Building Enclosure Fundamentals - Building...

BuildingEnclosureFundamentals:AconciseintroductionDrJohnStraube,P.Eng.UniversityofWaterloo&RDHBuildingScience

Thisshortdocumentintroducesthetechnicalaspectsofbuildingenclosures,witha

focusonhigh-performancewalls.Thefunctionsrequiredofallbuildingenclosures,

commercialandresidential,arepresentedfollowedbyconcisedescriptionsof

controlfunctions.

IntroductionThebuildingenclosureisdefinedasthephysicalcomponentofabuildingthat

separatestheinteriorfromtheexterior:itisanenvironmentalseparator.In

practisethebuildingenclosuremustprovidethe“skin”tothebuilding,i.e.,notjust

separationbutalsothevisiblefaçade.Unlikethesuperstructureortheservice

systemsofbuildings,theenclosureisalwaysseenandisthereforeofcritical

importancetoowners,theoccupants,andthepublic.Theappearanceandoperation

oftheenclosurehaveamajorinfluenceontheinteriorenvironmentandonfactors

suchascomfort,energyefficiency,durability,andoccupantproductivityand

satisfaction.

Todesignafunctionalenclosure,thedesignermustunderstandwhatthe

environmentalconditionsoneithersideoftheenclosureare.Climate,ameasureof

long-termweather,andextremeweatherevents(hurricanes,extremecolddays,

etc.)canbequantifiedandclassified.Twomapsareprovidedbelow(Figure1)for

combinedtemperatureandhumidityandannualrainfall.

©2014-2017JohnStraube Reproductionallowedfornon-profitteachingonly

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Figure 1: Climate and Rainfall Maps

EnclosureFunctionsIngeneralthephysicalfunctionofseparationrequiredofthebuildingenclosure

maybefurthergroupedintothreesub-categoriesasfollows:

1.Supportfunctions,i.e.,tosupport,resist,transferandotherwiseaccommodateallthestructuralformsofloadingimposedbytheinteriorandexteriorenvironments,

bytheenclosure,andbythebuildingitself.Theenclosureorportionsofitcanbean

integralpartofthebuildingsuperstructure,usuallybydesignbutsometimesnot.

2.Controlfunctions,i.e.,tocontrol,block,regulateand/ormoderatealltheloadingsduetotheseparationoftheinteriorandexteriorenvironments.Thislargelymeans

theflowofmass(air,moisture,etc.)andenergy(heat,sound,fire,light,etc.).

3.Finishfunctions,i.e.,tofinishthesurfacesattheinterfaceoftheenclosurewiththeinteriorandexteriorenvironments.Eachofthetwointerfacesmustmeetthe

relevantvisual,aesthetic,wearandtearandotherperformancerequirements.

Afourthbuilding-relatedcategoryoffunctionscanalsobeimposedonthe

enclosure,namely:

4.Distributionfunctions,i.e.,todistributeservicesorutilitiessuchaspower,communication,waterinitsvariousforms,gas,andconditionedair,to,from,and

withintheenclosureitself.


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Almostallenclosuresmustsatisfysupport,control,finishanddistributionfunctions.

However,onlythesupportandcontrolfunctionsmustbeprovidedovertheentiresurfaceoftheenclosure:controlandsupportfunctionsmustcontinueacrossevery

penetration,everyinterface,andeveryassembly.Thelackofthisrequired

continuityisthecauseofthevastmajorityofenclosureperformanceproblems.The

needforfinishanddistributionvariesovertheextentoftheenclosure.Itisrather

unlikelytofindanenclosurethatrequiresafinishontheinteriorandexterior

everywhere.Itisevenmoreunlikelytofindabuildingthatimposesthedistribution

functionontheenclosureoveritsentiresurface.

Thesupportfunctionisofprimaryimportance.Withoutstructuralintegrity,theremainingfunctionsareuseless.However,theindustryhasreachedahighlevelof

understandingandaccomplishmentinthisarea.Supportsystemshaveevolvedfrom

massiveelementspiercedatafewlocationstoefficientprimarystructuralsystems

(suchassteelorconcreteframes)withlightweightframedinfillandsheathing.The

trendtolightweightenclosuresislikelytocontinueasthedemandformore

resource-efficientbuildingsgrows.

Forphysicalperformance,themostcommonrequiredenclosurecontrolfunctionsinclude

• rain,• air,• heat,• vapour,• fire&smoke,• sound,• light(includingview,solarheat,anddaylight),• insects,• particulates,and• access.

Buttherearemanymore.Likesupport,thesecontrolfunctionsarerequired

everywhereforacceptableorgoodperformanceandhencecontinuityofthese

controlfunctions,especiallyatpenetrationsandconnections,iscriticaltoa

successfulenclosure.Themostimportantcontrolfunctionwithrespecttodurability

israincontrolfollowedbyairflowcontrol,thermalcontrol,andvapourcontrol.The

leveloffireandsoundcontrolrequiredvarieswithcoderequirementsandthe

owner’sdesires.


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Unlikethecontrolandsupportfunctions,whichrelyoncontinuitytoachieve

performance,thefinishfunctionisoptional,andmaynotbeneededinsomeareas.Forexample,abovesuspendedceilingsorinserviceorindustrialspaceswherethe

finishisoftendeemedunimportant.Exteriorfinishisoftentermed“cladding”,but

thetermisimprecise,sincecladdingsystemsandmaterialsoftenincludessome

controlfunctions(suchasUVcontrol,solarcontrol,impactresistance,etc.)while

alsodeliveringthefinishfunction.

Theservicedistributionfunction,abuildingfunctionoftenimposedontheenclosure,largelyservicestheadjacentinteriorspacesandonlyneedstobemet

wherethereisaserviceorutilitytobedistributed–largeproportionsofmost

enclosuresdonotneedtofulfillthisbuildingfunction.Thedistributionfunctionof

thebuildinghoweverusuallyimpactsthecontrol-relatedfunctions.Forexample,

serviceentrancespenetratetheentireenclosure,andpipes,ductsandwiresthat

runthroughinsulatedstudspacescanseriouslyreducetheperformanceof

insulationinstalledhere.

Confusionabouttheclassificationofthefunctionalrolesofenclosurecomponents

andmaterialsisfartoocommon,andthisconfusioncancauseseriousperformance

anddurabilityproblems.Forexample,

• vinylwallpaperisoftenappliedasafinish,butinfactfulfillsthecontrol-

relatedfunctionofvapourdiffusioncontrolandactsasalow-permeance

ClassIvapourcontrollayer.Thisunintentionalcontrolofdiffusioncan,and

toooftendoes,createseriousmouldproblemsinair-conditionedbuildings.

• drywallisoftenseenasfulfillingafinishfunction,butinfact,thepaintis

moreoftenthefinishandthedrywalloftenservesasacontrollayerforfire,

sound,andairflow.Ifadesignerorbuilderstopsthedrywallabovea

suspendedceilingbecauseafinishfunctionisnotneededhere,therequired

fire,sound,andairflowcontrolwillalsobemissing.

• athick,self-adhered,bituminousmembraneisoftenusedtodrainwaterand

controlairflowinhigh-performanceassemblies.However,thismembraneis

alsoaverylowpermeancevapourcontrollayer,andlocatingitonthe

outsideofallormostoftheinsulationinacoldclimatecanleadtodamaging

coldweathercondensation.

Fromamorepracticalpointofviewitisusefultodividethefunctionsofthe

enclosureintovarioussub-categoriestowhichwecanassignactualproductsand


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sub-systemassemblies.Withrespecttothecontrolfunction,thesearetermed

controllayers.

Themostimportantandcommonlydefinedenclosurecontrolfunctionlayersare,in

approximateorderofimportance:

1. water/raincontrollayer(e.g.,drainageplane&gaporwaterproofing),

2. airflowcontrollayer(e.g.,anairbarriersystem),

3. thermalcontrollayer(e.g.,insulation,radiantbarriers,etc.),and

4. vapourcontrollayer(e.g.,vapourretarderorvapourbarrierasrequired).

Eachcontrol“layer”maybeasinglematerial,orasub-assemblyofmaterialsthat

togetherprovidethecontroldesired.Inmanycases,twoormoreofthecontrol

functionsareprovidedbyasinglematerialorlayer(e.g.,amembranemayprovide

waterandaircontrol,orsprayfoaminsulationmayprovidevapor,air,andthermal

control).

The“Perfect”EnclosureFigure2showsanidealizedenclosurewiththefourcontrollayerslabelled,along

withcladding,supportandinteriorfinishfunctionlayers.

Figure 2: Idealized enclosure showing the four primary control layers


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Thewater,airandvaporcontrollayersareshownasthinlinestoindicatethatthey

can,inreality,bequitethin(e.g.lessthan1/16”or1mm)andstillperformtheir

functionsverywell.Somematerials,suchasprecastconcrete,canbeusedasa

water,airandvapourcontrollayeronlyiftheyarethicker,e.g.,4”or100mm.

Thesupportandthethermalcontrollayerareshownasthickercomponents,

becauseinpractisetheselayersneedtobethicker(e.g.,wellover1”or25mm)to

performtheirfunction.Dependingonthespanandtoalesserextentthemagnitude

oftheloads(snow,wind,dead,etc.),thesupportstructurewillusuallybeinthe

rangeof3”to8”(75to200mm)forwallswithaspanheightof8’to20’(2.4to6m).

Dependingonitsmaterialproperties,theclimate,andbuildingdesign,thethermal

controllayerwillusuallybe2”to10”thick(50-250mm).

Figure3depictsthespecial,butcommonandpractical,caseofanassemblythat

collapsesthewater,air,andvaporcontrollayersintoasinglephysicalmaterial

(typicallyapolymersuchasaplasticorbituminousmembrane),andprovidesan

optionalservicedistributionspaceandinteriorfinish.

Figure 3: Idealized enclosure with service distribution and finishes

RainControlRaincontrolisthemostimportantpracticalcontrolfunctiontoprovide.Thereis

littleacceptanceorforgivenessforfailure.

Therearethreerecognizeddesignstrategiestocontrolrainpenetrationwithinand

throughtheenclosure(Straube&Burnett1999,CMHC1999):


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1. Drained/Screen2. Mass/Storage3. PerfectBarrier

Thecategorizationdescribedisindependentofmaterialsordesignintentandis

basedsolelyonthemethodbywhichawallsystemcontrolsrainpenetration.

Figure 4: Examples of Drained/Screen Assemblies

Historically,thebestperformanceforwaterpenetrationcontrolhasbeenfrom

drained/screenassemblies.Someexamplesofdrainedwallsystemsincludecavity

walls,brickandstoneveneer,vinylsiding,metalpanels,two-stagejoints,and

drainedEIFS.Screened/drainedwallsassumesomerainwaterwillpenetratethe

outersurface(hencethecladding“screens”rain)andremovethiswaterby

designinganassemblythatprovidesdrainagewithinthewall.

Alldrained(aka“rainscreen”)systemsmusthavefivecomponents:

1. arainscreenorcladding(onethatleaksrain)

2. adrainagegap(oftenaclearairspace),

3. adrainageplane(watercontrollater,awaterrepellentplane,WRB.),

4. flashingatthebasetodirectwateroutwards(waterproof),and

5. drainholes(weepholes)toallowwateroutofthedrainagegap.


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Figure 5: Examples of Mass/Storage Wall Assemblies

Masswallsrequiretheuseofanassemblyofmaterialswithenoughsafestorage

capacityandmoisturetolerancetoabsorballrainwaterthatisnotdrainedor

otherwiseremovedfromtheoutersurface.Theinfiltrationoccursthroughmicro

cracksandporesintheassemblyandisdrivenpredominantlyfromgravityand

capillaryforces.Windisgenerallynotasmuchofafactorduetothesizeofthe

cracksandtherelativemagnitudeofthecapillaryforces.Inafunctionalmassor

storagewall,evaporativedryinganddiffusioneventuallyremovesmoisturebefore

itreachestheinnersurfaceofthewall.

Figure 6: Examples of Perfect Barrier Assemblies


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Perfectbarriers,thethirdtypeofraincontrolstrategy,stopallwaterpenetrationat

asingleplane.Suchperfectcontrolrequiredtheadventofmodernmaterials.Some

examplesofperfectbarrierwallsaresomewindowframes,andsomemetaland

glasscurtainwallsystems.Becauseitisdifficulttobuildandmaintainaperfect

barrierwall,mostwallsaredesignedas,orperformas,imperfectbarrierwall

systemsofeitherthemasstypeorthescreenedtype.However,somesystems,

usuallyfactorybuilt,providewallelementsthatarepracticalperfectbarriers,and

whilethepanelsthemselvescanbeconsideredperfectbarriers,thejointsareoften

susceptibletoinfiltration.

Thejointsbetweenperfectbarrierelementsmaybealsodesignedasperfect

barriers(e.g.asinglelineofcaulking).Suchjointshaveapoorrecordof

performanceandshouldnotbeusedtocontrolrainentry.Alternately,forlarge

panels,suchasprecastpanelsandwindow-walljointsitbecomespracticaltodesign

thejointsdrainedsystems.


Themostimportantjointisoftenthejointbetweenwindows,doors,curtainwalls

andthewallinwhichtheyareinstalled.Thisjointshouldbedesignedasatwo-

stagedrainedjointwithflashingatthebottomtocollectandrejectwateroutward.


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MasonryVeneer,MetalPanel,andEIFSCladdings

Masonryveneer,metalpanelsanddrainedEIFSsystemsareverycommonlyused

drainedassembliesthathavedemonstratedreliableperformanceformanagingrain

water.However,sincetheassembliesaremadeupofmultiplelayersandaresite

constructed,certainelementscanaffecttheperformanceoftheassemblies.

Ithaslongbeenknownthatwaterisabletopenetratethroughmasonryveneersdue

toacombinationofgravity-drivenwaterleakageandcapillaryforces.Thewater

controllayerinadrainedsystemiscalledthedrainageplaneorwaterresistivebarrier(WRB).Itisthelastlayeravailabletoresistfurtherwaterpenetrationintotheassemblyandisintendedtoexcludeallthewatertowhichitisexposed.The

WRBismostcommonlymadeofengineeredbuildingwraps,asphaltimpregnated

buildingpaper,fluidorsheet-appliedmembranes,orwater-resistivesheathingwith

tapedorsealedjoints.Thewaterresistivebarrierneedstobeinstalledwithno

discontinuitiesand/orallseams,joints,andpenetrationsshinglelappedtodrain

liquidwateroutandawayfromtheassembly..

Asignificantamountofforgivenessisbuiltintothesystemduetotheairspace

betweenthebrickandthebackupwallconstruction.Thisairspaceactsacapillary

breakanddrainagegap.Mostoftherainwaterthatpenetratesthroughthecladding

drainsdownthebacksideofthecladdingandoutattheweepholelocationssothe

actualmoistureloadontheWRBisminimalinmostcases.


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AirFlowControlAirmovementthroughanenclosureassemblyisdrivenbyairpressuredifferences

betweentheinteriorandtheexterior.Theseairpressuredifferencesarecausedby

acombinationofwind,stackeffect(naturalbuoyancyofair),andmechanical

pressurization.Thecombinedeffectoftheseforcescanbequitesignificantandlead

toalargevolumeofairmovement.Tocontrolairmovementthroughtheassembly,

acontinuousbarriertoairmovementisrequired(theaircontrollayer).The

locationoftheairbarrierintheassemblyisnotstrictlyimportant(thisisnotthe

caseforconvectiveloopsdiscussedlater).Theairbarriercanbeontheinterior,

exterior,atalocationinthemiddle,ormadeupofanentirecompositeassembly.In

orderfortheairbarriertobeeffectiveitmustbecontinuousinthefieldofthewall

aswellasattheconnectionstoothercomponents(suchasatfoundation,roof,and

windowsanddoors).Thelast30yearsofpracticalfieldexperiencewithairbarrier

performancehasresultedintwooutcomes,difficulttoproveinthelab,but

repeatedlydemonstratedinthefield:airbarriersinstalledtotheexteriorofthe

primarysupportstructureandinteriorpartitionsarefareasiertobuildreliablyair

tight,andfully-adheredorclamped(e.g.notloose)membranesaresuperiorto

loose-appliedmembranes.Alltypesofsystemscanwork,butthosetwolessons

resultineitherhigherperformance,lowerriskoffailure,orlowercostforthesame

performancerelativetootherapproaches.

Figure 9: Air barriers on the exterior of the primary framing are preferred for practical reasons, especially in light-framed wall systems.

Theairbarriercanbeasinglematerialoralayerofmaterialsactingtogetherasa

compositetoresisttheimposedairpressureloadswithoutrupturing.Materials,

suchassheetpolyethyleneortapedfoil-orkraft-paperfacings,whichhistorically

wereintroducedasvaporbarriers,havedemonstratedpoorperformanceasanair

barrier(duetotheirinabilitytoresistairpressureloads).Masonrymaterials(such


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asmasonryblock)donotfunctionasairbarriersbecausetheyaretooairporous,

butthickelastomericpaintscanrendermasonryairtightandprecastconcreteis

denseandexceptionallyairtight.Materialssuchasfluid-appliedorsheet

membranesfully-adheredtoasolidsubstrate(exteriorgypsum,masonry,concrete)

havedemonstratedhighlevelsofperformanceinpractice(duetothestructural

natureofthecombinedmembraneandsolidsubstrate).

Itiscriticalthattheairbarrierbecontinuousfromoneassemblytoanother(roofto

wall,walltofoundation,andwindow/doortowall).Iftheairbarrierisnot

continuous,thenleakageofairwilloccur.Thepathofairmovementwilldependon

thedesignoftheassembliesandthelocationofthediscontinuity.Condensation

withintheassemblywilloccurifmoisture-ladenairmovingthroughtheassembly

comesintocontactwithacoldsurface(surfacetemperaturebelowthedewpoint

temperatureoftheair).Theremustbeairmovement(overallairtightness)and

coldsurfacesalongthepathofflowforinterstitialcondensationproblemstooccur.

Figure 10: A mechanically –attached membrane air-water barrier held tightly to the wall with furring strips provides good performance. Nails or screws through the furring are self-sealing, unlike nails

and screws driven through the thin membrane.


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ThermalControlAcontinuousandeffectivethermalcontrollayerisrequiredtoprovidegood

thermalcomfort,reducetheriskofcondensation,andreduceoperatingenergy

consumption.

Allmaterialsandlayersinabuildingassemblyhavesomeresistancetoheatflow.

However,materialswithanR-valueofabout2/inchormore(k-valuelessthan0.07

W/m∙K)aredeliberatelyusedinbuildingassembliesfortheirabilitytoretardthe

flowofheat.Thesebuildingproductsarecalledthermalinsulations.Threematerial

categories,polymericfoams,mineralfibre(i.e.,glass,slag,orrockwool),andorganic

fiber(e.g.,cellulose,cotton,polyester,etc.),areusedtoproducealmostallcommon

buildinginsulationproducts(Table1).Insulationsareusuallysolidmaterials,but

radiantbarriersthatcontrolonlyradiationheattransferacrossairspacesarealso

sometimesused,especiallyinglazingsystems(thelow-ecoatingsthat

commonplace).

Insulationproductscanbeproducedinatleastfivecommonphysicalforms:loose,

batt,roll,board,andspray(Table2).Eachformhasadvantagesanddisadvantages,

andsomematerialscanbepurchasedinallfiveforms,andsomeonlyinone.

MaterialCategory

Examples Moisture

Fire VaporPermeance

AirPermeance

Mineralfiber Fiberglass,stone,slag Tolerant

Non-combustible high high

OrganicFiber Cellulose,cotton,wool,straw Sensitive

Combustible high high

Plasticfoam Polystyrene,polyurethane,polyisocyanurate

Tolerant

Combustible Low-medium

low

Mineralfoam Foamglass,pumice,airkrete,aerogel

Tolerant

Non-combustible low low

Table 1: Four material categories, with a range of physical attributes, describe most insulation products used in buildings

Highperformancebuildingenclosuresgenerallyrequirecontinuousthermalcontrol

layersontheexteriorofthestructuretoensurecontinuity.Thislayercanbeboard

orsprayfoamaswellassemi-rigidfiberglassorrockwoolinsulation.Iforganic-fiber

productsareused,theycanonlybeusedinsideoftheenclosureassembly’swater

controllayerortheywillbedamagedin-service.

Locatingallinsulationasacontinuouslayerontheexteriorofthestructure

effectivelyeliminateslow-costfibrousbattandloose-fillinsulationsfroma

designer’spalette.Suchlow-densityfibrousinsulationsarestillimportanttofill


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interiorvoidsandcansupplement(neverreplace)theexteriorinsulationlayer,especiallyforwood-framedassemblies.Toensureallspacesandvoidsinframing

cavitiesarefilledproperly(tocontrolconvectiveloops),sprayorblowninsulationis

preferredwhencavityfillsareappropriate.Wheneverairandvapour-permeable

insulationisaddedtoacavity,extracareisrequiredinthelocationandselectionof

thevapourcontrollayerandthelocationoftheaircontrollayer.

Cellularfoamplasticsareoftenusedwhenairimpermeableand/ormoisture

tolerantmaterialsarerequired.Theyarethemostcommontypeofsemi-rigid

insulation.Productssuchasexpandedpolystyrene(EPS),extrudedpolystyrene

(XPS),andfacedpolyisocyanurate(“polyiso”orPIR)havelongbeenusedbehind

claddingsandoutboardofthewatercontrollayer.Polyisoshouldnotbeusedin

applicationswhereitcanbeimmersedinwaterforlongperiodsoftime:thisisnota

concernforabove-gradewalls,butdoeslimititsusebelowgrade.Extruded

polystyreneisthematerialthathasthehighestresistancetowater,andshouldbe

usedwhenrepeatedorextendedwaterimmersionand/orlong-termhighvapour

pressuredrivesareexpectedinservice.

Form Installation LimitstouseLoose pouredorblown maysettle,easilycompressedBatt frictionfit heldinplacebyfriction,easilycompressedRoll frictionfit/mechanicallyattached asforbatts

Board mechanically,adhesivelyattached resistanttomechanicalpressure

Spray sprayinplace stickstoadjoiningsurfaces,resilient

Table 2: Insulations of can take on a wide range of forms, which may limit how they are used.

Expandedpolystyrene(EPS,sometimescalled“beadboard”)isverysimilarto

extrudedpolystyrene(XPS)especiallyathigherdensities,althoughithasalower

thermalresistance,andabsorbsmorewater.Widelyusedoutsidethewatercontrol

layerinabove-gradewallassemblies,itcanalsobeusedveryeffectivelybelow

gradeandhaslongbeenacceptedintheCanadianbuildingcodeforthisuse.

Experiencehasshownthanitcannotbeusedinprotectedmembraneroofs(i.e.,

abovealowsloperoofmembrane)withoutabsorbingtoomuchwater.

Medium-densityclosed-cellspraypolyurethane(ccSPF)isanincreasinglycommon

productthatisspray-appliedtoappropriatesubstrates.Ithasevenbeenusedunder

slabsbysprayingitdirectlytotheearth.Thisproductcanactaspartoftherain,air,

vapour,andheatflowcontrollayersofanassemblyifcareistakentomaintain


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continuityattransitionsandthepotentialcracksduetomovement(duringservice)

orcoolingshrinkage(duringinitialcuring).

Insulationproductsareoftenselectednotjustonthebasisoftheirfireandmoisture

resistance,butalsobecausetheymayassist,ortaketheprimaryrole,incontrolling

airandvapourflow.

Mostrigidfoamboardproducts,especiallythosewithfacers(e.g.thealuminum

faceronpolyisointendedforwalls,facedEPSboard),canactaspartofthewater

and/orvapour,and/orairflowcontrolsolongasotherrequirements,suchas

structuralsupport,durability,andcontinuityatjoints,aremet.


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Material Form UsesVapor

PermeanceAir

Permeance

Fiberglass loose packedintospaces,laidonceilings high high

batt

frictionfitbetweencloselyspacedframing,laidonceilings high high

spray

partiallyfillingirregularlyshapedvolumes high high

semi-rigidboard ductinsulation high high

Stonewool loose

high high

batts asforFG high high

spray

partiallyfillingirregularlyshapedvolumes high high

semi-rigidboard

cavityinsulaton,coveringinteriorsurface,roofs high high

Polyisocyanurate(PIC) board,foil-faced

sheathing,coverssurfaces,cavitywalls,insideroofs,ontopofroof low low

glassorpaper-faced med low

ExtrudedPolystyrene(XPS) board

sheathing,coverssurfaces,cavitywalls,insideroofs,ontopofroof,underslab,wetzones low-med low

faced low low

ExpandedPolystyrene(EPS) board

sheathing,coverssurfaces,cavitywalls,insideroofs,ontopofroof med low

faced low lowCellulose loose packedintospaces,laidonceilings high high

sprayorpacked

partiallyfillingirregularlyshapedvolumes high med

Fiberboard board drysheathingroofingfloors high med

Straw balesself-supportingwhendense,andsupportscladding high med

open-cellspraypolyurethane(ocSPF) spray

partiallyfillingirregularlyshapedvolumes,adheredtoundersidepouredontoceilings high low

closed-cellspraypolyurethane(ccSPF) spray AsocSPF,butmorewaterresistant med low

Table 3: Different Insulation Products, Their Uses, and Air-Vapor Permeance Properties


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Figure 11: Extruded Polystyrene (XPS) insulated used as continuous insulated sheathing for a

house. Note the red tape used to create a continuous water-air control layer

Semi-rigidfibrousinsulationboardscanonlyactasheatflowcontrollayers,

althoughtheycanprovideadrainagepath(i.e.,theyprovideadrainagegapinthe

largeairvoidsbetweenfibers)forrainwater,eitherbydesignorinactuality1.In

manyapplications,air-andvapour-permeablemineral-fibresemi-rigidinsulations

(MFI),suchasfiberglassandrockwoolcanbeusedasexteriorcontinuousinsulation

andhaveaverylongtrackrecordofgoodperformance.Toperforminunsupported

applications,fiberglassproductsgenerallyrequireaminimumdensityintherange

of2.5-4poundspercubicfoot(40-70kg/m3),whereasrockwoolshouldhavea

densityofoverabout3pcf(50kg/m3).SuchMFIproductstendtobelessexpensive,

andarealwaysmorefireresistant,thanfoamedplastics.

1 Semi-rigid mineral fiber insulation (MFI), particularly rockwool, provides a drainage path in the fibers of the outer 1/8” – ¼” (3-6 mm) without materially affecting its thermal resistance. MFI have been used in drained masonry cavity walls with great success for over 50 years in most parts of the world and have been widely deployed as below-grade drainage layers with some insulating value.


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Figure 12: Closed-cell spray polyurethane foam (ccSPF) has been used for decades as a combined

thermal insulation, air barrier material, drainage plane, and vapor control product. Note the extensive use of transition membranes at penetrations.


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Figure 13: Rockwool semi-rigid board insulation used as continuous exterior insulation in an

institutional building over a fluid-applied asphaltic air-water barrier over CMU


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Layer R-value RSI(m2K/W)Emptystudspace0.75”-6”(20-140mm) 0.85–1.4 0.15–0.25m2K/W

Emptyspace(3.5”/90mm) 1 0.18m2K/W

Emptyspace(5.5”/140mm) 1 0.18m2K/W

CMU8”/200mmnormalweight 2.0 0.35m2K/W

Insulationmaterial R-value/inch k(W/mK)

Batt(mineralfiber) 3.4-4.3 0.032-0.042

Extrudedpolystyrene(XPS) 5.0 0.029

Polyisocyanurate(PIC) 5.5-6.0 0.024-0.027

Expandedpolystyrene(EPS) 3.6-4.4 0.033-0.040

Semi-rigidmineralfiber(MFI) 3.6-4.4 0.033-0.040

Sprayfiberglass 3.8-4.3 0.034-0.040

Closed-cellsprayfoam(2pcf)ccSPF 5.8-6.2 0.023-0.025

Open-cellsprayfoam(0.5pcf)ocSPF 3.6 0.040

Aerogel 10 0.015

VacuumInsulatedPanels(VIP) 18-30 0.005-0.008

Table 4: Representative Insulation Material Properties and their R-values

RadiantBarriersandLow-ECoatingsSomethermalcontrolproductsaredesignedtoprimarilycontrolradiationheat

transfer.Radiantbarriersheetscanbeprovidedasasheet,beadheredtosheathing

andinsulationproducts,orbepartofarooforsheathingmembrane.Transparent

metalliccoatingsarecanbeappliedtoanyofthesurfacesofglazing.

Inallcasestheefficacyofradiationcontrolcanbemeasuredintheinfra-redregion

byemissivity.Lowemissivity,low-e,surfacesneitheremitnorabsorbenergy

effectivelyintheinfra-redtemperature(from-40°F/-40°Ctoover200°F/90°C).

Normalbuildingmaterialshaveanemissivityofaround0.90,or90%.Low-e

surfaces,almostalwaysbasedonsomemetalliccompound,haveemissivitiesofwell

below0.20,andthebestcoatingshavevaluesof0.03orevenlower.Alow-e

surfacewillreducetheradiationthattransfersacrossanairspaceregardlessof

whichsideoftheairspaceitisapplied.Applyingalow-ecoatingtobothsideswill

haveonlyaverysmalladditionalbenefit.Inallcases,aradiantbarriermusthavean

airspaceassociatedwithittohaveanybenefittothermalcontrol.


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Themostpowerfulapplicationofradiationcontrolisintheformoftransparent

coatingsonglass,theubiquitouslow-ecoating.Thecoatingisusuallyappliedtoone

ofthesurfacesfacingasealedairgapinaninsulatedglazingunit(IGU).Inthis

location,thesurfaceisprotectedfromcorrosion,condensation,dust,anddirt,and

thereforetheemissivityofthecoatingisprotectedfromchange.Low-ecoatings

increasetheapparentthermalresistanceofa½”(13mm)airgapinasealedIGUby

fromR1toR2.Whenthegapisfilledwithalessconductivegassuchasargon,the

benefitisevenlarger,boostingtheR-valueofadouble-glazedIGUfromaroundR2

toaroundR4.Somenewerglazingsystemsareaddingalow-ecoatingtotheinside

faceoftheglassaswell:thisimprovesR-valueandoccupantradiantcomfortbut

doesresultincolderglasssurfacetemperatures.

Low-emissivitycoatingsorfilmsareoftenappliedtocarriersheets,insulation

boards,orsheathing.Thesecoatingmayinitiallyhaveemissivitiesaslowas0.05,

butthevaluetendstoriseinserviceasdust,corrosion,andagingincreasesthe

emissivity.Somelow-epaintsandcoatingsmayhaveemissivitiesashighas0.30:

thebenefitofthesecoatingsareprettysmall.

Asradiationemissionincreaseswiththefourpowerofabsolutetemperature,

radiationismoreimportantathightemperaturesthanatcoldtemperatures.Hence,

radiantbarriers/low-ecoatingsareslightlymoreimportantathightemperatures

thanlowtemperatures.Asheatiscarriedbyrisingair,heatflowupwardacrossa

horizontalairspaceisdominatedbyconvection,whereasheatflowdownwardis

dominatedbyradiation:theimpactoflow-ecoatingswillbeveryimportantinthe

lattercaseandnottheformer.Allofthesevariedfactorsmeansthereisnoone

correctapparentthermalresistanceforanairspacewithlow-ecoating,oraradiant

barrierproductexposedtoanairspace.

ThermalBridging

Whenheatflowsatamuchhigherratethroughonepartofanassemblythan

another,thetermthermalbridgeisusedtoreflectthefactthattheheathasbridgedover/aroundthethermalinsulation.Thermalbridgesbecomeimportantwhen:

• theycausecoldspotswithinanassemblythatmightcauseperformance(e.g.,

surfacecondensation),durabilityorcomfortproblems.Thisisparticularily

importantinbuildingswithhigherinteriorhumidityduringwinter.

• theyareeitherlargeenoughorintenseenough(highlyconductive)thatthey

affectthetotalheatlossthroughtheenclosure.Thiseffecthasbecomerather


22

significantashigherR-valuesintherestoftheenclosureincreaseinhigh

performancebuildings.

Thermalbridging,especiallybysteelframing,orattheintersectionofwallcorners

withroofsandfloors,projectingstructuralelementslikecantilveredbalconiesand

perimeterconcreteslabsoftencausescoldinteriorsurfacetemperaturesandthus

condensationaswellasunwantedenergylossandgain.Attachedfiguresprovidea

schematicofhowtemperaturesatstudsandnearcornerscancauselowsurface

temperatures.Inthecaseofthesteelframingshown,anexteriortemperatureof–

10ºCcanresultininteriorsurfacetemperaturesof5to10ºCatstuds,andbelow

freezingatfloortowallcorners.

Allenclosuresshouldbedesignedtoavoidalargenumberandextremethermal

bridges.Themosteffectivesolution,exteriorcontinuousinsulation(e.g.,insulating

wallsheathingssuchassemi-rigidstonewoolandrigidfoam),arequiteusefulfor

“blunting”thermalbridgesandalsoofferenergysavingbenefits,andimprove

resistancetoexfiltrationcondensation.Claddingattachmentsandwindowframes

requirethermalbreaks,whereasmoststructuralframingsuchassteelstudsand

beams,concretewallsandframes,andwoodshouldbecoveredwithinsulation.For

conditionssuchasbalconies,andbrickshelfanglesandcanopyprojections,either

structuralthermalbreaksshouldbeusedortheareaofhighlyconductivematerials

penetratingtheinsulationshouldbestrictlylimited.

High-PerformanceEnclosuresThekeytohighperformanceisthatthefourcontrollayersbeprovidedandthat

theybeascontinuousandunbrokenaspossibleacrosspenetrationsandtransitions.

Thedesignandconstructionteammustbeabletoidentifywhichmaterials/sub-

assembliesareprovidingeachofthefourcontrolfunctions.Oncethecontrollayers

ineachenclosurecomponent(e.g.,window,roof,wall)areidentifiedorspecified,

continuityanalysisisconductedbydrawingalineforeachcontrollayeraroundthe

entireenclosurethroughallpenetrationsandtransitions.Anyinterruptioninaline

isadefectthatmustberectified.

Figure14providesanexampleofacommontypeofbuildingenclosurefor

commercialandinstitutionalconstruction:asteel-framedprimarysuperstructure,

withlight-gaugesteelstudinfill,andabrickveneercladding.High-performance

aspectsinclude


23

• acontinuousair-water-vaporcontrollayerappliedtotheexterioroftheprimarystructureandenclosuresupport(toensureitiseasyandpracticalto

achievecontinuity),and

• acontinuouslayerofthermalcontrolontheexterioroftheair-watercontrol

layer,uninterruptedbythermalbridges(especiallythesteelstructure).The

insulationwithintheframingisoptional,andoftennotworththeriskof

moistureproblemsforthesmallperformancegainprovided.

Figure 14: Steel-Framed, steel-stud infill enclosure with brick cladding

Differentcladdings,insulations,andcontrollayermaterialchoicesareminor

relativetotherobusttechnicalandpracticaladvantagesofthelayering.Ofcourse,

differentsolutionswillappeardifferent,e.g.,woodframingislessconcernedwith

thermalbridging,andconcretemasonryinfillwillalwayslocateallofitsthermal

controlontheexterior.


24

CommonWood-FramedAssembliesLoad-bearingwoodframedenclosuresarewidelyusedforlow-risehousing,and

increasinglybeappliedtobuildingsof4storeysandmore.High-risesteel,concrete

frameorevenheavytimberbuildingsof6to30storieshaveorarebeingbuiltwith

infill,non-loadbearingwoodframewalls,oftenprefabricatedpanelshungfromthe

framemuchlikeprecastconcreteorcurtainwallassemblies.

Becausewoodframingisthermallyfarsuperiortosteelframingorconcreteblocks,

itiscommontofillthestudspacewithinsulation.Forhigh-performanceenclosures,

theairandwatercontrollayersshouldinmostcasesbelocatedoutboardofthe

woodframingandsheathing—thisprotectsthemoisture-sensitivewoodandhas

theverysignificantpracticalbenefitofmakingcontinuityatpartitionwalls,floors,

andservicedistributionpenetrationsmucheasiertoachieve.

Acommonandflexibleformofahigh-performancewood-framedenclosure:thecontrollayersareallidentifiedandcontinuous,allmaterialsarewidelyavailable,andconstructionissimple.Increasingthethermalperformanceoftheassemblytomeetdifferentclimatechallengesandenergyperformancetargetsisachievedsimplybychangingthethicknessoftheexteriorinsulation.


25

CommonCommercialAssembliesThecontrollayerscanbeidentifiedinallfunctionalbuildingenclosures:thequality

ofthecontrolmaybemarginal,orthecontrolmaybeprovidedbyacollectionof

materials,butitmustbepresentortheenclosureisnotfunctional.Thefigurebelow

providesexamplesofnumeroussystemsalongwiththeidentifiedcontrollayers.


26

HistoricandAlternateConstructionAlthoughcontrollayersarecommonlydefinedandevenlabeledassuchinmodern

enclosuredesigns,theyareoftenmoredifficulttoidentifyinexistingandhistoric

buildingsandsomealternatemodernapproachestobuilding.Olderbuildingsused

masonryastheprimarywatercontrollayers(i.e.,thestorageormassapproachto

raincontrol)aswellasthesupportfunction(Figure15).Airflowcontrolwasoften

alayerofinteriorplasterorexteriorstucco,althoughsufficientlythickand

impermeablemasonrycouldfulfillthatrole.

Amasonrywallretrofitforimprovedthermalcontroloftenusessprayfoamasboth

athermalcontrollayerandaircontrol,andrequiresanewinteriorfinish/fire

controllayerintheformofgypsumboardonsteelstuds(Figure16).Alternatively,a

higherperformancelower-moisture-riskretrofitthatchangestheexterior

appearancecoulduseEIFSontheexterior,andemptysteelstudsontheinterior

(Figure17).Labelingthecontrollayersactsasadesignqualitycontroltool,aswell

asameansofeffectivecommunicationinconstructiondocumentation.


27

Figure 15: Historic solid masonry wall showing control layers

Figure 16: Interior retrofit of solid masonry wall showing control layers


28

Figure 17: Exterior retrofit of solid masonry wall showing control layers

Oneofthemorecommonmodernalternativesystemsthatisroutinelyandsuccessfullyusedinmodern,higherperformancebuildingsisarchitecturalprecast.Theuseofhighqualityconcreteallowsthesupportandprimaryairandwatercontrolfunctionstobelocatedontheexterior,andstillprovidegoodlong-termdurability.Fordurablehighperformancethechallengeremainstoensurethermalcontrol,avoidingcondensation,andproperjointdetailing.Anexampleofasuccessfuldesignofaprecastconcreteenclosuresystemisshowninthefigurebelow.


29

Figure 18: Examples of Architectural Precast

Numerousothersystemsexistthatmaybemoredifficulttoanalyze.InsulatedConcreteForms(ICF),StructuralInsulatedPanelSystems(SIPS),InsulatedMetalPanels(IMP),etc.However,eachofthesesystemsprovidessome,orallinthecaseofIMPs,ofthesupport,controlandfinishfunctions.

1 ReferencesStraube, J.F. and Burnett, E.F.P., "Rain Control and Design Strategies". J. Of

Thermal Insulation and Building Envelopes, July 1999, pp. 41-56.

Straube, J.F., High Performance Enclosures: Design Guide for Institutional

Commercial and Industrial Buildings in Cold Climates. Building Science Press,

Somerville,Massachusetts,2012. 320 pp.

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