six ways for condensation in buildings
DESCRIPTION
methods to prevent condensation in the building enclosureTRANSCRIPT
-
*WagdyAnis,FAIA,LEEDAP,Principal,Wiss,Janney,ElstnerAssociates,Inc.,Boston,[email protected]
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
-
Page2
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.
-
Page3
Figure1CondensationofwatervaporcarriedbyairexfiltratingbetweentheSIPSpanelscausedcondensationandrotofOSBwithinoneyearofconstructionofthisnatatorium.
Figure2Infiltratingwarmhumidairintothisairconditionedplenumcausedcondensationoncoldsurfacesofchilledwaterpipeinsulation.ControlofconvectionAirincontactwithcoolsurfaceswillbecooleddown.Coolairisheavierthanwarmairandwillsinkdown,beingreplacedbywarmeraircarryingmoisture,promotingaccumulationofcondensationonthecoolsurfacesduetoaconvectiveloop.Eliminatingandavoidingvoids(seeFig.4)adjacenttocoolsurfacesiskeytoreducingcondensationduetoconvection.Controllingconvectioncurrentswithinenvelopeassembliescausedbyconnectingaironthecoldsidetoaironthewarmsideofinsulationortheinteriorairbysealingtheinterior(Figure3).Thisisthetypicalmechanismofmoldformationininsulatedbasements,whereairthatisadjacenttoacoolconcretebasementwallcoolsdown,getsheavierandsinks,pullinginwarmhumidairatthetopoftheinsulatedwall.Typicalglassfiberinsulationislowdensityandpromotesconvection;denserinsulationssuchascellulose,rigidboardinsulationsandsprayfoaminsulationscaneliminatethisconvectionwhencarefullyinstalled.
-
Page4
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/[email protected])andjoinittogetherwithtapes,sealantsetc.,intoanassembly;orselectanassembly(ASTME2357orE1677)withamaximumairpermeanceof0.2L/s.m2@75Pa(0.04cfm/[email protected].)
Jointheairbarrierlayerofeachassemblywiththeairbarrierlayerofadjacentones,andtoallfenestrationanddoors,untilallenclosureassemblies(roof,walls,andbelowgradecomponents)areinterconnectedandsealed.
Sealallpenetrationsoftheairbarrierlayer. Theairtightlayerofeachassemblywillsupporttheentireairpressureofwind,stackeffectand
HVACfans.Ensurethattheairtightlayerisstructurallysupportedandcansupportthe
-
Page5
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.
-
Page6
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.
-
Page7
4. AirLeakageattheinterfaceofthewindowframetotheopaquewallsairbarriercausingcoolingofthewarmsideofathermallybrokenwindow.
Figure6Weepholesconnectinginteriorandexteriorenvironments.Thermalbreaksinpoorlyselectedlocation.
-
Page8
Figure7Frostonacurtainwallframeduetolackofprimaryairbarrierseal.
BelowGradeWallsandSlabsonandBelowGradeDeepgroundtemperatureinalocaleisnotunliketheaverageannualtemperature,withlocalvariationsduetoshadingfromvegetation,elevationorproximitytothecoast.ComparingtheannualaveragetemperaturewiththeAugustdewpointtemperatureoftheair(seeFig.8)isagoodindicationofwhethercondensationislikelytooccuronslabsandwallsofbelowgradestructures.Concreteishighlyconductiveandwillattainatemperaturesimilartothatofthegroundand,potentially,becomealikelycondensationsurface.Insulatingtheoutsideoftheconcretewallsandfloorsisthebestchoiceforkeepingtheconcreteabovethedewpointoftheindoorair.Intermiteinfestedareas,selectrigidinsulationthathastermiticidesincluded;thatrenderspoisoningthesoilunnecessary.Fullinsulationunderslabswithavaporretarderontopoftheinsulationinintimatecontactwiththeslabisthebeststrategyforadryslab.Selectionofinsulationdensityshouldtakeintoconsiderationmaximumpointloadswithanappropriatefactorofsafetytoavoidcrushing.Insulatingontheinsideofbelowgradewallsispossible,butitisbesttoinsulateusingadheredrigidinsulationsoastoavoidconvectionthroughfibrousinsulation(seeFig9).
-
Page9
Figure8ComparingalocationsaverageannualtemperaturewiththeAugustdewpointoftheairisa
goodindicationofwhetherbelowgradestructuresmaycausecondensationandmold.
-
Page10
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)
-
Page11
Figure11Steelstudsloweringtemperatureofinteriorsurfacescausingghosting
Figure12Theseexteriorsteelcolumnsareheatsinks,drawingheatfromthebeamsthatpenetratefromtheconditionedspace.Beamsinsidethebuildingweredrippingwet.
-
Page12
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