Indoor Air Quality andVentilation Strategies

for Cold Climates EEM-00450

BUILDING IN ALASKA

10-04/RS/1000 October2004TheUniversityofAlaskaFairbanksCooperativeExtensionServiceprogramsareavailable toall,withoutregardtorace,color,age,sex,creed,nationalorigin,ordisabilityandinaccordancewithallapplicablefederallaws.ProvidedinfurtheranceofCooperativeExtensionwork,actsofMay8andJune30,1914,incooperationwiththeU.S.DepartmentofAgriculture,AnthonyT.Nakazawa,Director,CooperativeExtensionService,UniversityofAlaskaFairbanks.The University of Alaska Fairbanks is an affirmative action/equal opportunity employer and educational institution.

INTRODUCTIONWhatconstitutesthedesignationcoldclimate?Forthepurposesofthisinformationalpublica-tion and to emphasize some of the difficulties withmechanicalventilationcontrolstrategies,wewillusethelowerlimitof8000-Fahrenheitheating-degree-daysasthelevelabovewhichwe consider the climate cold. The climate ofFairbanks, Alaska, far exceeds this 8000 de-gree-daylowerlimit,withanaverageheatingindex of 14,300-14,400 Fahrenheit heating-degree-days annually. This designation includes all buttheverysouthernmostportionsofAlaskainitsdefinition and much of the northern tier of U.S. states and Canada. This location provides aunique natural laboratory setting in which to not only discover difficulties with ventilation control and the general control of indoor airquality in cold climates, but to also suggest and testnewstrategiesfromourresearchutilizingexisting, inhabited homes in our community.Thisisthesourceofourmostrecentresearchresultsreportedinthispaper.The8000Fahren-heit-degree-dayboundary forclassifyingcold

climates also provides a large reference zonefor most regions of the planet north of 60degreeslatitude.

Residential Ventilation–Why Do We Need It?With the emphasis in modern cold climatehousing on air-tightness and energy efficiency, a house is typically lacking adequate air exchangewithout theadditionofmechanicalventilation. In addition, traffic noise, particu-late pollution (such as forest fire smoke in the northduringthesummer),pollen(aproblemfor asthmatics and highly allergic people),and the maintenance of a healthy level ofrelativehumiditycanbebettercontrolledwithmechanical ventilation. A range of humidityconsidered both achievable and healthful innorthern housing is the range from above30% to 50%. Although relative humidities abit higher are still considered healthful, ourpresent ability to prevent condensation oncoldersurfacesislimitedbymodernwindowtechnologies.Keepingwindowsfreeofconden-sationatourmostextremeperiodsofcold isnotfeasiblewithwindowsofR-valuesof4or

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Figure1: An exhaust-only ventilation system (Alaska Housing Manual, 2000).Although not shown inthis figure, exhaust-only systems typically provide for supply air by small inlet vents, such as theFresh80,abrandnametubularplasticductwithanexternallouvertoprovideinletairtomakeupfortheairbeingexhausted.Noheatexchangetakesplace,andtheexhaust-onlysystemnotonlyinducesairattheoutdoortemperature,butalsoputsanegativepressureontheentirehouse,possiblyleadingtoinducedpollutants,suchasradon.

less. Finally, the air quality in homes can often only be ensured by mechanical ventilation,designedforthathome.

COMMON VENTILATION SYSTEMS FOR RESIDENTIAL USEIt isappropriate to reviewthecommontech-nologies in use for residential ventilation tounderstand the options for control that theyafford.Thesesystemsinclude:

1. Unbalanced Mechanical Ventilation Systems–Exhaust-Only

Thistypeofsystemtypicallyemploysasinglefan,hopefullystrategicallyplaced,toexhaustair fromaresidence,withair inletsplacedinrooms typically requiring supply air, such as bedrooms(Figure1–supplyductsnotshowninfigure). Placement of the inlets is crucial because ofthepressuredynamicsofthebuilding.Thisis particularly so in cold climates becausethe temperature differentials are larger andthe consequent pressure differentials across

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the building shell are enhanced. Exhaust-onlysystemsalso induceaconstantnegativepressure inside the building–with respect tooutdoors–andinduceradonfrombelowgradewalls along with other soil gasses. For theseimportant reasons, most building scientistsin northern regions have rejected exhaust-only technology for cold climate ventilation.

Supply Ventilation System with Point Source Exhaust•Supplyfanbringsinoutsideairandmixesitwithairpulledfromacommonarea

(livingroom,hallway)toprovidecirculationandtemperingpriortosupplyingtocommonarea.

•Runtimeisbasedontimeofoccupancy.•In supply ventilation systems, and with heat recovery ventilation, pre-filtration is

recommendedasdebriscanaffectductandfanperformancereducingairsupply.•Kitchenrangehoodprovidespointsourceexhaustasneeded.

BathroomFan

FromCommon

Area

KitchenRangeHood

ToCommon

Area Filter

OutsideAir

Figure2: Non-integratedsupplyandmulti-pointexhaustventilationsystem(uncommoninthenorth,butcouldbeadaptedhere).Credit:BuildingScienceCorporation,2004,usedwithpermission.

A second type of unbalanced mechanicalventilation–supplyventilationwithpointsourceexhaust–isshownanddescribedinFigure2.

2. Balanced Heat Recovery Ventilation (HRV)For approximately two decades, improve-ments in balanced heat recovery ventilationhave resulted in steadily gaining adoption

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of this mechanical ventilation option (Figure3). A generic cross-sectional view of a heatrecovery ventilator (also called an air-to-air heat exchanger) is shown in Figure 4.These systems have been particularly usefulin Canada, Alaska and the northern UnitedStates.Thebiggestbarriers towideadoptionseem to be initial cost and a satisfactorycontrol strategy. Cost of installation is nearlyalways less fornewconstruction,as theduct

Exhaust

Exhaust

Supply Exhaust

KitchenRangeHood

OutsideAirInsideAir

Balanced Ventilation System with Heat Recovery via an Air-to-Air Heat Exchanger•Theventilationsystemhasaseparateductsystemnotintegratedwiththeheating

andA/Csystem.•Runtimebasedontimeofoccupancy.•Exhaustsaretypicallyfrombathroomsandsuppliesaretypicallytobedrooms.•In supply ventilation system, and with heat recovery ventilation, pre-filtration is

recommendedasdebriscanaffectductandfanperformancereducingairsupply.

Figure3:AHeatRecoveryVentilationSystem.Credit:BuildingScienceCorporation,2004,usedwithpermission.

distribution system and design integrationintothestructurearemuchsimplertoincludeat the time of construction. But satisfactoryperformance of these systems is not alwayscertainduetoimmaturecontroltechnologies.

3. Heat Wheel HRV with latent heat recovery.Thisthirdtypeofheatrecoveryventilationsys-temusesarotatingheattransfersurface,whichtransitsbetweentheoutgoingexhaustandthe

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incoming cooler, dryer air. Consequently there iscrosspollutionoftheairstreams.Thistypeofsystemisnotrecommendedeventhoughitsolves the condensation and humidity issuesinsomeinstances.

How Much Ventilation Do You Need?ASHRAE 62-2isaresidentialventilationstan-dard.Thisstandardrecommends7.5cubicfeetperminute(cfm)ofventilationairperpersonbasedonthenumberofbedroomsinthehouseplusone.Inaddition,anASHRAEcompliantdesign must add .01 cfm for each square foot of floor area. As an example:

A three bedroom 1500 sq. ft. house would take:

7.5x4plus1500x.01=45cfm

This could be supplied in many ways, andwe’ll talk about the control mechanisms fordoingso.

RESIDENTIAL VENTILATION CONTROL STRATEGIESEither by inference or by consensus of engi-neering,wehaveagreedtoreproduceindoorstheoutdoorclimateof the tropicalsavannah.

CoolAirSupply

CoolAirExhaust

ExhaustFromHouse

WarmAirSupply

Figure4:Acut-awayviewofatypicalHeatRecoveryVentilator,singlecore.Credit:AlaskaHousingManual,2000.

These conditions are typified by a relative humidityrangefrom30to60percentandthetemperaturebetween65°to70°F(18°to21°C).There are probably many reasons why thisdesign strategy and set point condition wereagreed upon. But it’s hard to imagine thatourevolutionaryemergencefromthetropicalsavannah was not a strong influence in this resultingagreement.

Residential control strategies have consequent-ly been developed to try to keep the relativehumidity in the rangeof30 to60percent. InAlaskawehavetoslidetowardthedrysideofthis rangeandnotexceed50percent relativehumidity indoors. Present window technolo-giesdonotallowustotolerateindoorrelativehumiditieshigherthan50percent.

Allcoldweatherventilationstrategiesarein-herently dehumidification processes. In a heat-ingclimate,warmmoistairisbeingexhaustedandreplacedbycoolerdrieroutsideair.

Another aspect of indoor air quality affected by ventilation strategy is radioactive radongas. When present in the soil gas, radon infil-tratesintoahousedrivenbythesamemech-anisms that cause the infiltration of cool dry

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outdoor air into a house. Transport of radonintoahouseisprimarilyduetoairleakage(aircontainingradon)causedbydifferentialpres-suresbetweenthehouseandsoil5. Infiltration can take place anywhere there are pressuredifferencesandopeningsinthebuildingenve-lope.Housestypicallyleakairinlowandoutatthetopofthebuilding.

Theventilationrateofahouseatanygiventimeisa functionof theopenings in the structureandtheforcescausingairtomovethroughthehouse.Theeffectoftheairbuoyancyisoftenreferred to as the stack effect and this effectis significant during the heating season in Fairbanks.

First, let’s look at some research:During the winter of 2000, researchers at theUniversityofAlaskaFairbanksdidaseriesofexperiments and research tests to study theresultsofvariousventilationsystemsandtheircontrolled operation on the indoor air quality ofseveralhomesintheFairbanksarea.

They recorded indoor radon concentrations,indoor temperatures, and outdoor tempera-tures at two Fairbanks homes for the periodbetweenthespringof1999andthespringof2000.Theyalsomeasureddifferentialpressuresacrossthebasementslabatoneofthehousesduring the winter months. The purpose wasto demonstrate the seasonalvariability of in-door radon concentrations, the variation ofindoor radon concentrations with outdoortemperature and the effectiveness of subslabdepressurization systems in the Fairbanksarea. TSI instruments were also used to ac-quire data consisting of relative humidity, carbonmonoxide,carbondioxide,andindoortemperaturemeasurementsintwelvedifferenthomesinInteriorAlaska.Inallbutaveryfewperiodsofmeasurement,homeswereoccupiedandinroutineuse.Housesweremeasuredfortwo-week intervals for each season, typicallyrotatingthroughadatacollectionintervalevery10-12weeksfornearlythreeyears.RadondatawastypicallygatheredsimultaneouslywithaSunNuclearradonmonitorandsupplemented

withvariousradontestkitsforcorroboration.Air leakage tests were typically done duringthe autumn or winter, and a mix of blowerdoor tests and carbon dioxide dilution testswere utilized to measure air leakage undervariousoutdoorconditions.

RESEARCH RESULTSFigure5 isaplotof indoorrelativehumidityand the simultaneous indoor/outdoor tem-perature differential in two different houses.However, the time periods are contiguousand the contrast in sustained indoor relativehumidity levelsbetweenthetwohouses isanimportant indicator of the importance of airleakage.HouseR-Shas50%morenaturalairleakage than House R-A. The temperaturedifferentialduringtheperiodofmeasurementforHouseR-Swasmarkedlygreaterindicatingit was much colder outside during that timeperiod.

ThelatterhalfofthemeasurementperiodforHouseR-A(1/28to2/8)comparessomewhatcloselytotheperiod(1/13to1/28)forHouseR-S. The relative humidity in house R-A isconsistently15-20%higher,andinahealthfulrange (ca. 45%). Air leakage is an importantcontrollingfactorinthisdifference.Theblowerdoorleakageratesforthetwohouseswere2.5ACH50forHouseR-Aand3.7ACH50forHouseR-S(ACH50meanstheAirChangesperHourat50Pascalspressuredifference.50Pascalsisacommontestpressureforblowerdoortests).This50%greaterairleakageappearstoexceedacrucialboundarysinceHouseR-Aisabletosustainhealthfulhumidity levels,andHouseR-Sisnotabletodoso.

A strong correlation was found between theindoor radon concentrations and differentialpressures across the slab at House R-A (Fig-ure 6). It is generally accepted that pressuredriven flow dominates as a radon entry mech-anism.Thisdemonstratesthecorrespondencebetween the pressure differences across thebuildingenvelopeandtheentryofradonbear-ingsoilgasassuming that the soilgas radonconcentrationsremainedstable.InHouseR-A

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therewasnopositivecorrelationbetweenin-door-outdoortemperaturedifferencesandtheradon concentrations. This was unexpected.Inworkwithotherhomes,strongcorrelationswere observed between the indoor-outdoortemperature differences and indoor radonconcentrations. In a study of passive ventila-tioninahouseinSapporo,Japan,Fukushimanotedthatthe leakagerateofanaveragesizeairtight house increases with indoor-outdoortemperature difference7. It was expected thatthe temperature differences would be a goodindicatorforthepressuredifferencesacrossthebasementslabandalsoanindicatorofsoilgasinfiltration.

The infiltration rates of the other houses tested inthisstudywerebetween0.3to0.6airchangesper hour (ACH) as measured by tracer gasmeasurementsattemperatureslessthan19.4°F(-7°C)andblowerdoormeasurements.Tracergas measurements indicated the infiltration

Figure5: RelativeHumidity(%)indoorsisdirectlyrelatedtotheoutdoor-indoortemperaturedifferenceand the "leakiness" of the house. A leaky house, such as house R-S here, cannot maintain ahealthfullevelofrelativehumidityatlowoutdoortemperatures."Dt"heremeanstemperaturedifference.

rate of House R-A to be between 0.1 and 0.2ACHat19.4°F(-7°C).Therangeofdifferentialpressuresacross theslabwasfrom0 to18Pawithonly2%ofthedatapointsgreaterthan10Pa. In thishouse theoperationof thekitchenfan, 2 bathroom fans, clothes dryer, and oil-fired boiler in various combinations could reducethepressureinthehousebyasmuchas11Pa.Otherconfoundingfactorswouldbetheopeningofthegaragedooronthegroundlevelandtheoccasionalwind.

CONCLUSIONSThereareseveralfundamentalconclusionsonecan draw from our results. First, the greaterthetemperaturedifference(i.e., thelowertheoutdoor temperature is), the greater the airleakage (infiltration) rate is likely to be, and a fraction of that infiltration will be soil gas that possibly contains radon or other pollutants.Second,theconcentrationresultingfromradonbearing soil gas induction is very strongly

(%) °F

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Figure6:RadonConcentration&PressureDifferentialAcrossBasementSlabvsTimeinHouseR-A.

correlatedtothepressuredifferenceacrosstheslab (Figure 6). This clearly indicates that airleakageresultingfromthepressuredifferenceisacrucialfactorinradoninduction.Blockingairandsoilgasentrybylimitingleakagefromthe building can go a long way towardcontrollingradoninductionandothersoilgaspollutantsfromenteringthehome.

Houseswithexhaust-onlyventilationshowaclear tendency to induce radon. So exhaust-onlyventilationisworkable,buthastheaddedriskofpossiblyinducingradonindangerousamountsintothehomeifitisonaradonrisksite.Forthisimportantreason,weurgecautionin using exhaust-only ventilation systems inAlaska.

Theresultsofthisresearchcorroboratethatwehavemadeprogresstowardacontrolstrategyfor ventilation systems. Very good control ofair leakage, and consequent control of indoor relative humidity is important. This controlofleakagemakesthehouselessresponsivetodifferentialpressuresacrossthebuildingenve-lope.That is what air sealing accomplishes! Large pressure differences should not be in-

ducedontheshellofabuildingbyventilationsystems, as this will induct soil gas, outdoorair pollutants, and would generally limit theability to control the air flow and pressure dif-ferenceacrosstheshell.Thisisanothermajorreasonwhymostbuildingscientistsrejecttheconcept of exhaust-only ventilation in verycoldclimates.

HouseR-Aisanexampleofahouseinwhichthe natural infiltration is well controlled. The lack of a positive correlation between theindoor-outdoor temperature difference andthe pressures across the envelope indicatesthat–in this house–the ventilation is drivenless by natural infiltration and predominantly byoperationof thevariousappliances in thehouse,andtheopeningsofdoorsandwindows.Ideally a house would have provisions toreplaceairremovedbyappliancestominimizethedepressurizationofthehouse.Itissimplya tight house. Operation of appliances cancause substantial depressurization, and thishas a more significant impact on this house thanhouseswithgreaterair leakagerates. IninteriorAlaska,thepredominantnaturalforcedriving air infiltration is buoyancy resulting

Pa °FpCi/l

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from temperature differences. The averageoutdoortemperatureislessthan19.4°F(-7°C)for approximately five months out of the year1.

SowhatcanweconcludeaboutventilationforAlaskanhomesandhowtocontrolit?Herearesomeconcisestatementsaboutventilationanditscontrol:

• Controlled ventilation REQUIRES an air-tightbuildingenvelopeandducts.

• Exhaust ventilation can work, but it is diffi-culttocontrolandhasseriousliabilitiesforinducting pollutants and bad outdoor airintothehouse.

•Balancedventilation,whichcanbesinglepoint, multi-point (i.e. fans at locationswhere exhaust is commonly necessary),integrated with a central fan system orforcedairheatingsystem,orwithHeatRe-covery Ventilation (HRV) is the preferredapproach. Control based on maintainingrelative humidity above 25% is recom-mended.

Installation and selection of a ventilation system for your house is not considered ex-pendable, and should be done by a competent ventilation contractor who is capable of designing the ductwork and certifying the system with tests. Although these services are available in Alaska, there is no certifi-cation of ventilation installers. This is regrettable, and Extension, Alaska Building Science Network, and others are working to remedy this situation. For the present, seek experienced installers and consult with your contractor, the ABSN (www.absn.com) or local homebuilders association at the time of construction.

Ventilation system cost estimatesTheseestimatesarefromBuildingScienceCor-porationfornationallaborratesfor2003:

•Central-integrated-fan system $320 total:$65 fan recycling control, $65 motorizeddamper,$30ductparts,$160labor.

•Multi-pointsupply$800total:$250supplyfan with filter and two inlet ports (outside air and recirculation air) and one outletport,$150ductsandgrilles,$400labor.

•HRV, approximately $1.25 per square foot of building floor area, installed at the time of construction. Perhaps 20% more in retrofit.

REFERENCES1. Alaska Climate Website, http://climate.

gi.alaska.edu2. Alaska Housing Manual (2000), Alaska

HousingFinanceCorporation,98pp.plus5majorappendices.AlsoonCDavailablefromUAFCooperativeExtensionService.

3. BuildingAmerica,EEBAHousesThatWorkTraining Guidance, Ventilation, Chapter14, Midwest Research Institute, NationalRenewable Energy Laboratory Division,Golden,CO.Preparedby:BuildingScienceCorporation,Westford,MA.AlsoavailableonCD.

4. Hartman, C. and Johnson, P., 1978, Envi-ronmentalAtlasofAlaska,InstituteofWa-terResources,UniversityofAlaska.

5. Nazaroff,W.W.1992,Radon Transport from Soil to Air, Reviews of Geophysics,May,Vol.30,No.2,pp.137-160.

6. R. Seifert, J. Schmid, R. Johnson, andD.Wilkinson, (2000) Indoor Air Qualityand Ventilation Strategies, paper present-edattheThirdInternationalColdClimateHeating,VentilationandAirConditioningConference at Hokkaido University, Sap-poro,Japan,November,8pp.

7. Fukushima, A. and Enai, M., A Feasibil-ity Study on Passive Ventilation in Airtight Houses in Cold Regions,ProceedingsoftheFifth International Symposium on ColdRegionDevelopment,Anchorage,Alaska,May4-10,1997.