Service Life of Building Envelope Components

Construction and Building Materials 16(2002) 83–89

0950-0618/02/$ - see front matter� 2002 Elsevier Science Ltd. All rights reserved.PII: S0950-0618Ž02.00003-X

Service life of building envelope components: making it operational ineconomical assessment

Claus Rudbeck

Rockwool International AyS, Hovedgaden 584, DK-2640 Hedehusene, Denmark

Received 4 April 2001; received in revised form 6 August 2001; accepted 9 January 2002

Abstract

Recently, standards and guides have been developed to assess the service life of buildings. During design and operation ofbuildings, focus is mainly on the cost of constructing and operating the building than on its service life and tools; making servicelife operational in economical assessments are therefore needed. Development of such a tool is performed after a description ofthe current standards and suggestions for their improvement. Use of the tool is illustrated by performing an economical assessment,including the effects of service life, on an innovative low-slope roofing insulation system. The assessment reveals that theinnovative roofing system has a lower total cost than traditional systems as it is prepared for repair and maintenance.� 2002Elsevier Science Ltd. All rights reserved.

Keywords: Service life prediction; Economical assessment; Prepared for repair and maintenance

1. Introduction

During the last 10 years, national standards have beendeveloped in order to assess the expected service life ofbuilding materials and constructions and work is stillprogressing at the international level. Besides the currentand upcoming standards, several methodologies havebeen developed or suggested at the national, internation-al or individual level. By using these standards ormethodologies, designers can estimate the service lifeof material or components or estimate the risk of failurethroughout the entire life of the material or component.However, during design or operation of buildings, thebuilding designer or operator is more focused on thecost of constructing, operating, maintaining and replac-ing components than on the service life of these com-ponents. Therefore, tools that can translate service liferelated parameters into economical terms are needed.

2. Standards, guides and methods for assessing serv-ice life

2.1. National standards

Service life prediction or demand for durability hasbeen treated in numerous countries in their standards

E-mail address: [email protected](C. Rudbeck).

and building codes. Of the available standards andbuilding codes, the following three are the most known:Japan—Principal Guide for Service Life Planning of

Buildings w1x.Great Britain—Guide to Durability of Buildings and

Building Elements, Products and Componentsw2x.Canada—Standard S478: Guideline on Durability in

Buildings w3x.The Japanese guide states that service life may be

predicted for the whole building, parts of the buildingor its elements, components or equipment. The end ofthe service life is determined by physical deteriorationor by obsolescence. Assessment of physical deteriorationis based on an assessment of the deterioration level atthe end of the service life and the length of the servicelife. Based on these values, an annual physical deterio-ration, valid for one climate, can be calculated. Calcu-lation of the predicted service life under other stresses(level of use, climate etc.) and with other materialqualities is performed by including factors in an equationwhich links the predicted service life to the referenceservice life. The reference life is added to or multipliedby these factors, which describe the influence fromenvironmental agents, quality of work, quality of mate-rials etc.

84 C. Rudbeck / Construction and Building Materials 16 (2002) 83–89

The British guide recommends that service life isattained by reference to previous experience with asimilar construction, measurements of the natural dete-rioration rate or results from accelerated tests. It isrecommended that more than one approach be used, asthe methods may be imprecise. There is a lack ofinformation regarding whether or not one method issuperior to the others. Another issue not being addressedis the possibility of calculating service life for a structureunder one set of conditions using information for otherconditions.From the beginning the Canadian guide drew on work

done during the development of the BSI guide, but indue time evolved in width and depth. Unlike the othertwo guides, the Canadian guide also includes renovatedbuildings. Information is given regarding when to usewhich service life assessment method and whether morethan one method should be used. Like the BSI guide,the Canadian guide does not offer calculation routinesfor calculating service life.

2.2. International standards

At the international level several organizations(ISO,CIB, RILEM, EOTA and ASTM) are working with theassessment of service life. Work in ISO is conducted inISOyTC59ySC14 ‘Design Life’ with the aim of devel-oping an international standard series titled ‘Buildingand Constructed Assets-Service Life Planning’ of whichsix parts are planned. These are titled:Part 1—General PrinciplesPart 2—Service Life Prediction PrinciplesPart 3—AuditingPart 4—Data formattingPart 5—Life Cycle CostingPart 6—Environmental SustainabilityOf these, the first part of the ISO-15686 seriesw4x

describes the method for calculation of the EstimatedService Life for a Component(ESLC) based on aReference Service Life of the Component(RSLC).According to the ISO standard, the RLSC should bebased on experience, building codes or test results. Toobtain the ESLC under specific conditions, the RSLC ismultiplied with several factors, thereby taking intoaccount quality of materials, design, site work, indoorand outdoor environment, operating characteristics andmaintenance level. In short, this approach is oftenreferred to as the ‘Factor method’.In the joint venture between CIB W80 and RILEM

TC-140 ‘Prediction of service life of building materialsand components’, the focus was on integration of exist-ing prediction and service life techniques, tools andmethods with information technologies being developedfor the construction industry. To ensure integration ofstate of the art regarding service life prediction a ‘Guideand Bibliography to Service Life and Durability

Research for Building Materials and Components’w5xis to be developed. Besides treating service life predic-tion, the guide offers a long literature list with a shortdescription of the referred publications. Work in thesuccessor CIB W80yTC-175 is performed in four taskgroups named: damage functions and environmentalcharacterization, factor methods, information technologyand reliability and probabilistic methods.Another group in CIB, namely CIB W94 Design for

Durability, aim to develop a design methodology whichmakes it easy for designers to include durability andservice life prediction in the design process. Likewise,the focus in CIB W94 is on the production of guidelinesfor presentation of research results in publications, whichwill ease the communication between researchers andpractitioners.

2.3. Individual recommendations

The standards are not the only available informationregarding service life prediction, as a number ofresearchers have developed methods which are based oneither a structural engineering approach, a probabilisticapproach or methods that are further developments ofthe deterministic approaches which the national andinternational standards are based on. A survey of meth-ods belonging to any of the three approaches wasperformed by w6x. The following conclusions wereobtained:Structural engineering approach: Although the relation

between the structural engineering and service life pre-diction has been recognized, none of the examinedstructural engineering approaches reveal a potential forfurther development.Probabilistic approach: Two probabilistic approaches

are described, one approach using a mathematical func-tion (Weibull) to describe the performance of a com-ponent over time and one approach using discreteMarkov chains. The latter of these two have successfullybeen used to predict the performance through time ofroad pavement and bridges. However, one major disad-vantage with using Markov chains is that the methodrequires a large number of similar building envelopecomponents which are subjected to the same climaticinfluences, a demand which cannot be met very oftenin the building sector as almost every building is aprototype, being different from other buildings.Deterministic approach: Variations of the method

specified in the Japanese guide and a later ISO standardhave been described and examined, e.g. byw7x. Insteadof using modifying factors, the variations introducestatistical functions to describe the influence of theindooryoutdoor climate, quality of materialsywork etc.and as such combine the deterministic and probabilisticapproach.

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3. Discussion of standards and guidelines

As seen from the previous chapter, different methodsexist which can be used to assess the service life of abuilding component under specific environmental con-dition, usage patterns etc. Common to all the describedmethods is that measured performance through time forcomponents is needed to determine the accuracy of themethods.The two approaches, deterministic and probabilistic,

both have their advantages and disadvantages. Oncedeveloped, deterministic methods in the proposed formatare easy to use as they only require that a designer canassess the influence of the different factors by using asmall table and that the designer can perform simplearithmetic operations. However, the result of using oneof the deterministic methods is an exact service life ofa building, which is not correct as service life of identicalbuildings can best be described as a stochastic distribu-tion. The probabilistic methods are opposite the deter-ministic methods with regards to the result of themethods and the usability. Even when information nec-essary for using the probabilistic methods are present,the methods are very computing intensive, and it istherefore not an easy task for a designer to perform aservice life assessment for a component. However, asprobability is included in the methods, this is reflectedin the results of the methods so the designer can seeaverage values and spread for the service life of acomponent.An issue that does not receive much attention in any

of the guides and standards is the coupling of the servicelife of a component to its economical performance. Theimpact of the service life on the economical performanceof a component is growing in importance as parts ofbuilding codes and regulations are changed. An exampleon such a change is found in the public-funded buildingsin Denmark, where total economy is to be used as aperformance assessment toolw8x. Using this approach itis possible for a designer to use more expensive com-ponents during the construction phase if subsequenteconomical savings can be proved later on during thelife of the building.

4. Suggestions for improvement of standards andguidelines

As the economic performance of building componentsis an important factor, and as the economic performanceof the components is strongly linked to the service lifeof the component, improvement of standards and guide-lines should include the link between assessment ofservice life and assessment of economical performance.Assessment of the service life of buildings or com-

ponents may be performed using methods based onprobability or so-called ‘Factor methods’. As both of

the approaches have advantages and disadvantages, anatural conclusion would be to combine parts of thetwo methods into a ‘Factor method’ where probabilityhas also been included. A combination of these twoapproaches has been proposed byw7x and described indetail by w6x. The basis of the method is that the ESLCis calculated as being the RSLC multiplied with anumber of factors, as proposed inw4x. However, unlikethe ISO standard, the factors are not constant numbers,but instead stochastic distributions of the influence ofthe different factors. The result of using the method istherefore an ESLC given as a stochastic distribution.The stochastic distributions that describe the influenceof the different deterioration aspects on components arenot developed yet, but can be when data of sufficientquality and quantity are available.

5. Translating service life into economical terms

Based on the outlined approach it is possible, withrelative ease, to calculate the estimated service life fora component. However, for many of the actors in theconstruction phase and operation phase of a building,the focus is on having a building which fulfill thespecified performance levels while still having a lowcost. It is therefore important that the service life of acomponent can be translated into economic terms. Basi-cally, this is done by combining the proposed servicelife assessment method with a tool called net presentvalue (NPV) calculation, a tool which is well-knownand well-used in all kinds of economical assessment.NPV is a method which allows the user to discount allcosts to a base time thereby taking into account inflation,increase in price level etc. The method makes it possibleto compare the total cost of components with differentdistributions of cost of investment, operation, mainte-nance and replacement through time. NPV of the totalcost throughout the life of a component can be assessedusing Eq.(1).

NPV sNPV qNPVTC Investment Operation

qNPV qNPVMaintenance Replacement

yNPV (1)Resale

where NPV denotes the NPV of the different partsx

(investment, operation, maintenance, replacement andresale) of the cost through the life of the component. Itshould be noted that maintenance covers traditionalmaintenance of components which is performed atregular intervals, e.g. repainting, window cleaning etc.Substitution of one component with another, e.g. re-roofing, is treated as a replacement. It should also benoted that the resale value is often referred to as thescrap-value of the component, i.e. the value of thecomponent at the end the assessment period, which,under Danish conditions, should not exceed 30 yearsw8x. However, as the service life of the building may be


Fig. 1. Section showing the normal build-up of a low-slope roofinginsulation system using a concrete deck and rigid insulation boards.

longer, say 60 years, this should also be reflected in thecalculations.As the cost of the component can be expressed as

costs occurring at single predetermined points in time(investment and resale) or costs occurring with a certainperiodicity (operation and maintenance), equations forcalculation of NPV of these costs is easily found in theliterature, e.g.w9x. However, cost of replacement, whichis the final part of Eq.(1), does not occur with a certainperiodicity or at single predetermined points in time.Instead, replacement of a component is performed at theend of its service life, thereby requiring a link to themethods of service life prediction.For a given component, the stochastic distribution of

the ESLC has to be calculated using results fromexperiments before the NPV of the cost of replacementcan be assessed. Following the construction of thestochastic distribution a Monte Carlo simulation is usedto predict the point in time where replacement is needed,an information which can then be used to calculate thecost of replacement expressed as a NPV. The principleof a Monte Carlo simulation is that a large number ofrandom numbers(representing an equal number ofidentical constructions) are inserted into the stochasticfunction with the result being the service life of allthese constructions. These results are then used todetermine the point in time where replacement of thecomponent takes place and the associated NPV of thereplacement cost. In theory, this is performed for anunlimited number of components to improve the qualityof the cost calculations. The total assessment of theNPV of the replacement costs is performed by comple-tion of the following steps:

1. ESLC (added to ESLC or previous components) iscompared to the service life of the building. If thesum of ESLC is higher than the service life of thebuilding, go to step 4.

2. NPV of cost of replacement of component iscalculated

3. Go to step 14. Sum of NPV of replacement of component during

the life span of the building is added and stored forlater processing

5. New identical building is considered, and the calcu-lation starts from step 1 unless a sufficient amountof calculations has already been made. In that case,go to step 6

6. The results from step 4 are compared and furtherprocessed

The result from the six steps described above is aseries of NPV of replacement costs for the componentduring the life span of the building. An example of sucha series of values will be shown later on.Such a series of NPVs is a valuable piece of infor-

mation regarding a component as the series show the

minimum replacement costs and the maximum replace-ment cost throughout the life of the building(or anotherspecified period). The financial consequences of unex-pected replacements through a period can therefore notcome as a surprise for the clientyoperator and moneycan then be set-aside early on for replacement. It is thesame mentality that lies behind insurance: The policyowner is insured against financial disaster by settingaside money(paying the insurance premium) in time.

6. Including service life in an economical assessmentof a dryable low-slope roofing insulation system

To illustrate the use of the method which has beendescribed, the methods has been used to assess theeconomical performance of a dryable low-slope roofinginsulation system, an economical performance which isthen compared to the performance of a traditional low-slope roofing insulation system.

6.1. Description of traditional low-slope roofing insu-lation system

Low-slop roofing insulation systems are often usedon commercial, institutional or industrial buildings, asthe use of pitched roofs would pose severe difficulties.If pitched roofs were to be applied to large industrialbuildings, the construction of such roofs would bedifficult especially with regard to the valleys, which areneeded to collect and transport water to the gutters.Low-slope roofs are constructed with layers of mineralwool or polystyrene insulation being placed on top ofthe deck which is normally made of concrete or steel.A section of a typical build-up of a low-slope roofinginsulation system is shown in Fig. 1.The normal application of a low-slope roofing insu-

lation system begins after the completion of the deck.To hinder vapor transport from the inside into theinsulation, a vapor retarder is placed on top of the deck.


Table 1Investment costs for different parts of dryable roofing system with 195 mm insulation

Building materialycomponent Euroym2

Low slope roof insulation system incl. vapor retarder and roofing membrane 61.2Moisture indicator produced in strips placed per 1 meter on the roof 0.3Inlets and outlets for ventilation system 0.3Construction of grooves in the insulation panels 2.2Total costs 64.0

The vapor retarder maybe either a polyethylene foil ora water permeable vapor retarder. On top of the vaporretarder, the thermal insulation is placed. To carryphysical loads from activities at the roof surface, rigidinsulation is preferred. The last part of the system is theroofing membrane, which acts as a water and vaporbarrier.A typical failure for low-slope roofing insulation

systems is if the roofing membrane is penetrated, therebymaking it possible for water to enter the insulation layer.As the insulation is located between two vapor tightlayers (the vapor retarder on the underside and theroofing membrane on the topside) the water has nomeans of escaping from the insulation if a traditionalvapor retarder is used. Rudbeckw6x identified threepossibilities of repair in case of such failures, but neitherof these would reduce the future risk of failure for thesystem. These three possibilities of repair includedreplacement of the entire insulation system, addition ofnew insulation and membrane on top of the existingconstruction and installation of roofing vents.

6.2. Description of dryable low-slope roofing insulationsystem

A way to improve the performance of a low-sloperoofing insulation system is by its two major problems,i.e. detection of moisture and facilitation of excessivemoisture removal.As seen in Fig. 1, the insulation is applied in several

layers to decrease air movement and to increase work-ability. Water that enters the insulation system willnormally locate itself in the insulation layer just abovethe vapor retarder. To detect if excessive moisture ispresent in the insulation system, thin metal wires areembedded in the vapor retarder in parallel, and ameasurement of the electric resistance between such twowires can then be used as a detection system. If metalwires in the vapor retarder are unwanted, they can alsobe incorporated by construction of coaxial-like cablewith a metal core, a metal screen and a moisture-accumulating material in between. Such a cable maythen be applied at points of special interest. To facilitatethe removal of moisture, grooves are created in thedown-facing side of the lowest insulation panel. Ifexcessive moisture is detected in the insulation, forced

ventilation with outside air is applied to the grooves.The size of the grooves is 30=50 mm so they decreaseneither the thermal nor the structural performance of theinsulation systemw6x. Investigations using numericalmodeling and experimentsw6x have also proved thedrying capabilities of the system.

6.3. Economical parameters for low-slope roofing insu-lation systems

As suggested previously, an economic assessmentshould include a calculation of the NPV of the total costfor a building component for a specific period. Calcu-lation of the NPV of the total cost is performed usingEq. (1). A number of assumptions are given for thecalculation of net present value● The real interest rate is 2.9% throughout the 60 years.● The energy price is 0.0677ykWh throughout the 60

years.● Service life of membrane can be described as a

normal distribution with mean value 22 years and astandard deviation of 7 years. This is based oninvestigations by Marcellus and Kylew10x.

● Economical write-off of value of roofing insulationsystems is linear.

● Resale value of roofing insulation system is notincluded in the calculations as the roof is withoutvalue at the end of the 60-year life of the building.A small error is introduced as the resale-value of theroofing membrane should be included, but the impacton the net present value calculation is very small inthis case. If shorter building service life is used, theresale value should be taken into account to avoiderror.The cost of construction for the dryable roofing

system is given in Table 1 with the prices being obtainedfrom a Danish price book for building componentsw11x.The investment cost of764ym can be compared with2

the investment cost of761.2ym for a traditional low2

slope roofing system. As the investment cost is due inyear 0, at the beginning of the service life of thebuilding, these values are also equal to NPV .Investment

Net present value-life spanŽ .1y 1qreal interest rateŽ .

sAnnual cost*real interest rate

(2)


Table 2Net present value of the difference cost aspects and the total cost forthe traditional roofing system and for the dryable roofing system. Bothsystems use 195 mm of insulation

Aspect Net present valuew7ym x2

Traditional Dryablesystem system

Investment 61.2 64.0Operation(heating of building) 21.2 15.9Maintenance 9.8 9.8Replacement 72.0 36.0Total cost 164.2 125.7

Fig. 2. Distribution of net present value of the replacement cost for a traditional roofing insulation system during a 60-year period. The costs areassessed using a Monte Carlo simulation.

NPV of the operational cost for the two componenttypes, taking into account thermal loss through thecomponents, is assessed using Eq.(2).The annual cost is based on calculation of the heat

loss through the component. Under Danish climaticconditions the annual heat loss through a traditionalroofing insulation system amounts to 17.2 kWhym for2

the traditional system and 12.9 kWhym for the dryable2

roofing insulation system. Using Eq.(2), the net presentvalue of the operational costs may be obtained.NPV of the maintenance cost is assessed using Eq.

(2) assuming the annual cost of maintenance, coveringvisual inspection of the roof surface etc. to be70.35ym .2

NPV of the cost of replacement of parts of the roofinginsulation system is performed using the approach,which was outlined, in Section 5. The cost of replace-ment of the roofing membrane and the insulation forthe traditional system amounts to772ym . Using the2

approach outlines in Section 5, the net present value ofthe replacement cost for the traditional system is foundto be in the interval between723 and7230ym with2

the average being772ym . A visual representation of2

the net present value of the replacement cost is shownin Fig. 2.Using a similar approach for the dryable roofing

insulation system, the net present value of the replace-ment cost is found to be between712 and7115ym2

with the average value being736ym .2

The net present value of the total cost for the tworoofing systems is shown in Table 2.The conclusion, after having compared the net present

value of the total cost of the two systems, is that the

dryable roofing system is to be preferred as it has asignificantly lower net present value of the total cost.The difference between the costs of the two systems ismainly due to difference in cost of replacement.

7. Conclusions

An overview of current methods and standards usedfor predicting service life of buildings is given. A reviewis provided that contrasts the less practical against themore useful aspects of national and international stan-dards and guides. One improvement needed in themethods for predicting service life of building envelopecomponents is to include life-cycle cost assessment.A method which links service life prediction with

economical assessment has been formulated. The meth-od is based on a coupling of net present value assessmentand a service life prediction system. The service lifeprediction system is the ‘Factor method’, found in the


upcoming ISO-standard 15686, which has been extendedto include a probabilistic approach utilizing Monte Carlosimulations.The method has been used to assess the economical

performance of an innovative dryable roofing insulationsystem for low-slope roofs. The system was designed tobe prepared for repair and maintenance in case of leaksin the roofing membrane followed by water intrusioninto the insulation. The economical performance of thesystem was compared with the economical performanceof a traditional roofing insulation system for low-sloperoofs. During the life-cycle of the under-lying building,the innovative roofing system performed much betterwith a lower total-cost being the result even though atraditional economical assessment would have found itto be more expensive.

References

w1x AIJ, The English Edition of Principal Guide for Service LifePlanning of Buildings. Architectural Institute of Japan, Tokyo,Japan, 1993.

w2x BSI, Guide to Durability of Buildings and Building Elements.BS 7543:1992. British Standards Institution. London, UK,1992.

w3x CSA, Standard S478-95: Guideline on Durability in Buildings.Canadian Standards Association, Rexdale, Canada, 1995.

w4x ISO, Buildings-Service Life Planning-General principles. ISOyDIS 15686-1. International Standards Organization, Geneva,Switzerland, 1998.

w5x Jernberg, P., Minutes of CIB W80yRILEM TC 140-TSLyRILEM-SLM meeting in Capri, Italy May 1997. InternationalCouncil for Research and Innovation in Building and Construc-tion, Rotterdam, The Netherlands, 1997.

w6x Rudbeck, C., Methods for Designing Building Envelope Com-ponents Prepared for Repair and Maintenance, Ph.D.-thesis,Report R-35, Department of Buildings and Energy, TechnicalUniversity of Denmark, 1999.

w7x Moser K. Towards the practical evaluation of service life-illustrative application of the probabilistic approach, in Dura-bility of Building Materials and Components 8: Service Lifeand Asset Management. NRC Research Press, Ottawa, Canada,1999. p. 1319–29.

w8x TRAMBOLIN, Program for calculation of total economy forhousing by calculation of present value(in Danish). Ministryof Housing and Urban Affairs, Denmark, 1998.

w9x ASTM, E917-94 Standard Practice for Measuring Life-CycleCost of Buildings and Building Systems. American Society forTesting and Materials, PA, USA, 1994.

w10x Marcellus, R., Kyle, B., Towards better decision making forroofing systems. URL ftp:yyftp.tech-env.comypubyReliabltyBetterRoofingDecisions.pdf, Internet document dated 1997-12-20, (1998).

w11x V&S Prices. Buildings-Brutto(In Danish: Husbygning-Brutto).V&S Byggedata, Denmark, 1999.

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Service Life of Building Envelope Components