*WagdyAnis,FAIA,LEEDAP,Principal,Wiss,Janney,ElstnerAssociates,Inc.,Boston,MAwanis@wje.com

BEST2WB34SIXWAYSFORCONDENSATIONINBUILDINGSWagdyAnis,FAIA,LEEDap*

Abstract:Sixofthewayscondensationcanoccuroncoldsurfacesinbuildingsarediscussedhere.Theyare:

1. AirLeakage,2. Diffusion3. Convection,4. Thermalbridges5. Fenestration6. Groundcontact

Eachofthesewayswillbediscussedinthispaperandsolutionssuggestedtomitigateandavoidtheproblemsfromoccurring,Preventingcondensation,ormoreaccurately,avoidinganincreaseinthemoisturecontentofbuildingmaterialstolevelsthatcansupportthegrowthofmicroorganisms,especiallymoldswithintheenclosureandoninteriorsurfaces,isfundamentaltoavoidinghealthandairqualityproblemsinbuildings;itisalsoessentialinavoidingprematuredeteriorationofenclosurecomponents.ThegrowthoramplificationofmicroorganismsinbuildingsnotonlyresultsinbiodeteriorationofsusceptibleconstructionmaterialsbutalsoleadstotheproductionofallergensandmicrobialVOCs(thecauseofmustyodors)thatcanaffectoccupanthealthandairquality.Acomplexmicrobialecology(e.g.,mitesfeedonmoldandskinparticles;otherorganismsfeedonmites)candevelopinoronconstructionmaterialsthatarechronicallywetordamp.Allergensassociatedwithmoldsandarthropods(theirfecesandbodyparts)growinginchronicallywetconstructionnichescanentertheindoorenvironmentandposearisktosensitiveoccupants.Themoisturecontentofbuildingmaterialsincreasesduetowatervaportransportacrossenclosureassemblies,eitherbyinfiltrating,exfiltrating,orconvectingaircomingincontactwithsurfacesthathaveatemperaturelowerthanthedewpointofthemovingair,and/orbydiffusionduetoadifferenceinwatervaporpressureacrosstheassembly,orbycapillarytransportthroughthemicroscopicvoidsinbuildingmaterials.Properlydesignedenclosureassemblieshavegreaterdryingpotentialthanwettingpotential.Theyachieveamoisturebalanceovertime.Whenthatbalanceisnotachievedproblemsoccur.Manybuildingdesignsdonotgetsufficientlyscrutinizedtoensurethatthedesignsareappropriatefortheclimatetheyareexpectedtoweather.Thermalbridgesareconductivematerialsthatpartiallyorcompletelypenetratetheinsulatedenclosure.Theheatlossthatresultscandroptemperaturesofinteriorsurfacestolevelsbelowthedewpointoftheindoorairandpromotecondensation

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Buildingenclosuresareoftendesignedwithoutaproperunderstandingoftheperformanceoftheassemblywhensubjectedtotheexteriorweatherandinteriorboundaryconditions.Coderequirementsmayevenimposesolutionsthatareproblematicsuchasrequiringvaporretardersprescriptivelyincircumstancesthatignorelocalconditions.Waterresistivebarriersthatmaybetoovaporpermeableundercertainweatherconditions.Prescriptivecriteriaincodesareslowlybeingimproved,butthesubstitutionofasinglematerialinanassemblycanradicallychangehowtheassemblyperformsovertime.BuildingenclosuresshouldbedesignedbyaknowledgeabledesignprofessionalusingdesigntoolsreferencedintheHandbookofFundamentals(ASHRAE,2009chapter25)1,inordertoavoidthelikelihoodofmoisturerelatedproblems.ControlAirLeakageAirleakageisidentifiedasthebiggestcauseofcondensationinbuildings(CBD23A.J.Wilson,1961).Condensationofmoistureininterstitialcavitiesfromexfiltratingairinnorthernclimates,orfrominfiltratinghothumidairinsouthernclimates,cancausecondensationandmoldgrowththatcanpromotehealthproblemsandprematurebuildingdeterioration.Unlikethemoisturetransportmechanismofdiffusionduetoavaporpressuredifference,airpressuredifferentialscantransporthundredsoftimesmorewatervaporthroughairleaksintheenvelopeoverthesameperiodoftime(Quirouette,1985).Thiswatervaporcancondensewithinthebuildingenclosureinaconcentratedmanner,dependingonthepathway,andtheinternaltemperaturedistributions.Therearethreemajorairpressuresonbuildingsthatcauseinfiltrationandexfiltration:

WindPressure StackPressure(sometimescalledchimneyeffect,orbuoyancy) HVACFanPressure,includingappliancessuchasclothesdriers.

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Figure1CondensationofwatervaporcarriedbyairexfiltratingbetweentheSIPSpanelscausedcondensationandrotofOSBwithinoneyearofconstructionofthisnatatorium.

Figure2Infiltratingwarmhumidairintothisairconditionedplenumcausedcondensationoncoldsurfacesofchilledwaterpipeinsulation.ControlofconvectionAirincontactwithcoolsurfaceswillbecooleddown.Coolairisheavierthanwarmairandwillsinkdown,beingreplacedbywarmeraircarryingmoisture,promotingaccumulationofcondensationonthecoolsurfacesduetoaconvectiveloop.Eliminatingandavoidingvoids(seeFig.4)adjacenttocoolsurfacesiskeytoreducingcondensationduetoconvection.Controllingconvectioncurrentswithinenvelopeassembliescausedbyconnectingaironthecoldsidetoaironthewarmsideofinsulationortheinteriorairbysealingtheinterior(Figure3).Thisisthetypicalmechanismofmoldformationininsulatedbasements,whereairthatisadjacenttoacoolconcretebasementwallcoolsdown,getsheavierandsinks,pullinginwarmhumidairatthetopoftheinsulatedwall.Typicalglassfiberinsulationislowdensityandpromotesconvection;denserinsulationssuchascellulose,rigidboardinsulationsandsprayfoaminsulationscaneliminatethisconvectionwhencarefullyinstalled.

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cold side warm side

spaces allow air movement

cold side warm side

spaces allow air movementspaces allow air movementspaces allow air movement

Figure3Aconvectiveloopdepositscondensationonthecoldsideofthewall

Figure4Airspacesduetoshapeofstudscreatepathwaysforconvectionofair.DesignAirtightnessoftheenclosureIncludeacontinuousairbarriersystem2inthebuildingenclosure:

Selectamaterialineachopaquewall,floorandroofassemblythatmeetsamaximumairpermeanceof0.02L/s.m2@75Pa(0.004cfm/ft2@0.3w.g)andjoinittogetherwithtapes,sealantsetc.,intoanassembly;orselectanassembly(ASTME2357orE1677)withamaximumairpermeanceof0.2L/s.m2@75Pa(0.04cfm/ft2@0.3w.g.)

Jointheairbarrierlayerofeachassemblywiththeairbarrierlayerofadjacentones,andtoallfenestrationanddoors,untilallenclosureassemblies(roof,walls,andbelowgradecomponents)areinterconnectedandsealed.

Sealallpenetrationsoftheairbarrierlayer. Theairtightlayerofeachassemblywillsupporttheentireairpressureofwind,stackeffectand

HVACfans.Ensurethattheairtightlayerisstructurallysupportedandcansupportthe

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maximumpositiveandnegativeairpressuresitwillexperience,withoutrupture,displacementormechanicaldamage.Stressesmustbesafelytransferredtothestructure.

ConvectionAirgapsadjacenttocoolorcoldsurfacescanpromoteconvectionwithinawallassembly.Coldairisheavierandsinks,pullinginwarmhumidairtoreplaceitanddepositmoistureonthecoldsurface.Thisisespeciallytrueinverticalorslopingassemblies.Thecoldersidecanbethesheathingorprecastconcreteincolderclimatesortheinteriordrywallinwarmerclimates.Eliminatingtheairspaceononeorothersideoftheinsulationcanbeeffectiveinpreventingtheseconvectiveloops.Fibrousinsulation,however,whichismostlyair,willalsoallowtheseconvectiveloopstohappen.Denserinsulationsincludingcellulose,rigidboardinsulationwheninstalledinamannertopreventairgaps,orsprayfoaminsulationcanbeespeciallyusefulinavoidingthisproblem.DiffusionAvaporretarderwithappropriatepermeancefortheapplication3shouldbeplacedonthepredominantlyhighvaporpressuresideoftheassembly.Todesignassembliesforappropriatediffusioncontrol,hygrothermalanalysisusingeitherthesteadystatedewpointorGlasermethodsdescribedintheHandbookofFundamentals,(ASHRAE,2009Chapter25)orbyusingamathematicalmodelthatsimulatestransienthygrothermalconditions(theASTMmanual,MoistureAnalysisandCondensationControlinBuildingEnvelopes(Trechsel,2001)reviewsthesemodels).Usersofsuchmethodsmustunderstandtheirlimitations,andinterpretationoftheanalysisresultsshouldbedonebyatrainedpersontoreasonablyextrapolatefieldperformanceapproachingthedesignresults.TheInternationalEnergyAgencyAnnex14(IEA,Hens1991)hasestablishedthatasurfacerelativehumidityof80%averagedoveraperiodof30daysrepresentsareasonablethresholdfordesignerstoachieveasuccessfulbuildingenclosureassemblyfortemperaturesbetween40Fand120F(5Cand50C).Thisisathresholdabovewhichmoldcangrowandbuildingassembliesdeteriorate.WindowandSkylightSelectionFenestrationshouldbeselectedcarefullybydesignerstoavoidcondensation.Fenestrationisselectedtakingintoaccounttheinteriorboundaryconditionsandexteriorweatherconditions;fromachartdevelopedbytheAmericanArchitecturalManufacturersAssociation(AAMA)4,(seeFig.5)theappropriateCondensationResistanceFactor(CRF)forthewindoworskylightisdeterminedbycomparingtheindoorrelativehumidityagainstthe99.6%winterdesigntemperature.

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Figure5CondensationResistanceFactorcurves(AAMA)Fenestrationwithagoodthermalbreak(>1/4thickoflowconductionmaterial)thatminimizetheamountofexteriormetalexposedtocoldusuallyperformbest,andfromacondensationresistanceperspective,mayoutperformnonmetalunits(nonmetalunitsmayhaveimprovedUfactorsthough).Theedgespaceroftheinsulatingglassunitisusuallythemostconductive(andcoldest)locationinawindowassembly.Anewgenerationofwarmedgespacersthatincludethermallybrokenaluminumspacers,stainlesssteelspacersandnonmetallicglassfiberreinforcedplasticspacersareincreasinglybeingusedtoimprovethethermalperformanceoffenestration.InertgasfillssuchasArgonandKrypton,andmultiplelowEfilmsreduceheattransmissionfurtherbyreducingconvectionandradiation.WindowandskylightmanufacturersgenerallycanprovideNationalFenestrationRatingCouncil(NFRC)5simulationsusingthesoftwareTHERMthatshowshowaspecificselectionofwindow,spacerandglass,withvariousgaseousfillwillperform.ItisalsoimportanttonotethatsomenonmetalwindowsthathaveanimprovedUfactormayhaveaworseCRFthanametallicthermallybrokenwindowwithahigherUfactor.CustomdesignsareoftenrequiredtobeverifiedusingtheTHERMandWINDOWsoftware,andvalidatedbyphysicallaboratorytestingaswell.Commonproblems,inadditiontopoorselectionofawindow,andtheirdesign,constructionandinstallationinclude:

1. Poorlocationofthermalbreak,ornothermalbreakatall.2. Thewarmsideofathermallybrokenwindowframeexposedtocoldtemperatures.3. Weepholesthatcommunicatebetweentheindoorandoutdoorenvironmentsresultinair

leakageofcoldairintothewindowframe.

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4. AirLeakageattheinterfaceofthewindowframetotheopaquewallsairbarriercausingcoolingofthewarmsideofathermallybrokenwindow.

Figure6Weepholesconnectinginteriorandexteriorenvironments.Thermalbreaksinpoorlyselectedlocation.

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Figure7Frostonacurtainwallframeduetolackofprimaryairbarrierseal.

BelowGradeWallsandSlabsonandBelowGradeDeepgroundtemperatureinalocaleisnotunliketheaverageannualtemperature,withlocalvariationsduetoshadingfromvegetation,elevationorproximitytothecoast.ComparingtheannualaveragetemperaturewiththeAugustdewpointtemperatureoftheair(seeFig.8)isagoodindicationofwhethercondensationislikelytooccuronslabsandwallsofbelowgradestructures.Concreteishighlyconductiveandwillattainatemperaturesimilartothatofthegroundand,potentially,becomealikelycondensationsurface.Insulatingtheoutsideoftheconcretewallsandfloorsisthebestchoiceforkeepingtheconcreteabovethedewpointoftheindoorair.Intermiteinfestedareas,selectrigidinsulationthathastermiticidesincluded;thatrenderspoisoningthesoilunnecessary.Fullinsulationunderslabswithavaporretarderontopoftheinsulationinintimatecontactwiththeslabisthebeststrategyforadryslab.Selectionofinsulationdensityshouldtakeintoconsiderationmaximumpointloadswithanappropriatefactorofsafetytoavoidcrushing.Insulatingontheinsideofbelowgradewallsispossible,butitisbesttoinsulateusingadheredrigidinsulationsoastoavoidconvectionthroughfibrousinsulation(seeFig9).

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Figure8ComparingalocationsaverageannualtemperaturewiththeAugustdewpointoftheairisa

goodindicationofwhetherbelowgradestructuresmaycausecondensationandmold.

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S.O.G

SEALANT AT ALL CONSTRUCTION JOINTS

TREAT AND SEAL ALL CRACKS & PENETRATIONS

VAPOR BARRIER AND INSULATION KEEPS SLAB WARM AND REDUCES CHANCES OF CONDENSATION

CRUSHED ROCK AND PERFORATED PIPES RELIEVE GAS PRESSURE AND PROTECT BASEMENT S.O.G. FROM MOISTURE TRANSFER

DAMPPROOFING AND DRAINAGE INSULATION RELIEVE GAS PRESSURE AND KEEP BASEMENT WALLS WARM AND DRY.

CONNECT PERIMETER DRAIN AND UNDERDRAIN SYSTEMS TO A GAS-TIGHT PVC PIPE THROUGH ROOF. ADD IN-LINE DRAFT FAN IF NECESSARY

Figure9InsulateoutsidebelowgradestructuresThermalBridgingThermalbridges,duetoconductivematerialsthatpenetrateorinterruptthethermalinsulationlayer,causeadropintemperatureoftheinteriorsurfaceincoldclimates;buildingswithexoskeletons,orbuildingswithexteriorstructuralcomponentsareatypicalexample.Thiscancausecondensationduetocoldertemperaturesandresultinmoldgrowth.Itcanalsocausedepositionofparticulatesontothecoldinteriorsurfacescalledghosting.Inthebuildingshownbelowinacoldclimate,interiorhumidity,candlesmokeandthermalbridgingcombinedtocauseghostingofthesteelstuds;(seeFig.10and11).

Figure10THERManalysisshowingtemperaturevariationcausedbythermalbridgingbysteelstuds

(Anis,2007)

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Figure11Steelstudsloweringtemperatureofinteriorsurfacescausingghosting

Figure12Theseexteriorsteelcolumnsareheatsinks,drawingheatfromthebeamsthatpenetratefromtheconditionedspace.Beamsinsidethebuildingweredrippingwet.

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Figure13Asteelbeamtosteelbeamstructuralthermalbreak,oneofmanydesignsavailable.Structuralmembersthatareoutsidethethermalinsulatedenclosure(seeFig12)canbeheatsinksdissipatingheatintotheenvironmentanddroppinginteriortemperaturesbelowthedewpoint.Smallerthermalbridgesincludingmetalfastenersandfurringmemberscanalsobeaproblem;itisimportanttorememberthatstainlesssteelhasaboutonethirdtheconductivityofcarbonsteelandonetenththatofaluminum.Conclusions:Keepingenclosurecomponenttemperaturesabovethedewpointoftheaircomingincontactwithit,controllingairmovement,vaporpressuresandthermalbridgesinbuildingenclosureassembliesiscriticaltoavoidingcondensationin/onbuildingenclosures.Knowledgeableprofessionalsshouldfollowsoundbuildingscienceprinciplesinthechoiceanddesignofenclosureassembliesfortheapplicationsandclimatesinvolved.References:1HandbookofFundamentals;ASHRAE2009,Chapter252AirBarrierSystemsinBuildings,WholeBuildingDesignGuidewww.wbdg.org;Anis,W.FAIA3InternationalBuildingCode2009,InternationalCodesCouncil4www.aamanet.org5www.nfrc.org