enclosure of a UFAD plenum, or anHVAC zone or an acoustical partition withinthe plenum, it shall be done so in such away as to maintain an airtight seal. Whenthe cable or wire is in a conduit, both theannular space between the conduit and theclosure material shall be sealed and theconduit end shall be plugged to prevent airfrom leaking through the conduit. Thespecific methods of achieving such seals arethe responsibility of the design professionals.

Where ducts or mechanical devices arelocated such that they create potentialobstructions to the routing of cabling, aminimum of 4 inches (100 mm) of verticalclearance shall be provided to allow thecables to cross above the ducts. Whereductwork is run, care should also be taken toallow space for air diffuser extensions, andelectric power and voice/data terminationboxes that extend downward into the UFADplenum.

Cable management and planning for systemswith underfloor air require the same funda-mental considerations as for RAF systemswithout underfloor air as described inSection 5.0, with some additional consider-ations resulting from the fact that 1) thehorizontal pathway serves as an air distribu-tion path, and 2) the cables share the spacewith mechanical ducting, possibly piping,and other devices.

13.1 Cables in Air DistributionPathways (Plenums)

The National Electrical Code (NEC)addresses the requirements of power andvoice/data cables installed in air distributionpathways (“plenums”). These requirementsinclude:

Power wiring shall be protected inmetallic conduits or raceways.Voice/data cabling shall be insulatedwith “plenum rated” insulation.All cabling shall be routed for theminimum length possible and voice/datacable length shall not exceed 90 meters.All cables shall be color coded by service(phone, data, etc.) and clearly markedfor plenum use.

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A fundamental caution when planning aRAF with UFAD was stated in Section 3.0of this Guideline, to wit: “With a raised accessfloor, an unanticipated conflict cannot beresolved during construction by the equivalent ofdropping a ceiling in an area.” Thus, allpossible interferences must be anticipated

and resolved during the planning and designprocess. Additionally, since future technol-ogy is unknown at this time, spaces, clear-ances, routing and other geometrics thatmay be required in the future will need tobe accommodated by the underfloor airsystem design.

14.0 Requirements for Documentation

14.1 Specifications for UFAD,including a “Mockup”

If Masterspec© Guide specification Section10270 is used, careful reference to theEvaluations will provide guidance to selec-tion of raised access floor types in responseto project-specific conditions. However,Section 10270 is not specifically directed atUFAD, and will require appropriate modifi-cation. Related sections will also requirecoordination and modification to ensure alltrades involved in providing airtight ple-nums are alerted to the importance of thisrequirement.

In addition to the items specified in Section6.1 of this Guideline, special attentionshould be devoted to:

Requiring a pre-installation meeting to:Emphasize the importance of sealingthe plenum.

Ensure proper coordination ofsystems intended to be placed underthe floor with the floor installationitself.Emphasize the need to keep theunderfloor space clean.

Requiring manufacturer’s certification insubmittals of performance requirementsspecified, especially seismic.Clear detailing on design and shopdrawings of critical areas such as edgeconditions.Requiring that a full-size mockup beconstructed, on-site or off-site, of a largeenough segment of the UFAD to providea basis of standard of workmanship. Theperformance of the mockup shall betested in accordance with the procedurein Section 15.2.1. As stated in Section15.1, “It is highly recommended thatthe same craftsmen (or at a minimumthe same foremen) who are to execute

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the work on the building participate inbuilding, inspecting and pressure testingthe mockup. The mockup may be apart of the actual installation or it maybe a separate structure, the size of whichshould be no less than 1,000 square feet,if off-site, or two structural bays of thebuilding if a part of the actual construc-tion. As a minimum, the mockup shallcontain a representative quantity of thebuilding and system elements that arereasonably expected to impact theUFAD plenum performance includingbut not limited to:

Structural concrete work and details.Structural concrete sealants, sealercoats and finishes.Exterior walls.Elevator shafts, stairwells, escalatorareaways.Partition and solid walls finished tofloor.Partition and walls extended tostructure.Floor panels.Access floor penetrations.Electrical power and communica-tions outlets with connections, wireand conduit.Supply air diffusers, grilles, VAVboxes and controllers with thermo-stats.

Other temperature control deviceswith thermostats such as underfloorfan-powered VAV boxes withterminal reheat.Underfloor plenum dividers.Electrical and piping penetrations,conduits and cables.Structural column and shear wallpenetrations.Thermostat wiring details.Floor finish applications.

Specifying spare panels, stringers,pedestals and accessories for replace-ments.Citing all related sections of the specifi-cations that will be affected by creatingan airtight, clean plenum with appropri-ate moisture handling provisions. Theseshould all refer to the Raised AccessFloor/UFAD section as a “RelatedSection”. These may include, but not belimited to:

Division 3 – Concrete – Cast-in-placeconcrete and Floor topping sectionsshall include treatment of all cracksand joints, and the application of asealer coat on the slab under thefloor system as appropriate. Thissealer coat must not be confusedwith the sealer that may be appliedas part of the concrete curingprocess. A separate sealer coat shall

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be applied prior to the installationof the UFAD system, including theRAF framework, cables and wiring,and ductwork.Division 5 – Metals – Architecturaljoint systems section with expansionjoints must include need for floorjoints to be air tight.Division 7 – Thermal and MoistureProtection - Sealants must specificallybe designated for the plenum spacesand be selected accordingly; EIFS, ifany and if in any way connected tothe plenum, should refer to theconstruction needed to seal the EFISframing space from the plenum; fire-resistive joint systems (fire stops)must refer to the air-tightnessrequirements of such joints occur-ring in plenum slabs.Division 8 – Doors and Windows –Any door systems such as sliding,folding, etc. that must penetrate theplenum should reference to the needfor maintaining an air tight condi-tion with a barrier or bulkhead, ifthose are provided in these sections.Division 9 – Finishes

Gypsum board assemblies andshaft wall; other similar parti-tion, furring and similar assem-blies must specify that these

finishes must seal to the slab.Gypsum board must also betaped at the joints within theplenum. Acoustic, security, fireand smoke resistance characteris-tics must also be specified to becontinuous under the floor inthe plenum, including underdoor openings.Carpet tile should be specifiedto have joints occur over panelsrather than at panel joints.Carpet type should be selectedto generate minimal lint andother contaminants that couldpotentially fall or be inducedinto the plenum. Adhesivemust be used and applied thatwill not gum up panel screws.Ample replacement tiles mustbe specified to provide forchanges in floor service unit anddiffuser locations.

Division 10 – Specialties - Varioustypes of operable partitions, ifpenetrating the plenum, must beisolated with barriers or bulkheadsto prevent leakage. If resting on theraised access floor, adequate supportof their weight must be shown andspecified. A central vacuum clean-ing system shall be installed with

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multiple locations to facilitatecleaning of the UFAD plenum.Division 11 – Equipment – Anyequipment specified in this divisionthat may penetrate the plenum and/or rest on the slab under the plenummust be provided with barriersaround the equipment in theplenum or other measures to sealand prevent leakage. If equipment issupported on the RAF, adequatesupport must be shown and speci-fied, and equipment locations mustbe coordinated with floor diffusers,grilles, service units, etc.Division 12 – Furnishings - Anyfurnishings specified in this divisionmust be coordinated with floordiffusers, grilles, service units, etc.Division 13 – Special Construction -Security and fire detection andsuppression sections must reflect therequirements for the underfloorplenum, including coordinationwith other systems designed for theplenum penetrations of the plenumby wire, conduit or other devices forthese systems must be sealed andairtight;Division 14 – Conveying Systems –Any system that penetrates theplenum such as wheelchair lifts,

escalators, moving walks, etc. mustbe specified and shown to beisolated from the plenum by barriersor bulkheads to prevent air leakage.Elevator shafts must be detailed toshow separation from plenum byair-tight construction, including atthe entrance sills.Divisions 15 – Mechanical - Floordiffusers and grilles should be in thisdivision rather than in the RaisedAccess Floor section of Division 10.Emphasis is needed on providingcoordination drawings of underfloorductwork and locating floor diffuserscoordinated with furnishings,equipment, etc.Division 16 – Electrical – Floorservice units for power, data andcommunications should be in thisdivision rather than in the RaisedAccess Floor section of Division 10.The allowable air leakage of theseunits must also be addressed.Coordination drawings may beneeded for underfloor cable trays, ifany, locations of floor service unitsand coordination with furnishings,equipment, etc.

If other guide specifications are used,attention to the same issues as above is

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necessary. Proprietary specifications (anddetails) from individual manufacturers is notin compliance with federal acquisitionregulations and thus prohibited, since itcould exclude appropriate alternative brandsand eliminate meaningful competition andperhaps result in higher costs.

14.2 Drawing Details for UFAD

RAF with or without UFAD shall be shownon the drawings generically with the detailsfocusing on the project-specific interfaceswith other building components andsystems. Typically, details shall include:

Edge conditions with air seals at parti-tions, columns, walls, and built-upadjacent floors.Seals of partition finishes as they abutthe plenum slab. J-bead and sealantshall be shown and called out whereveran air seal is needed.Seals at plenum slab perimeter, includ-ing at firestopping.Seals of expansion joints and otherpenetrations.Air barriers or bulkheads around pen-etrations of plenum such as sliding firedoors, rolling doors, stairs, lifts, equip-ment, etc.Edges and air seals and thresholds underelevator entrances, stair landings, ramps,

etc.Partitions mounted on top of raisedaccess floors.Special acoustic properties and acousticseals.Special trim and other details for raisedfloors in existing buildings, especiallywhere ornate wall, door and windowconditions occur.

Where more than one type of raised accessfloor, pedestal height or system occurs, plansshall clearly show the extent of each type orcondition.

Locations of underfloor equipment, if any,with respect to floor grid or above-floorfeatures shall be shown.

Locations of floor service outlets, registers,grilles and diffusers, if any, with respect tomodular furniture or other above-floorfeatures shall be shown.

14.3 Evaluate ConstructionDocuments before Release

Because of the special details that are re-quired for RAF with UFAD, additionalquality control of the construction docu-ments shall be conducted. In addition tothe Checklists provided in Appendix A.3 of

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the P-100, a Supplemental Review Checklistfor RAF with UFAD is to be provided.

14.4 Operations and MaintenanceDocuments

The Property Manager shall be aware, byinstructions and documentation from theraised floor manufacturer and the ContractDocuments before, during and/or afterinstallation, of the special issues involved inthe operation of a building with RAF withUFAD, including:

Monitoring and supervision of changesto systems under the floor.

Abandoned electrical circuits shallbe removed completely, and anyopenings left in the walls or floor ofthe plenum must be sealed.When new wiring, conduit orcabling is run, penetrations of theplenum shall be sealed as theoriginal installation.

Periodic cleaning of the underfloor spaceis essential, especially where panels havebeen removed for maintenance andrepair.Training and supervision of O&Mpersonnel in relocation of device outlets,floor registers, grilles and diffusers, whenworkstation furniture, partitions or

equipment are moved, and replacementof panels previously pieced with floordiffusers and service units with solidpanels, and restoration of carpet tilekeeping the joint pattern intact. Thesemeasures are necessary to preserve theoriginal integrity of the plenum.Every time panels are removed for serviceor replacement, they and adjacent panelsshould be checked for damage, especiallyto edges.Proper training of maintenance person-nel in cleaning products and in handlingand protecting panels when removingand replacing them is required.Repair and/or replacement of floorpanels with damage such as:

Excessive deformation from impactso that adjacent panels do not meettightly.Loss of panel edges allowing open-ings between panels.

Procedures for cutting openings in floorpanels, sealing edges, etc. shall bespecified.To ensure plenum pressure, air leakagetests should be conducted in any areabeing modified that is 2,500 sf orgreater (see Section 15.2.3 for additionaldetails).

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14.5 As-Built Drawings

Along with the transfer of responsibility tothe ownership team, and usually as part ofthe commissioning process, it is common toprovide the owner with key documents andrecords for reference in operating the facility.Benefits of UFAD systems include theability to relocate or to provide electricpower or voice/data outlets and air supplyoutlets as the need arises. To gain maximumbenefit from the ease of these tasks mandatesthat the owner maintain up-to-date recordson the location and routing of all cables,pipes and devices.

The As-built Drawings should be main-tained, preferably in electronic form, andeach time a modification is undertaken, thedrawings should be updated. It is highlyrecommended to remove abandoned cables,wires, or ductwork or indicate on the As-built Drawings that they have been aban-doned in-place.

When replacing cables, the color codingformat used in the construction phaseshould be adhered to. Any deviation fromthis format shall be indicated on the As-built Drawings.

The air tightness of the underfloor plenummust be maintained at all times. Whenremoving a conduit or cable that passesthrough the plenum wall, slab floor or theraised floor, the opening must be sealed withan approved fire rated material. When newcables or conduits are being installed, theannular openings through which they passmust be sealed tightly (see Sections 11.2,13.2 and 15.2.2). All modifications shall beindicated on the As-built Drawings.

Another advantage of a UFAD system is thatas the spaces are reconfigured, the air outletscan be relocated by simply relocating theraised floor panel that contains the outlet.Although it may seem unnecessary to recordthe types and locations of the outlets on theAs-built Drawings, as they can be “seen,” itis quite possible that they could be coveredwith a carpet tile or piece of furniture.Thus, all outlets locations and types shouldbe recorded on the drawings.

Care should be taken if outlets are added soas not to upset the air balance or overloadthe fan motor.

If supply outlets are thermostatically con-trolled, the location of the thermostat thatcontrols each outlet should be clearlymarked on the As-built Drawings.

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The Property Manager shall be sure thatfirst responders – especially fire fighters –know of the presence of the RAF withUFAD so that in the event of an emergency,they are aware that they must proceed withcaution in areas with the raised floor (seeSection 11.5).

14.6 Housekeeping

A visual inspection shall be made periodi-cally to assess the cleanliness of the UFADplenum. At the completion of construction,the UFAD plenum surfaces are sealed andthoroughly cleaned. However, over timethey may become contaminated withairborne dust, insect colonies, vermin,microbial growth, moisture, or other foreignmaterials. It is necessary that any suchinspection be planned in such a manner thatall areas can be visually inspected. The

cleaning of UFAD plenums should only bedone by those who specialize in this work,since there is always the possibility ofcausing damage to cable layouts or connec-tions, mechanical devices, and inherentdanger to individuals working in the plenumdue to the presence of electric power devices.

14.7 Warranty Period

As the integrity of the UFAD plenum can becompromised by contractors and operatingpersonnel during and after initial occupancy(see Sections 14.4 – 14.6), an extendedwarranty that covers the UFAD plenum andspecifies acceptable air leakage rates from theplenum shall be coordinated with theContracting Officer to ensure continuousperformance after Substantial Completion.It is recommended that the warranty beextended for a minimum of five (5) years.

15.0 Inspection, Testing and Commissioning

Sections 11 – 14 include considerableinformation concerning the coordination ofthe multiple trades in assuring the airtightness of the underfloor air plenums.Also, Sections 11.2 and 15.2 providedetailed information and performancecriteria on plenum air leaks. To achieve therequired performance, diligence is required

from multiple layers of responsibility ininspecting the work as the constructionactivities proceed.

15.1 Inspection

In general terms, the seven levels of inspec-tion for RAF with UFAD are the same as

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identified in Section 7.1. Some such levelsof inspection are understandably moreformal than others. In general, the top threelevels (5, 6 and 7) should require a formalityof inspection and “approval” of some nature.This is particularly true when the successfulachievement among numerous trades isnecessary for the optimal performance of thecompleted UFAD system.

It is highly recommended that the samecraftsmen (or at minimum the same fore-men) who are to execute the work on thebuilding, participate in building, inspectingand pressure testing the mockup (see Section14.1).

15.2 Testing

Following the completion of the construc-tion and the various levels of inspection andapprovals, the system is ready to be tested.The mockup (see Section 14.1) shall betested prior to the actual construction of theUFAD plenum.

15.2.1 Test Procedure for Mockup

The mockup shall be tested prior to theconstruction of any of the permanentbuilding UFAD systems by the followingprocedure.

1. A test fan shall be provided which shallhave the capability of supplying variousairflow quantities from shutoff to 120%of design airflow quantity required forthe mock-up test and shall be driven bya variable speed inverter. The test fanshall be installed with an airflow teststation and the supply duct connectedinto the plenum with an adhesive sealedpressure tight connection.

2. A static pressure sensor-controller shallbe inserted into the plenum and cali-brated against a calibrated static pressuresensor immediately adjoining it. Thecontroller shall be arranged to controlthe speed of the test fan.

3. The UFAD plenum shall be installedcomplete with a representative numberof supply diffusers, electrical poweroutlets, and voice/data outlets. Thesupply diffusers, whether automaticallyor manually controlled, shall be in thefully closed design position.

4. The fan shall be operated to hold thetest pressure in the plenum. The testpressure shall be the design operatingstatic pressure for the system.

5. The system shall be operated for 24hours, with the measured static pressure

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(in. w.g.) and airflow rate (CFM)recorded 24 times (nominally eachhour). The sum of the 24 flow rates(CFM) shall be divided by 24. Thisaverage value will be considered the sumof the Category 1 and Category 2leakage (see Section 11.2.1) and calledthe å leakage.

6. With the test fan off, the floor panel andedge joints, the supply air diffusers andthe cable floor connectors shall betightly sealed with mastic and sealanttaping, and the test repeated for 24hours with hourly readings, and themethod of averaging repeated. Thisvalue will represent the Category 1leakage.

7. Subtracting the Category 1 leakage ratefrom the å leakage rate will represent theCategory 2 leakage rate.

8. The leakage rates shall be compared tothe allowable rates from the Table 15-1.If the rates are found to exceed the tablevalues in either category, steps shall betaken to re-inspect, utilizing test smokeif necessary, determine sources or causesof the leakage, repair or correct andretest - repeating this process until therates are within the Table 15-1 limits.

9. The systemic corrections that arerequired for the mockup to bring it intocompliance with the test limits shall beincorporated into the constructionprocess and procedures for the remain-ing UFAD plenums in the building.

Table 15-1. Maximum allowable UFAD plenum air leakage rates in mock-up and buildingfloor plenums, when measured at design operating static pressure.

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3. The static pressure sensing componentof the BAS shall have been installed andcalibrated before the test. An indepen-dent, calibrated static pressure gaugeshall be installed adjacent to eachpermanent sensor.

4. All supply air diffusers, of both auto-matically or manually controlled types,shall be closed to their minimum designpositions and the fan shall be operatedon pressure control - controlling at thedesign static pressure in the plenum for24 hours.

5. During the test, the supply air quantity(CFM) and the static pressure shall beread and recorded approximately onceeach hour (time noted) for 24 hours andrecorded. The flows for each hour shallthen be added and the sum divided by24 to obtain the total å leakage.

6. If the total å leakage exceeds the allow-able maximum in Table 15-1, theconstruction shall be inspected, testedwith test smoke if necessary, causesrepaired or corrected, and the systemretested until the rates are within theTable values.

15.2.2 Building Floor Plenum Tests during Construction

The lessons learned in step 9 in Section15.2.1 shall be disseminated to all the tradesinvolved in the construction of the plenumsas supplemental information, not to be abasis for a change in the contract price.They should also be distributed to allinspection and approval authorities on theproject.

The permanent building floor plenums shallthen be tested by the following procedures.These test results shall be professionallycertified by a nationally recognized Testing,Adjusting, and Balancing Contractor oragency (i.e., NEBB, AABC, or TABB):

1. The testing shall be performed after theconcrete surfaces of the plenum havebeen sealed, and all mechanical andelectrical devices, equipment, cables,racks, diffusers, power connectors andvoice/data connectors and floor finisheshave been installed.

2. The permanent air-handling systemshall have been installed, inspected andsuccessfully tested.

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15.2.3 Floor Plenum Tests duringWarranty

All penetrations, modifications and repairsto the UFAD plenum are to be documentedon the As-built drawings (see Sections 14.4– 14.6). To ensure long-term performanceof the UFAD plenum, the following addi-tional air-leakage testing shall be conductedwhenever an area exceeding 2,500 ft2 ismodified, and during a period within thirty(30) days prior to completion of the war-ranty period (see Section 14.7). These testresults shall be professionally certified by anationally recognized Testing, Adjusting,and Balancing Contractor or agency (i.e.,NEBB, AABC, or TABB):

1. Steps 3 – 5 in Section 15.2.2 shall berepeated for a contiguous floor area of atleast 1,000 ft2, which shall be selectedby the owner.

2. If the total å leakage exceeds the allow-able maximum in Table 15-1:2.1 The plenum of the selected area

shall be inspected, tested with testsmoke if necessary, causes repaired orcorrected, and the system retesteduntil the rates are within the Tablevalues.

2.2 Steps 3 – 5 in Section 15.2.2 shall

be repeated for two additionalcontiguous floor areas of at least1,000 ft2, which shall be selected byGSA.

3 If the total å leakage exceeds the allow-able maximum in Table 15-1 for eitherof the areas selected in 2.2, all of theUFAD plenums in the facility shall beinspected, tested with test smoke ifnecessary, causes repaired or corrected,and the system retested until the ratesare within the Table values.

15.3 Testing, Adjusting and BalancingAir System

After the leak testing is completed in accor-dance with 15.2.2 or 15.2.3, the Air andWater System Testing, Adjusting andBalancing (TAB) Contractor or agency shallperform the TAB work in accordance withthe Project Specifications.

15.4 Commissioning

The UFAD system requires no special orunusual steps in the commissioning process.The commissioning agent shall serve aninspection and documentation role asdefined in the level 6 inspection step (seeSection 7.1).

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FEDERAL BUILDING, OKLAHOMA CITY, OKLAHOMA

Photographer: Timothy Hursley

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Appendices:A-1: DefinitionsA-2: Economic ConsiderationsA-3: UFAD Systems

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Appendix A-1: Definitions

Building Life-Cycle Cost (BLCC) Model:A computer program developed by theNational Institute of Standards and Technol-ogy to provide economic analysis of pro-posed capital investments that are expectedto reduce long-term operating costs ofbuildings. It is especially useful in evaluat-ing the costs and benefits of energy conserva-tion projects in buildings.

Conventional Air Distribution (CAD):Transportation of supply air from air-handling units to occupied zones byductwork in ceiling spaces, wall cavities, orfloor cavities (See Chapter 5 Section 4 of P-100).

Category 1 Leaks: Leaks of cool condi-tioned supply air from the underfloorplenum into other building cavities, andthen either from the building or into returnair passages back to the air- handling unitsor building exhaust air system.

Category 2 Leaks: Leaks of air throughraised floor system components into thespace to be conditioned.

Appendix

Air Handling Unit (AHU): A fan assem-bly, which is located in a mechanical equip-ment room and may include heating andcooling coils and filters. The AHU providessupply air through conventional orunderfloor air distribution (See Chapter 5,Section 4 of PBS P-100).

Building Automation System (BAS): Adirect digital control (DDC) system forproviding lower operating costs and ease ofoperation. The BAS shall adjust buildingsystems to optimize their performance andthe performance with other systems in orderto minimize overall electric and fuel con-sumption of the facility (See § Chapter 5,Section 16 of PBS P-100).

Benefit-Cost Analysis: A systematicquantitative method of assessing the desir-ability of investments, projects, or policieswhen it is important to take a long view offuture effects and a broad view of possibleside-effects. The benefit-cost analysisprovides a ratio of the life cycle benefits ofthe alternative to the life cycle costs of thealternative.

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Cavity: An opening or hollow volumebeneath the raised access flooring.

Conventional Flooring (CF): A finishedflooring assembly without underfloor cavitiesor air plenums for cable management.

Cost-Effectiveness: A systematic quantita-tive method for comparing the costs ofalternative means of achieving the samestream of benefits or a given objective.

Diffuser: A circular, square, or rectangularair distribution outlet located in the ceiling,wall, or floor, and comprised of deflectingmembers to discharge supply air in variousdirections and planes, and arranged topromote mixing of primary air with second-ary room air (SMACNA HVAC Testing,Adjusting, and Balancing, 2002).

Emergency Power Off (EPO) Station: Asecured panel located at each major point ofegress in a zone and used by emergencypersonnel to disengage electrical circuits.

Equipment-Centric: Refers to concentra-tion of electronic equipment such as com-puters, telecommunications equipment, etc.

Energy Efficiency: Building energyefficiency is defined as the ratio of energyrequired to provide the specified exposurecriteria to the energy consumed for thispurpose. Energy efficiency focuses onminimizing energy waste rather than mini-mizing energy consumption.

Grille: A louvered or perforated covering foran air passage opening which can be locatedon a wall, ceiling or floor (SMACNA HVACTesting, Adjusting, and Balancing, 2002).

Leadership in Energy and Environmen-tal Design (LEED™): A national consen-sus-based, market-driven building ratingsystem designed by the U.S. Green BuildingCouncil to encourage the development andimplementation of green building practices.

Life Cycle Benefits: The overall estimatedbenefits for a particular alternative over thetime period corresponding to the life of theprogram, including direct and indirectinitial benefits plus any periodic or continu-ing benefits resulting from operation andmaintenance.

Life-Cycle Cost (LCC): LCC applies toboth equipment and projects, and includesall costs from project inception to disposal of

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equipment. Life cycle costs are determinedby an analytical study of total costs experi-enced during the life of equipment orprojects. The objective of LCC analysis is tochoose the most cost-effective approach froma series of alternatives so that the least long-term cost of ownership is achieved. LCCanalysis helps justify equipment and processselection based on total costs rather than theinitial purchase price of equipment orprojects.

Occupied Zone: The region normallyoccupied by people within a space, generallyconsidered to be between the floor and 1.8m (6 ft) above the floor and more than 1.0m (3.3 ft) from outside walls/windows orfixed heating, ventilating, or air-condition-ing equipment and 0.3 m (1 ft) frominternal walls. (ASHRAE Standard 55-2004, also see Chapter 5, Section 3 of P-100).

Optimize: To select the solution thatminimizes or maximizes an objective func-tion over a range of options.

Outlet Flow Rate (CFM): The volumetricairflow rate (cubic feet per minute) deliveredby the device at the design static pressure(in. w.g.).

Pedestal: Vertical support members forraised access flooring panels.

Plenum: An air compartment connected toone or more distributing ducts (SMACNAHVAC Systems Testing, Adjusting andBalancing, 2002).

Pressure, Static: The normal force perunit area that would be exerted by a movingfluid on a small body immersed in it if thebody were carried along with the fluid.Practically, it is the normal force per unitarea at a small hole in a wall of the duct orplenum through which the fluid flows(piezometer) or on the surface of a stationarytube at a point where the disturbances,created by inserting the tube, cancel. It issupposed that the thermodynamic proper-ties of a moving fluid depend on staticpressure in exactly the same manner as thoseof the same fluid at rest depend upon itsuniform hydrostatic pressure (SMACNAHVAC Systems Testing, Adjusting andBalancing, 2002).

Psychrometrics: The subscience of thethermodynamic properties of moist air. It isused to show how changes in heating,cooling, humidification, and dehumidifica-tion can affect the properties of moist air,

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and provides data necessary to analyzeprocesses relating to air distribution.

Raised Access Flooring (RAF): A fin-ished flooring assembly that also providesaccessible horizontal pathways for cablemanagement through cavities underneathmodular floor panels, which are supportedabove the sub-flooring by an understructure.

Secure Compartmented InformationFacility (SCIF): An accredited area, room,group of rooms, building, or installationwhere secure and/or classified informationmay be stored, used, discussed, and/orprocessed.

Sound Attenuator: A device or equipmentthat prevents, reduces, or absorbs sound.

Stringers: Horizontal support members forraised access flooring panels.

Sunk Cost: A cost incurred in the past thatwill not be affected by any present or futuredecision. Sunk costs should be ignored indetermining whether a new investment isworthwhile.

Supply Air: Air leaving an air handlingunit that has been cleaned and thermally

treated to achieve the indoor design criteriafor the occupied zones (See Chapter 5,Section 3 of PBS P-100).

Temperature, Dew Point: The tempera-ture at which moist air becomes saturated(100% relative humidity) with water vaporwhen cooled at constant pressure (ASHRAEStandard 55-2004).

Temperature, Dry Bulb: The tempera-ture of a gas or mixture of gases (e.g., air)indicated by an accurate thermometer aftercorrection for radiation (SMACNA HVACSystems Testing, Adjusting and Balancing,2002).

Temperature, Operative: The uniformtemperature of an imaginary black enclosurein which an occupant would exchange thesame amount of heat by radiation plusconvection as in the actual non-uniformenvironment (ASHRAE Standard 55-2004).A practical procedure to calculate theOperative Temperature is provided inAppendix C of ASHRAE Standard 55-2004for air velocities up to 1 m/s (200 fpm).

Testing, Adjusting and Balancing (TAB):A process to verify that all heating, ventilat-ing and air-conditioning (HVAC) water and

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air flows and pressures meet the designintent and equipment manufacturer’soperating requirements (SMACNA HVACSystems Testing, Adjusting and Balancing,2002).

Throw: The horizontal or vertical axialdistance an airstream travels after leaving anair outlet before the maximum streamvelocity is reduced to a specific terminal

value; e.g., 200, 150, 100 or 50 fpm (1.0,0.75, 0.5 or 0.25 m/s) (SMACNA HVACSystems Testing, Adjusting and Balancing,2002).

Underfloor Air Distribution (UFAD):Transportation of supply air from air-handling units to occupied zones throughplenums under raised access flooring (RAF)(See Chapter 5, Section 4 of PBS P-100).

Appendix A-2: Economic Considerations

Economic considerations are paramount toassure maximum value to the federal govern-ment. To ensure that this value is maxi-mized, a benefit-cost method shall beutilized in which the life cycle costs andbenefits of RAF are compared to the lifecycle costs and benefits of a facility withconventional flooring and horizontal path-ways for wiring and cabling distributionabove suspended ceilings. The RAF iseconomically preferred if the benefit-costratio of the RAF exceeds that of the conven-tional floor.

Analogous considerations are required forUFAD. As with the case of RAF, a benefit-cost method shall be utilized in which the

life cycle costs and benefits of UFAD inappropriate areas with horizontal pathwaysfor wiring and cabling distribution belowthe RAF, and as modified for UFAD, arecompared to the life cycle costs and benefitsof a facility with overhead or sidewall supplyair (i.e., conventional) air distribution(CAD) and horizontal pathways for wiringand cabling distribution below the RAF,assuming RAF has been selected. UFAD iseconomically preferred if the benefit-costratio of UFAD exceeds that of CAD.

RAF for office areas is only one element thatmust be assessed in determining value, andRAF may increase or decrease this value.UFAD is one of the four optional Perimeter

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and Interior Heating and Cooling Systems tobe considered in defining the HVACBaseline System (see Chapter 5, Section 4 ofPBS P-100), and is one of several optionsthat must be assessed in determining value.UFAD may increase or decrease this value.In addition to first cost impacts on HVAC,cable management, and flexibility concerns,economic considerations shall include theimpact that utilization of RAF/UFAD mayhave on health, safety, security, andsustainability (PBS P-100).

Assessing RAF and UFAD economic andrelated factors can be difficult as relativelylittle reliable, objective information isavailable in the published literature. Fur-ther, much of what is available is imprecisewith respect to the differences between RAFwith and without UFAD.

This Appendix describes methods forquantitative analysis and also providesinformation relating to benefits and coststhat can be used in qualitative assessments.If quantitative analysis is not possible for aspecific project, the qualitative methods shallbe used.

A-2.1 Life-Cycle Cost and Benefit-CostMethods

Life-cycle cost (LCC) is an analytical methodthat is required by GSA as it is mandated bylaw (PBS P-100). LCC analysis sums, over adesignated study period, all costs related tothe owning and operating of a building orbuilding system, adjusted for the time-valueof money. These costs include those relatedto initial capital investment, operating,maintenance, and repair costs, energy costs,and other costs. LCC estimates are calcu-lated in present-value dollars; that is, allfuture costs are discounted to present valueand summed. Thus, LCC provides a sys-tematic method for comparing projectalternatives with different streams of coststhat provide the same benefits in a time-equivalent manner.

Many established guidelines and computer-based tools effectively support LCC analysis.The National Institute of Standards andTechnology (NIST) has prepared the LifeCycle Costing Manual for the Federal EnergyManagement Program (NIST Handbook135), and annually issues discount factors

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for LCC analysis. NIST has also establishedthe Building Life Cycle Cost (BLCC)computer program to perform LCC analyses– see The NIST “Building Life-Cycle Cost”Program, Version 4.3: User’s Guide andReference Manual, National Institute ofStandards, October 1995. The NISTmaterials define all required LCC method-ologies used in GSA design applications(PBS P-100). LCC information can also beobtained from the Center for the BuiltEnvironment (CBE) UFAD Cost AnalysisModel, which is documented in CBE UFADCost Analysis Model: UFAD First Cost Model,Issues and Assumptions, Center for the BuiltEnvironment, University of California,Berkeley, May 2004; and from StandardPractice for Measuring Life-Cycle Costs ofBuildings and Building Systems, AmericanSociety for Testing and Materials (ASTM)E917. LCC is defined in the Code ofFederal Regulations (CFR), Title 10, Part436, Subpart A: Program Rules of the FederalEnergy Management Program (PBS P-100).

Life-Cycle Cost without Benefit-Costanalysis is incomplete as it assumes nodifference in benefits between options underconsideration. Benefit-Cost methods utilizethe results of the LCC to permit a decisionto be made as to the alternative preferred oneconomic grounds.

For Raised Access Flooring:

The ratio of the total life cycle RAFbenefits to the LCC of the RAF iscomputed, and the ratio of the total lifecycle benefits of the conventional system(i.e., conventional flooring with horizon-tal distribution above suspended ceil-ings) to the LCC of the conventionalsystem is computed. The two ratios arethen compared, and the alternative withthe highest ratio – the largest economicbenefits relative to costs – is preferred oneconomic grounds.At the Final Concept Phase (see Appen-dix A-3 of PBS P-100), the benefits andcosts of using RAF over the life of theproject shall be compared to the benefitsand costs of the conventional system,including overhead cabling, and thesystem with the most favorable benefit-cost ratio shall be used. Note that RAFwith underfloor cable management mayinvolve higher first costs than a conven-tional floor with overhead cable manage-ment, but may reduce LCC.

For Underfloor Air Distribution

Two ratios are computed: 1) the ratio ofthe total life-cycle benefits of UFAD tothe life-cycle costs of UFAD; and 2) the

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ratio of the total life cycle benefits ofCAD to the LCC of CAD. The tworatios are then compared, and thealternative with the highest ratio – thelargest economic benefits relative to costs– is preferred on economic grounds.At the Final Concept Phase (see Appen-dix A-3 of PBS P-100), the incrementalbenefits and costs of using UFAD overthe life of the project shall be comparedto the benefits and costs of CAD,assuming RAF has been selected, andthe system with the most favorablebenefit-cost ratio shall be used. UFADwith underfloor cable management mayinvolve higher first costs than CAD, butmay reduce LCC.

Guidance for selecting economic methods toevaluate investments in building systemsand to measuring net economic benefits canbe obtained from Standard Practice forMeasuring Net Benefits for Investments inBuildings and Building Systems, ASTM E1074 and Standard Guide for SelectingEconomic Methods for Evaluating Investmentsin Buildings and Building Systems, ASTME1185. Additional information on applyingbenefit-cost methods can be obtained fromPractice for Measuring Benefit-to-Cost andSavings-to-Investment Ratios for Buildings andBuilding Systems, ASTM E964; Practice for

Measuring Internal Rate of Return andAdjusted Internal Rate of Return for Invest-ments in Buildings and Building Systems,ASTM E1057; Practice for Measuring Pay-back for Investments in Buildings and BuildingSystems, ASTM El121; and “Guidelines andDiscount Rates for Benefit-Cost Analysis ofFederal Programs,” OMB Circular A-94Revised, October 29, 1992.

As described above, benefit-cost methods areapplied subsequent to the LCC analysis, butthis benefit-cost calculation is complicatedby two factors. First, while potential RAFand UFAD benefits relating to productivity,workplace flexibility, energy consumption,churn cost savings, etc. are well publicized,the potential risks and costs are neither wellknown nor quantified.

Second, RAF cannot readily be analyzedwithin the structure of the currently avail-able, mandated NIST LCC model andBLCC computer program. RAF is notexplicitly included in currently availablestate-of-the-art LCC methods, including theNIST BLCC method that is required to beused in GSA design applications. However,RAF considerations can be included in LCCparameters relating to annually recurring,escalating costs (such as service or mainte-nance which involves increasing amounts of

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work, and frequent replacements thatescalate at a rate different than inflation) andoperating, maintenance, and repair costs.

BLCC can be used to evaluate UFAD, whichmay have higher initial costs but loweroperating-related costs over the project lifethan the lower-initial-cost CAD option,while providing comparative levels of service(reliability, safety, occupant comfort, con-formance with building codes, noise levels,useable space, etc.). With BLCC, the life-cycle costs of UFAD and CAD are computedand compared to determine which has thelowest life-cycle cost, and UFAD can beanalyzed within the structure of the cur-rently available, mandated NIST LCCmodel and BLCC computer program (PBSP-100).

To perform the required LCC analysis andthe subsequent benefit-cost comparison forRAF and the conventional floor alternativeand for UFAD and CAD, the followingparameters and associated quantitative or (ata minimum) qualitative values shall beconsidered:

Life cycle benefits, including IAQeffects, occupant comfort, productivity,workflow and workplace configuration

flexibility, churn savings, wiring/cablemanagement, waste reduction, tenantrollover, and space rental value.Life cycle costs, including installationcosts, energy costs, health risks andcosts, fire safety risks and costs (includ-ing fire protection costs and risks tooccupants and first responders), seismicloading risks and costs, risks and costs ofaccidents, security risks and costs,operating costs, maintenance andcleaning costs, and added space costs.

GSA must approve the set of parameters andrelated values prior to use in a specificproject. Once the LCC analysis has beenconducted, the costs of RAF and of conven-tional flooring and the costs of UFAD and ofCAD must be compared to the benefits ofeach alterative, and the alternative with themost favorable benefit-cost ratio shall beused (PBS P-100).

In certain cases, application of objectivebenefit-cost methods may indicate that RAFand/or UFAD is not the preferred alterna-tive. In these cases, the results of thebenefit-cost comparison must be weighedwith quality assurance and validationmethods when making design decisions,professional judgment must be applied, andrecommendations shall be submitted to GSA

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for approval (PBS P-100). Guidance can beobtained from Standard Practice for ApplyingAnalytical Hierarchy Process to MultiattributeDecision Analysis of Investments Related toBuildings and Building Systems, ASTM 1765.

A-2.2 Benefit Factors

GSA is committed to providing physicalwork environments that have acceptableIAQ, contribute to occupant comfort, andenhance work flow. GSA further recognizesthat productivity is greatly affected by theworking environment (PBS P-100). RAFand UFAD systems may increase productiv-ity by reducing churn time and by enhanc-ing configuration flexibility. Flexibility maybe improved if RAF/UFAD makes it easier torelocate or add power/data/voice outlets tothe work environment. This enhancedflexibility may increase productivity bymaking it easier to reconfigure workflow-oriented workspaces. The workflow mayrequire a team-centric furniture layout,individual work stations, or a combination ofthe two. RAF/UFAD may also enhanceproductivity in applications where require-ments are likely to change during the lifecycle of the core building and in cableintensive installations where cables need tobe accessed for change, reconfiguration,monitoring, etc. In addition, UFAD may

increase productivity by facilitating thermalcomfort and IAQ improvements.

The workflow process is likely to changemany times over the life of a federal facility,and many office and work areas are con-stantly changing: Agency missions andpriorities change, new staff are hired, newwork space is required, work areas areexpanded and reconfigured, moves are madeto new space, and equipment is added – seethe 1999 GSA report The Integrated Work-place: A Comprehensive Approach to Develop-ing Workspace. Computer equipment foreach work area, as well as other office equip-ment, increases the need for wiring. Itshould be determined whether RAF/UFADis likely to reduce major disruptions increating new workspace and getting allrequired equipment wired in. It should alsobe determined whether making thesechanges is easier with RAF/UFAD or withconventional flooring using overhead ca-bling.

While productivity gains with RAF/UFADare possible, they are not assured. Forexample, equipment turnover in manymodern equipment centric areas is aroundtwo years and creates the situation wheredata and power cabling is subject to nearlycontinuous change. The difficulty of access

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to cabling when it is under the raised flooror in the UFAD plenum can result in delay,costs, and reduced productivity associatedwith changing needs. Further, if the cablingis in the UFAD plenum, the air tightnessintegrity of the plenum must be assured.

Many claims have been made for UFADbenefits, but few have been validated. Thefollowing is a critical review of the some ofthe claimed benefits. It should be noted inthe following review that UFAD mightincrease or decrease benefits with respect toCAD, with or without RAF.

In new construction, UFAD first costs maybe equal to or lower than CAD first costs if araised floor has been selected. The factors thataffect UFAD first costs include: HVACconstruction sequencing and installationcosts; power, data/voice networking installa-tion costs; ductwork and duct materialscosts; HVAC controls costs; power for fans;sizes of chillers; building height impacts;and construction time and materials costs.It has also been reported that floor-to-floorheight may not need to be increased inUFAD projects.

UFAD may influence thermal comfort andIAQ. Thermal comfort may be greater with

UFAD systems if air velocities are low anddiffusers are designed for effective mixingwithout drafts. Indoor air quality may beimpacted by a number of factors, includingthe ability to relocate and add diffusers tomatch use patterns and the proximity of thediffusers to the individual’s breathing zone.The upward direction of air flow fromUFAD air theoretically removes equipmentcontaminants and heat directly throughceiling return air systems.

Flexibility may be improved if UFADsupports a greater level of spatial change.This enhanced flexibility may increaseproductivity if it makes it easier toreconfigure workflow-oriented workspaces.The workflow may require a team-centricfurniture layout, individual work stations, ora combination of the two. UFAD may alsoenhance productivity in applications whererequirements are likely to change during thelife cycle of the core building. Flexible andadaptive UFAD systems may reduce materialand component waste that is often necessaryin mechanical and electrical system modifi-cations.

It shall be determined whether UFAD orCAD can be better designed to allow for acontinuous reconfiguration of thermal zones

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and controllers, or with micro-zoning (onezone per workstation) that can be dynami-cally grouped and regrouped with control.User controls improve user satisfaction withthermal comfort. UFAD may enhance theability to relocate diffusers, which is criticalto churn/reconfiguration cost savings, and allsystems can be coordinated to make thispossible (carpet, outlets, diffusers, tethers).

Compared to CAD, UFAD has more supplyair diffusers and they are located closer tothe occupants. It shall be determined if thismay help to support the use of task/ambientconditioning, which provides occupantswith individual comfort control through theuse of fan-powered, directional air outletsthat increase local air movement. It shall bedetermined if UFAD may also have lowerambient noise levels due to low face velocityat the floor diffuser. Additional discussion ofpotential UFAD benefits can be obtainedfrom Energy Savings Potential of Flexible andAdaptive HVAC distribution Systems for OfficeBuildings, Air-conditioning and Refrigera-tion Technology Institute, March 2002.

There are several available reports on assess-ing and measuring productivity in federalworkplaces. These include the 1999 GSAreport Workplace Evaluation Study, whichdescribes the GSA cost per person model;

the 2001 GSA report Productivity and theWorkplace, which presents the workplaceperformance model and the GSA productiv-ity payback model; and the 2001 GSAreport People and the Workplace, whichdiscusses productivity measurement.

GSA is committed to incorporating prin-ciples of sustainable design and energyefficiency into all of its building projects,but sustainable design shall not adverselyaffect the health and comfort of buildingoccupants (PBS P-100). It shall be deter-mined whether RAF/UFAD contributes tothe sustainability of the building and toenergy efficiency by reducing energy require-ments and by reducing the total amount ofwaste generated over the building’s lifetime(PBS P-100). Waste may be reduced due tothe ease of reconfiguring office spaces withminimal structural perturbations – see thecurrent version of the Whole Building DesignGuide at www.wbdg.org for additionalguidance on estimating this waste reduction.

GSA is committed to minimizing use ofnonrenewable energy, and Executive Order13123 establishes a national program toreduce building annual energy consumptionby 35 percent from the 1985 baseline – seeGreening the Government Through Efficient

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Energy Management, Executive Order13123, June 3, 1999 (PBS P-100). GSAalso supports the federal government’sEnergy Star Buildings Program for achievingmetered consumption with the top 25percent of involved building categories (PBSP-100). Guidance on the Energy StarBuildings Program can be obtained from theEnergy Star web site at www.energystar.gov.However, little empirical evidence exists thatRAF (as distinct from UFAD) is likely tohave a measurable effect on energy costs.

All GSA projects must be certified throughthe Leadership in Energy and Environmen-tal Design (LEED™) Green BuildingRating System of the U.S. Green BuildingCouncil, and GSA’s sustainability objectivefor LEED™ certification involves exceedingASHRAE 90.1 (P-100, 2005: Section 1.7).RAF may contribute to increased LEED™points (PBS P-100). UFAD may impactLEED™ categories such as VentilationEffectiveness, Thermal Comfort, and Opti-mized Energy Performance, and may con-tribute to LEED™ points (PBS P-100).Additional guidance can be obtained fromthe U.S. Green Building Council’s Leader-ship in Energy and Environmental DesignBuilding Rating System, version 2.1 and the2004 GSA report GSA LEED™ Cost Study.

A-2.3 Risk Factors

Building occupant health and related healthcosts are major GSA concerns. Water, mold,contaminants (food, drinks, trash, etc.) androdent infestation in RAF cavities andUFAD plenums can cause potentially seriousindoor air quality and health problems.Contamination can result from lateral and/orvertical instability of the framing systemsand from improper sealing of cutouts andcan increase risks and costs. It is thusimportant to minimize sources and path-ways for liquids, water, water vapor, andcontaminant incursion into RAF cavities andUFAD plenums, including migrationthrough building envelopes.

If the UFAD plenum is airtight, as it shouldbe, the plenum will be virtually water tight.To avoid the possibility of structural failurefor applications involving framed structuralfloors (i.e. other than slab on-grade), theremust be a way to quickly/automaticallyremove standing water in the raised floorcavity caused by catastrophic leak, firesuppression, or other flood scenario.

Water, mold, and contaminant control,including minimizing sources and pathwaysfor liquid water, water vapor, and contami-

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nant incursion into RAF cavities and UFADplenums (e.g., migration through buildingenvelopes), is critical. In addition, care mustbe taken to assure the 1ong-term cleanlinessof the supply air in the UFAD plenums andto ensure the effective pressurization of theUFAD plenums.

In UFAD systems, the floor surface tempera-tures will likely be lower than in CADsystems, resulting in increased radiantexchange with the occupants and an in-creased risk of thermal discomfort. Accept-able air speed in the occupied space (asdefined in ASHRAE 55-2004) and theacceptable relative humidity range (asdefined in ASHRAE 62.1-2004) may bemore difficult to maintain with UFAD thanwith CAD. These values may be moredifficult to achieve in occupied areas withUFAD than in comparable areas with CAD,primarily due to the need to limit thesupply air temperature to not less than 63 –65oF dry bulb temperature and the airvelocities to no more than 50 feet/min (0.25m/s) through the floor diffusers in order tocreate the turbulent mixing needed inUFAD. Design guidance from athermophysiological response perspectivecan be obtained from the ASHRAE Hand-book of Fundamentals. Additional guidancecan be obtained from Thermal Environmental

Conditions for Human Occupancy, ANSI/ASHRAE 55-2004 and Ventilation forAcceptable Indoor Air Quality, ANSI/ASHRAE 62.1-2004.

GSA is committed to protecting human lifein federal buildings. Physical safety, lifesafety, and fire and smoke management arecritical in RAF cavities and UFAD plenumsthat contain water pipes and/or electricaland IT wiring/cabling. When cabling is runwithin an RAF cavity or UFAD plenum,issues relating to fire protection shall beaddressed. One way to address these is toenclose cabling in a fire-rated conduit,which may be metal or a special fire ratedpolymer.

Although protection and response criteriaare available for some extraordinary inci-dents, the characteristics of UFAD plenumspresent special issues that shall be addressed.Internal floods in UFAD systems can resultin additional structural loads; local seismicconditions or live (e.g., rolling) loads mayrequire heavy duty pedestals, stringers andcross-bracing; and special transitions at theinterfaces with non-raised flooring and othersurfaces are required to minimize accidents,flame spread, and air leakage. For UFADsystems, criteria for isolation and contain-ment of chemical or biological releases and

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fire and smoke control shall be defined,especially for pressurized plenums thatmaximize exposures to seated occupants.When water from floods or leaks accumu-lates in the UFAD supply plenum, the riskof electrical fires increases because the powerwiring is typically not waterproofed. “Emer-gency Power Off” (EPO) stations shall berequired for each fire zone in UFAD systemsin order to disengage all power in “wetplenums.”

In offices and other equipment-centric areasthere is often extensive cabling underneathraised floors and within UFAD plenumsinvolving large amounts of PVC insulation.This extensive cabling in the underfloorcavity or UFAD plenum increases the risk offire. Fires in such areas requiring extensivecabling/wiring in the underfloor cavity (suchas offices, data centers, laboratories, medicalfacilities, clean rooms, and telecommunica-tions facilities) can arise from problems withthe wiring, electrical distribution systemcomponents, and electronic equipment(computer hardware, power switchgear,overcurrent protection devices, etc). Theprobability of a fire occurring in such areas isestimated to be “somewhat likely” (0.0001 -0.01/yr). This implies that, of every 10,000equipment-centric facilities, every year a fireis “somewhat likely” in between one and 100

of them. Additional guidance can beobtained from the article “Business Interrup-tion Risk Assessment: A Multi-DisciplinaryApproach, by M.H. Long, in” DisasterRecovery Journal, 1997.

A fire in a raised floor area, particularlywithin the UFAD plenum, is serious due tothe mass of cabling and the highly toxic andcorrosive combustion product resulting froma PVC fire (burning plastics create extremelytoxic gasses). Further, cable fires under theraised floor or in UFAD plenums can beespecially dangerous because the hot toxicgasses can exit anywhere and can be moreconcentrated than re-circulated and dilutedgasses in re-circulated air. There is also ahigher probability that the gasses will exitwithin the breathing zone – during fires thestandard procedure is to crawl to safety, apractice that could be fatal if the fire is inthe raised floor or the UFAD plenum.

Fires in areas with RAF and/or UFAD can beespecially dangerous for occupants and forfire fighters, police, security staff, EMTpersonnel, and other first responders, sincefire-weakened access panels present obvioushazards that may be difficult to deal with –especially in dark, smoke filled environ-ments. Moreover, increasing the height ofthe raised floor – UFAD may require ple-

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nums 18" or higher, increases the potentialdanger to the occupants and the first re-sponders.

Risk control alternatives include a systemtripped by cross-zoned smoke detection andpre-action sprinkler systems, which couldreduce the risk of water damage from a failedhead. Beneath the raised floor or within theUFAD plenum, it may be necessary toinstall a “very early smoke detection system”in which the smoke induction associatedwith such a unit reduces the activation time.Other options include a carbon dioxidesystem or a similar environmentally friendlygaseous extinguishing system, improved firedetection (e.g., line detectors), and/orimproved passive fire protection (e.g., fire-resistant intumescent paints or fire stops).

The full load capability of an RAF or UFADraised floor is only realized when all of thepanels are in place, as the buckling (lateral)strength of the floor depends on the pres-ence of the panels. However, panels andeven rows of panels are routinely removed inareas when cabling changes or maintenanceis performed. This can lead to collapse ofthe plenum; but even one panel left openposes a significant and unexpected risk tostaff and visitors moving in the area. More-over, the risk increases with the height of the

raised floor, and UFAD typically requiresplenums that are more than 12" deep. Inaddition, equipment is frequently movedover the course of the lifetime over manyareas, and this creates the risk that the floorloading will be exceeded, leading to a floorcollapse.

RAF and UFAD increase the difficulty ofassuring compliance with seismic require-ments for equipment-centric areas. Support-ing equipment above the floor on a gridcompromises the ability to anchor equip-ment. This is a serious problem in caseswhere the capability to withstand seismicshocks is required. For example, in andaround Kobe, Japan during the earthquakeof 1995, data centers experienced an extraor-dinary range of earthquake damage. Manycenters which should have been operationalwithin hours or days were down for morethan a month when a large number ofsupposedly earthquake-rated raised floorsystems buckled, sending IT equipmentthrough the floor. Similarly, during theWorld Trade Center collapse of 2001, nearbydata centers which should have survived thetragedy were seriously damaged and experi-enced extended down time when impacts tothe buildings caused raised floor systems tobuckle and collapse.

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Safe, secure, cost-effective buildings are amajor GSA priority. Therefore, the RAF orUFAD design shall not increase risks ofcasualties, property damage, and loss ofcritical functions. Electronic security is akey element of facility protection (PBS P-100). RAF and UFAD have some inherentsecurity concerns, as electronic devices maybe installed in the plenum and, since theUFAD plenum will often be 18" or higher,it may even provide physical access. Thus,in facilities with high security requirements,RAF or UFAD shall not increase the risk of asecurity breach.

“Zinc whisker” contamination is anotherpotential RAF/UFAD problem that has ledto loss of critical functions. Zinc whiskersare micron-sized filaments that grow ongalvanized steel plates and, since zincconducts electricity, a filament falling on acircuit board is capable of causing a shortcircuit. Such contamination is especiallydifficult to detect because the filamentdisintegrates, leaving little or no trace. Thesource of this contamination is often thegalvanized coating on the underside of raisedfloor tiles and, while zinc whiskers have beenfound on a wide range of surfaces, accessfloor panels are of particular concern because

they have large surface areas and are oftenmoved during day-to-day operations. Tinand tin alloy whiskers can also cause prob-lems in electronic equipment.

The whiskers are fragile and easily break offthe panel. They can then get into theUFAD supply airflow and be carried to theequipment via the plenum beneath theflooring. Zinc whiskers cause failures inelectronic systems and can have severefinancial impact in equipment-centric areas.However, the problem is recognized in theindustry and advancements have been madein the fabrication of raised floor systemswhich can be designed to minimize theproblem of zinc whiskers.

Additional guidance on risk factors can beobtained from the following sources: Stan-dard Practice for Measuring Cost Risk ofBuildings and Building Systems, ASTM E1946; Risk Management Guidance for Health,Safety, and Environmental Security, ASHRAE,2003; and Guidance for Protecting BuildingEnvironments From Airborne Chemical,Biological, or Radiological Attacks, NIOSH,2002.

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RAF/UFAD and its continuing maintenancerequirements may limit its application tospecialized areas. Further, advancements inoptical data transmission between electronicequipment have reduced the requirements toinstall CAT 6 cables under floors and havemade overhead cable trays (used in conjunc-tion with bus ducts for power) an alternatesolution. This overhead solution may reducethe costs of stringing new power and con-nectivity.

UFAD may require significant changes inarchitectural, structural, mechanical, andelectrical loads and designs. For example,the height of the supply air plenum andchoice of using return air plenums can affectthe size and locations of windows, theseismic load imposed on the floor, and theexternal and internal thermal loads to bedissipated by the HVAC system. Neverthe-less, the exposure criteria to be maintainedin the occupied zones shall not be differentfor UFAD than for CAD.

Positive pressure within UFAD plenums canforce air movement into space perimeterpartitions, and costs may be incurred insealing the plenums. Effective perimeter airbarriers shall be treated the same asductwork. To assure effective ventilation airdistribution, dedicated ventilation systems

A-2.4 Cost Factors

It shall be determined if the first cost ofRAF/UFAD, including engineering, materialcost, fabrication, installation, and inspec-tion, is significantly higher than that of aconventional floor and/or CAD. In addi-tion, the maximum area that might beultimately utilized is normally built-outwith a raised floor whether or not thecurrent, near term, or even the actualultimate function requires the use of RAF.Extra costs associated with power and datacabling can also be incurred using RAF/UFAD. Stairs, elevator shafts, piping,standpipes, etc. may have to be lengthenedaccordingly and shall be considered “relatedcosts.”

Architecturally, RAF/UFAD can presentcostly challenges. As the height of the floorand plenum increases, ramps from finishedfloor level to access floor level increase inlength. Depending on the height of thefloor or plenum (which can be 12 inches ormore), stairs and landings may also have tobe provided to access the RAF/UFAD area.These ramps and stairs occupy valuablespace, increasing the cost per usable squarefootage. Weight limitations on access floorloading must also be considered. For manyapplications the initial high cost of installing

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shall be used to provide dehumidifiedpositive air pressure to the building, avoid-ing possible failure of large main air-han-dling equipment to control minimumoutside air intake, to distribute that ventila-tion air precisely, to dehumidify the build-ing, and to operate efficiently/stably at low-air volume conditions. These treatmentsmay involve additional costs.

With UFAD, exposure criteria may be moredifficult to control during occupancy thanwith CAD (i.e., uniformly mixed air inroom), as UFAD systems will incorporateeither vertical displacement or turbulentmixing strategies. These require additionaldata to assess system performance. There-fore, additional costs may have to be in-curred in UFAD design to ensure that thespecified exposure values are achieved duringoccupancy (i.e., effects of occupant distur-bance of airflow patterns).

Adequate sizes and locations of mechanicalequipment rooms, locations and accessibilityof equipment, and components in UFADplenums shall be assured to minimizedisruption of occupied space during mainte-nance (PBS P-100). Access to UFADequipment shall be facilitated by its place-ment in positions that will not be covered

by furniture/partitions. Equipment accessabove 10 feet from the service floor shall beavoided. Proper air-handling unit mechani-cal space shall be provided to enable equip-ment servicing, and adequate mechanicalspace (approximately 4 percent of the floorarea) shall be provided. This may involvesignificant additional costs.

Locating UFAD air-handling units withininterior core areas can cause distributionpath problems in that corridors are often notintended to have raised floor treatments. Toprovide an air path from core-located air-handling equipment, either the RAF mustbe extended to include corridors (withattended concerns for rolling load specifica-tions), or overhead distribution to spacedrop-downs must be considered. Analternative is to place air-handling equip-ment within the space being served, i.e. notin core locations. This may involve signifi-cant additional costs.

Selection of UFAD may not alleviate the useof suspended ceilings. Rather, UFADfacilities may require two plenums, theceiling plenum that contains plumbing,sprinkler systems, lighting, power wiringand return and exhaust air ductwork, andthe raised floor plenum that contains power

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wiring, hydronics, IT cabling, and supplyair. Thus, UFAD heights may be 18" orhigher and the height of the occupied zonevaries considerably in UFAD. The methodsof maintaining stratification shall be accord-ingly adapted. This may involve significantadditional costs.

Due to the higher supply air temperaturesin UFAD than in CAD, the maximumcooling capacitance of the UFAD systemmay be lower than CAD. The need toprovide greater levels of cooling in certainareas, such as conference rooms, may requireadding fan-powered diffusers, dedicatedducting to create conference room zones,and additional water-based cooling compo-nents, all of which may impact the cost ofthe system.

RAF cavities and UFAD plenums are notconvenient to clean, as dust, grit, trash, andvarious items normally accumulate underthe floor and within the plenum, andcleaning of access floor space used as an air

plenum is difficult. The complexity, diffi-culty, and accidental risk potential associatedwith cleaning this area can be costly prob-lems which will continue through the life ofthe facility.

The building flooring under the RAF/UFAD, which will usually be concrete, shallbe sealed to prevent the accumulation ofconcrete dust in the cavity. The cavity shallalso be sealed to prevent rodent infestation.

Additional guidance on cost factors can beobtained from the following sources: Stan-dards on Building Economics (Fifth Edition),ASTM; Green Buildings Costs and FinancialBenefits, Massachusetts Technology Corpora-tion, 2003; Good HVAC Practices for Residen-tial and Commercial Buildings, ACCA, 2003;Standard Guide for General Principles ofSustainability Relative to Buildings, ASTMWK5566, 2004; and The Costs and Benefitsof Green Buildings: A Report to California’sSustainable Building Task Force, 2003.

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Appendix A-3: UFAD Systems

During the closing years of the century,computer, data, and communication tech-nology had undergone extensive changes,and significantly impacted the workplaceenvironment. Area lighting was beingreplaced by a combination of environmentaland task lighting, convenience power wasrequired for personal computers, printers,charging cords for portable electronics. And,there was the network of cables required forcommunications, data transmission, signal-ing, safety and environmental controlsystems.

A logical method of distributing and manag-ing this multitude of wires was in an acces-sible underfloor cavity much like the com-puter rooms of fifty years earlier. Also, likethe computer rooms of fifty years earlier, thepresence of the underfloor cavity providedthe opportunity to use it as a supply airplenum to deliver conditioned air to thevicinity of the work stations for the purposeof air- conditioning for human comfort.

However, it is important to recognize at theoutset that, thermodynamically, there islittle similarity between the earlier process ofserving single, reasonably high density and

A-3.1 Functions and Characteristics ofUFAD Systems

When the RAF cavity serves the secondarypurpose of delivering conditioning air to thespace, the floor system is then called anUnderfloor Air Distribution (UFAD) System.The genesis of contemporary UFAD systemswas in main frame computer facilities in themid twentieth century, in which the com-puter room had numerous types of comput-ers and data processors with relatively highand constant heat dissipation rates. Themajor objective of the cooling system was toremove the heat from the machinery whilemaintaining a relatively constant dry bulbtemperature and relative humidity in thecomputer room. These facilities generallyconsisted of one room conditioned as asingle zone with conditioned air suppliedinto the floor cavity from a packaged air-conditioning unit or units. Air supplyoutlets were generally placed in the floorpanels near the heat sources in such loca-tions as to minimize discomfort to theindividuals “operating” the computermachinery. The cooling loads were usuallyhigh density and relatively constant.

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relatively constant machine cooling loadsand providing air conditioning for humancomfort from a single conditioning system tomultiple zones of control with widelyvarying loads.

Thus, when planning the design of a UFADsystem to provide air-conditioning forhuman comfort, a designer must decide onthe method to be employed to satisfy allvarying load conditions from full coolingloads, to no load, to full heating load. Thesystem must, at the same time, maintain therequired minimum ventilation rate at alloccupied times in each room or space to beconditioned while satisfying different partload requirements in adjoining spaces. Thedesign options and pitfalls relating to thisobjective are discussed in some detail inSection 12 of this Guideline.

An advantage of utilizing a UFAD system isits compatibility with displacement ventila-tion design in those climactic locationswhere this type of distribution can satisfythermal comfort and ventilation require-ments. Displacement ventilation systemshave been employed successfully in dryclimates and in those spaces in which thereis a space cooling load imposed primarily bythe occupants and appliances (i.e., not by

other internal cooling loads or by heat gainthrough the building envelope). Theassumption is that room air velocity is notnecessary to stimulate moisture evaporationor convective velocity to achieve sensible heatremoval for human comfort. With thesesystems, ventilation and conditioning air isintroduced at or near the floor at relativelylow velocities and at no more than about 5 –10oF cooler than the “comfort” temperatureat the breathing level (say, 5 feet above thefinished floor). As the air is heated by heatdissipation from the occupants and theappliances, the warming air rises, picking upheat, odors and contaminants and is expelledthrough a return air system at the ceiling.The concept is that there is little or novertical mixing and that the temperaturerises as the air mass rises, continually in-creasing in temperature. Ideally, then, thewarmer temperatures above the occupancyheight do not have to be controlled withinthe conditioned air temperature rise of thedelta T (i.e., difference between room andsupply air temperatures) that is needed inthe room for the cooling and ventilation.

When heat gain through the envelope issignificant or the indoor air relative humid-ity exceeds about 40% and some air velocityis necessary (e.g., 0.15 m/s or 30 feet per

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minute) to assist in the cooling process ofthe human body, displacement ventilationtype air distribution systems are normallynot utilized. Rather, other air distributionproducts and designs are available (i.e.,turbulent mixing) that are claimed toenhance the performance of displacementventilation systems by creating horizontalturbulent layers in the space while maintain-ing upper layer stratification by preventingvertical mixing.

Section 12 in Part 2 of this guideline in-cludes a discussion of the engineeringconcepts, both fluid and thermal, relating tothe design of the UFAD systems. In addi-tion to the space conditioning there are twounique challenges in the design. One is thepsychrometric need to cool the air to a 52°Fdewpoint temperature and then to provide asecondary conditioning process to raise thedry bulb temperature to about 63°F for theunderfloor supply plenum. The other is theimmense thermal mass of the buildingstructure that essentially forms part of theair passage. If this latter challenge is notaddressed in the system design, it will resultin a control time constant not normallyencountered in air systems.

In addition to the air-conditioning prin-ciples relating to UFAD systems there aresignificant physical issues that must beaddressed concerning the underfloor supplyair plenum. These issues, which are alsodiscussed in some detail in Part 2, includesuch concerns as water incursion, alarm anddrainage; air leakage into the conditionedspace and into building cavities; interfer-ences between mechanical and electricalsystem’s components and devices; acousticalisolation; fire safety and alarms, etc.

The most fundamental step in the decision-making process of whether or not to use anunderfloor air distribution (UFAD) systemshould always be directed toward theobjective of providing an air-conditioningsystem that will meet all of the thermalcomfort, indoor air quality, acoustic andvibration (i.e., environmental quality)criteria without compromise. As in theselection of any such system, the designstudy should start with analyzing the spacesthat are to be occupied. The two majorconcerns in this regard include, first, roomair distribution system and, second, thecontrol techniques.

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A-3.2 Room Air Distribution System

In order to analyze the capability of achiev-ing an acceptable air distribution system forspace cooling, one must recognize that thetheory of underflow air distribution issignificantly different from the overhead orsidewall techniques with which most engi-neers (and space occupants) are familiar.With overhead or sidewall supply diffusersor grilles, the concept is to use the device tomix the colder supply air with the room airwhile absorbing the heat and then extractingthe return air from the space at the roomtemperature. The ambient air velocityneeded for comfort is generated by theintroduction velocity of the supply air, andthe system is ideally designed so that thelower supply air temperature never impactsthe occupant. The ideal performance is tohave a homogeneous temperature through-out the occupied zone of the space and thesupply diffuser is designed to supply at areasonably high velocity (300 to 500 ft/min,1.5 – 2.5 m/s or higher).

With the underfloor distribution system,cold air is supplied through floor grilles ordiffusers at reasonably low velocities de-signed to achieve vertical stratification intemperature instead of mixing. The theory

is that the air then moves vertically upthrough the space absorbing heat andcontaminants as it flows upward to returnair inlets in the ceiling. In this model (i.e.,vertical displacement), the temperature ofthe air leaving the grille is at approximatelythe supply air temperature of 63°F and,dependent on the amount of horizontalmixing at the floor caused by the grille ordiffuser (i.e., turbulent mixing), the floorsurface temperatures may vary from 63 –72oF. Ideally, as the air moves upward bybuoyant effects, the air increases in tempera-ture such that it reaches the desired spacetemperature of, say 75°F, at about thebreathing level (5 feet), and continues toincrease to about 77 - 83°F as it enters theceiling return plenum. To prevent verticalmixing, which would negate the stratifica-tion, the outlet velocities must be keptreasonably low which limits the capacity ofcommercially available floor grilles anddiffusers to approximately 50 – 100 CFM(25 – 50 l/s).

A-3.3 Capacity Control Options

Control techniques must also be resolved inthe schematic design phase (i.e., FinalConcept, Appendix A.3, PBS P-100).Following the determination of space usages

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and a preliminary load analysis, the thermo-static control zones and thermostat locationsmust be established. Next, the method ofchanging (i.e. reducing) the capacity as theload reduces in any given zone must bedetermined. The two methods available areto either reduce the flow rate (CFM) or toreduce the temperature differential betweenthe room air and the supply air from theplenum (delta-T).

A benefit in UFAD system air distributionthat is not available in ceiling or highsidewall Variable Air Volume (VAV) systemsis the compatibility with displacementventilation. Since displacement ventilationtheory does not require a high outlet velocityto create mixing, the outlet flow rate (CFM)from a UFAD outlet that utilizes displace-ment ventilation can be reduced withoutconcern of “dumping” which results fromsuch a reduction with overhead or highsidewall systems. The minimum flow towhich a UFAD outlet can be “turned down”then is set by the minimum required tomaintain adequate ventilation air at thebreathing level. Below that minimum flow,the only option is some form of reheat whichmust be justified from an energy consump-tion perspective. (This is a method ofreducing the delta-T.) With an overhead

VAV system, the fan powered terminalachieves this by mixing warm air from theceiling plenum with cold supply air, but thisoption is not available in a UFAD system,since the plenum air and that immediatelyabove the floor is essentially at the supply airtemperature.

Another option available with overheadducted systems that is not available withUFAD systems is the re-setting of the supplyair temperature. With the UFAD system, ifall of the zones on a given air-handling unitare at less than design load, the supply airtemperature cannot be changed (raised) tosatisfy the flow requirements. The reason forthis is that the air pathways are massive (seeSection 12.2) and will require long periodsof time following a change in air tempera-ture to return to thermal equilibrium,during which time lack of space temperaturecontrol will be realized.

There are design options available foraddressing these issues, some of whichrequire the routing of ducting from theoutlet of the AHU to or near the flooroutlet(s). If one or more of these solutionsare elected, care must then be taken tocoordinate the design and routing of theductwork with the cable management.

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When UFAD is utilized with RAF, anadditional benefit is that there is usuallymore than ample space for all of the cablerequirements. However, the designers mustbe careful not to block cable access routeswith ductwork or with air flow controlpartitioning below the floor.

As all HAVC designers are aware, the heatcapacity equation is used in a first lawbalance to relate the cooling capacity ofconditioned air to the space sensible coolingor heating load at any given time. Thisequation, expressed for an air system (whereq is the load in BTU/hr, CFM is the supplyair quantity in cubic feet per minute, tr isthe room temperature, °F and ts is thesupply air temperature), is:

q = CFM (1.1) (tr – ts) Equation [1]

or

q = CFM (1.1) (Dt) Equation [2]

Then, at design load, the value thereof isinserted into the equation, the desired roomair temperature and supply air temperaturesare inserted and the equation is solved forthe needed air circulation quantity. This is astraightforward calculation for a “conven-

tional” air distribution (CAD) system,which maintains a homogeneous mix ofroom air at the room air temperature.However, for a UFAD system, which utilizesdisplacement ventilation or turbulentmixing, proponents of the technique main-tain that all of the heat that is “captured”above the “occupied” height of, say, 6 feet,does not have to be accounted for in thecalculation of the space load, if indeedvertical mixing does not occur. It has alsobeen published that this non-accounted forheat generally represents about 40% of thecooling load in the normal office environ-ment. The recommendation by manufactur-ers of UFAD diffusers and related products isthat 60% of the normally calculated coolingload be used in Equation [1] and that thiswould provide adequate cooling with thesame air at 63º F supply temperature as aconventional (mixing) system would requireat 55º F – considering a 75º F room.

The stratifying air would continue to heatabove the 6-foot level and the cooling coilwould, of course, “see” the same sensibleload in either case.

Once the system parameters have beenestablished at full load conditions, the onlytwo options available for changing the

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system cooling capacity with load variationsis to design the control system to effectchanges in either the (CFM) or the (Dt) asthe loads vary. Methods that have been used

in conventional or homogeneous tempera-ture, air mixing systems historically areshown in Table 1.

Table A-4.1. Types of Capacity Control Systems.

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There are hybrid systems that functionthermodynamically as one of the fourgeneric systems (i.e., heat-cool-off, dualstream, reheat or variable volume). Anexample would be, say, in a perimeter zone,combining a variable volume unit withminimum position terminal units andfinned tube radiation in the space. Thefinned tube heat would be controlled suchto always assure enough cooling load tosatisfy the minimum flow (thermodynami-cally, a VAV reheat system).

When considering options for space tem-perature control with UFAD systems, thesystem options are limited by physical(dimensional) limitations, thermodynamicopportunities, and products. The optionsavailable which can be used independentlyor in combination are variable air volumeand reheat and numerous manufacturesprovide products specifically to be installedin the surface of the floor system and in thefloor cavity.

Additionally, there are various hybridsystems that have been designed and em-ployed successfully including displacementventilation or turbulent mixing VAV withperimeter radiation in the conditioned spaceand ceiling mounted fan powered VAV

terminals that supplied the discharge airinto the underfloor supply air plenum.

A third hybrid system utilizes underfloorsupply ducts to a partitioned zone plenumprovided with a VAV terminal where the airenters the partitioned zone. With thissystem, air leakage from the plenum to thespace would not adversely impact theperformance.

A-3.4 Ventilation and HumidityControl

The baseline system required by the GSAstandard PBS P-100 consists of two verydifferent types of air-handling systems. Oneis a dedicated outdoor air unit (DOA) orventilation air conditioner (VAC) and theother is a recirculating air conditioning(RAC) unit. Both of these types of air-handling systems pertain equally to CADand UFAD systems (see Chapter 5, Section4 of PBS P-100).

The ventilating air-conditioning unit isrequired to be sized only for the ventilationair required for the zones served, as deter-mined by ASHRAE Standard 62. It isimportant to note that this unit (theseunits) is not to be sized to handle all of the

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circulated air but only the ventilationcomponent All of the ventilation air is to beintroduced to the building through this unit(or these units). The VAC unit is to provideall necessary cleaning (particulate and/orchemical) preheating, cooling, dehumidify-ing and volume control of the ventilation air.Another important feature is, to the extentfeasible, all humidity control for the build-ing is to be provided with this unit (usuallyby simple dewpoint control).

The second unit type is a simple recirculat-ing unit which simply recirculates air fromthe space or spaces, removes any spacegenerated particulate, provides any necessarysensible cooling or heating and supplies theair back to the space.

PBS P-100 further requires that the Ventila-tion Air Conditioning unit be used topressurize the building during occupied andunoccupied times. The recirculating unitsare required to be located in fan rooms onthe floors which they serve, and since theyhave no requirement for outdoor air they canbe located conveniently in interior equip-ment rooms in the core of the building.

In warm humid climates the dischargetemperature of the cooling dehumidifying

coils in the VAC units must be at or belowthe desired room dewpoint temperature(approximately 52°F) and it can be distrib-uted at that temperature to help satisfyspace sensible loads or reheated as necessarywhen there is no space sensible load. The aircan then be distributed directly to thespaces providing constant or controlledquantity ventilation air, or it can be distrib-uted into the return air stream of therecirculating units. The option selected isdetermined by the design engineer.

If the engineer elects to distribute theventilation air directly to the spaces, theUFAD VAV controls can be tight shutoffsince ventilation would no longer pose aconstraint on the system design.

Another advantage of the P-100 baselinesystem concept, insofar as UFAD systems areconcerned, is that the higher supply airtemperature required (63ºF) could simplybe the controlled coil discharge temperaturefrom the recirculating air-conditioning unitsince this unit is not required for buildingdehumidification.

If the system designer, through life cycle costanalysis (See Sections 2 and 9 and AppendixA-2 of this Guideline), can demonstrate it

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beneficial to provide the outdoor air throughmixing damper control at all of the airhandling units that serve the spaces, it willbe necessary to reduce the air to the con-trolled dewpoint temperature of about 52ºFand then increase it to the UFAD supply

temperature of about 63ºF. Since thiscannot be done with reheat (ASHRAEStandard 90.1) it will require the use of areturn air bypass system which adds bothcost and complexity and the resultantdecreased reliability.

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xi

June 2005

U.S. General Services AdministrationPublic Buildings Service

Office of the Chief Architect1800 F Street, N.W.Washington, D. C. 20405

www.gsa.gov

Smarter Solutions

Cover Photo: U. S. Courthouse, Mobile, Alabama