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Insulation Performance Comparison of Curtain Wall Systems with Existing Pipe Frames and Truss-Shaped Insulation Frames

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Energies
Authors:
Bo-Hye Choi at Ewha Womans University
Bo-Hye Choi
Seung-Yeong Song at Ewha Womans University
Seung-Yeong Song
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Abstract and Figures

The purpose of this study was to compare insulation performance between a base case that applied the existing steel pipe frame and an alternative case that reduced thermal bridging by applying a truss-shaped insulation frame (TIF) to a back frame type curtain wall. Insulation performance was compared by obtaining the effective U-factor and the lowest indoor surface temperature through a three-dimensional steady-state heat transfer simulation. In addition, mock-up tests were performed to compare the U-factors of the base case and alternative case. The simulation results showed that the effective U-factor of the alternative case was 36% lower than in the base case, a significant heat loss reduction. The lowest indoor surface temperature of the alternative case was 0.5 °C higher than in the base case, showing that the surface condensation risk also decreased. In the mock-up test results, the alternative case U-factor was 33% lower than in the base case, confirming the associated large heat loss reduction. For the base case, both the effective U-factor by simulation and the U-factor by the mock-up test were much higher than the design U-factor according to the Korean Design Standard, which neglects thermal bridging, indicating a significantly increased heat loss caused by this factor. For the alternative case, however, both U-factors were similar to the design U-factor.
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Energies2021,14,4682.https://doi.org/10.3390/en14154682www.mdpi.com/journal/energies
Article
InsulationPerformanceComparisonofCurtainWallSystems
withExistingPipeFramesandTruss‐ShapedInsulationFrames
Bo‐HyeChoiandSeung‐YeongSong*
DepartmentofArchitectural&UrbanSystemsEngineering,EwhaWomansUniversity,52Ewhayeodae‐gil,
Seodaemun‐gu,Seoul03760,Korea;bhchoi@ewha.ac.kr
*Correspondence:archssy@ewha.ac.kr;Tel.:+82‐2‐3277‐3913
Abstract:Thepurposeofthisstudywastocompareinsulationperformancebetweenabasecase
thatappliedtheexistingsteelpipeframeandanalternativecasethatreducedthermalbridgingby
applyingatruss‐shapedinsulationframe(TIF)toabackframetypecurtainwall.Insulationperfor‐
mancewascomparedbyobtainingtheeffectiveU‐factorandthelowestindoorsurfacetemperature
throughathree‐dimensionalsteady‐stateheattransfersimulation.Inaddition,mock‐uptestswere
performedtocomparetheU‐factorsofthebasecaseandalternativecase.Thesimulationresults
showedthattheeffectiveU‐factorofthealternativecasewas36%lowerthaninthebasecase,a
significantheatlossreduction.Thelowestindoorsurfacetemperatureofthealternativecasewas
0.5°Chigherthaninthebasecase,showingthatthesurfacecondensationriskalsodecreased.In
themock‐uptestresults,thealternativecaseU‐factorwas33%lowerthaninthebasecase,confirm‐
ingtheassociatedlargeheatlossreduction.Forthebasecase,boththeeffectiveU‐factorbysimula‐
tionandtheU‐factorbythemock‐uptestweremuchhigherthanthedesignU‐factoraccordingto
theKoreanDesignStandard,whichneglectsthermalbridging,indicatingasignificantlyincreased
heatlosscausedbythisfactor.Forthealternativecase,however,bothU‐factorsweresimilartothe
designU‐factor.
Keywords:thermalinsulation;thermalbridge;curtainwall;truss‐shapedinsulationframe
1.Introduction
1.1.BackgroundandObjective
Reducinggreenhousegas(GHG)emissionsisaglobalissue,andtheinternational
communityhasmadeeffortstoreduceGHGemissionsatthegloballevel.TheParis
Agreementsignedin2015dealtwithmeasurestoreduceGHGemissionsafter2020,and
theSouthKoreangovernmentalsoestablishedandannouncedthe“Revisionofthebasic
roadmapforachievingthenationalgreenhousegasreductiontargetin2030[1]”byre‐
flectingtheParisAgreement.Inthecaseofthebuildingsector,whichrepresents22%of
GHGemissionsfromSouthKoreaasof2017,areductiontargetof32.7%comparedtothe
expectedGHGemissionsby2030wasset.Inaddition,thereinforcementofbuildingper‐
mitstandardsfornewbuildings,improvementoftheenergyperformanceoftheexisting
buildings,improvementofequipmentefficiency,andexpansionofthedistributionofre‐
newableenergysystemswerepresentedasthemainmeasurestoreduceGHGemissions.
Accordingly,zeroenergybuildingcertificationhasbecomemandatoryfornewpublic
buildingswithatotalfloorareaof1000m
2
orlargersince2020,andthecertificationwill
bemandatoryforallnewbuildingswithatotalfloorareaof500m
2
orlargerafter2030
[2].
Thermalinsulationofthebuildingenvelopeisaveryimportantelementforzeroen‐
ergybuildings.Thebuildingenvelopethatrequiresthermalinsulationcanbemainlyclas‐
sifiedintotheexternalwall,roofandfloor.Sincetheexternalwallgenerallyrepresents
Citation:Choi,B.‐H.;Song,S.‐Y.
InsulationPerformanceComparison
ofCurtainWallSystemswith
ExistingPipeFramesandTruss‐
ShapedInsulationFrames.Energies
2021,14,4682.https://doi.org/
10.3390/en14154682
AcademicEditors:MonicaSiroux
andFrancescoNocera
Received:25May2021
Accepted:26July2021
Published:2August2021
Publisher’sNote:MDPIstaysneu‐
tralwithregardtojurisdictional
claimsinpublishedmapsandinstitu‐
tionalaffiliations.
Copyright:©2021bytheauthors.Li‐
censeeMDPI,Basel,Switzerland.
Thisarticleisanopenaccessarticle
distributedunderthetermsandcon‐
ditionsoftheCreativeCommonsAt‐
tribution(CCBY)license(http://crea‐
tivecommons.org/licenses/by/4.0/).
Energies2021,14,46822of17
thelargestareainbuildings,theinsulationperformanceoftheexternalwallisparticularly
important.Eventhoughtherearesomedifferencesdependingontheconstruction
method,mostexternalwallshavethermalbridgesinwhichinsulationispartiallydiscon‐
tinued.Thethermalbridgebecomesapaththroughwhichheatistransferredbetweenthe
indoorandoutdoorspace,therebyreducinginsulationperformanceandcausingmany
problems,suchasenergylossandcondensation.Inthecaseofthecurtainwall,inpartic‐
ular,theriskofthermalbridgeoccurrenceishigherthanthatoftheconcretewall,because
metalmaterialswithhighthermalconductivityareappliedandmanyjointsareinevitably
requiredbetweenvariouscomponents[3].Therefore,althoughthecurtainwallsatisfies
thedesignU‐factor(assumingone‐dimensionalheattransferandnotconsideringthein‐
fluenceofthethermalbridge)requiredbytheDesignStandardforEnergy‐EfficientBuild‐
ings[4],itsactualU‐factorishighlylikelytobelargerthanthedesignU‐factor.
Thebackframetypecurtainwall,inwhichexteriorfinishmaterialsareinstalledafter
theinstallationofgrid‐typeorstraight‐typeframesonthebuildingstructureasshownin
Figure1[5,6],hasbeenwidelyappliedasexternalwallsinvarioustypesofbuildingsdue
toitssimpleinstallationandlowcost.Ingeneral,framesaremadeofpaintedorgalva‐
nizedrectangularsteelpipesandfixedtothebuildingstructureusingfasteners(primary
connectors).Exteriorfinishmaterialssuchasmetalorstonearefixedtotheframesusing
brackets(secondaryconnectors).Insulationisinstalledbetweenframes(seeFigure1c)or
betweentheframeandexteriorfinishmaterial.
(a)(b)
(c)
Figure1.Configurationandexampleofbackframetypecurtainwall:(a)configuration[5,6];(b)
exampleofabackframe;(c)exampleofinsulationinstallation.
Kim,S.‐S.etal.[7]reportedthattheinsulationperformanceofmetalmaterialswith
highthermalconductivityisamajorelementthatdeterminestheinsulationperformance
Energies2021,14,46823of17
oftheentirecurtainwallastheinsulationperformanceofthevisionparts,suchaswin‐
dows,inthecurtainwallisreinforced.Song,J.‐H.etal.[8]dividedthebackframetype
curtainwallintopaneltypeandsheettypewallsdependingontheexteriorfinishmateri‐
alsandinsulationinstallationmethod,andevaluatedtheinsulationperformanceofeach
type.Theyreportedthatthejointsbetweenmetalpanelsandthesecondaryconnectorsact
asthermalbridgesforthepaneltype,inwhichmetalpanelscontaininginsulationare
fixedtoframes,andtheframesthemselvesalsobecomethermalbridgesforthesheettype,
inwhichmetalsheetsarefixedtoframesandinsulationisinstalledbetweenframes.Brent
Griffithetal.[9]evaluatedthethermalperformanceofacurtainwallthroughexperiments
andsimulationbyvaryingthespacingandmaterialofbolts.Theyreportedthatthespac‐
ingofboltsaffectedthethermalperformanceofthecurtainwall,andthatthethermal
performancewasreducedwhenthespacingwasnarroworsteelboltswereusedinstead
ofstainless‐steelbolts.Park,H.‐Y.etal.[10]evaluatedtheimpactsofmajormaterialson
theU‐factorofthecurtainwall.TheycomparedtheU‐factorbetweenthecaseinwhich
theinfluenceofscrewsthatplayedastructuralrolewasincludedandthecaseinwhich
theinfluencewasexcluded,andreportedthatthethermalbridgemustbeconsideredfirst
toimprovetheinsulationperformanceofthecurtainwall.TheodorosG.etal.[11]ana‐
lyzedtheeffectoffastenersthatareusedtofixexteriormaterialsandinsulationonthe
insulationperformanceofexternalwall,andreportedthatmeasurestoreducethethermal
bridgearerequired.Oh,J.‐M.etal.[12]replacedaluminummoldingonthesideofmetal
panelsthatareusedasexteriormaterialsinthecurtainwallwithplasticmolding,and
reportedthattheinsertionofathermalbreakerintotheexistingaluminumbracketsignif‐
icantlydecreasesthethermalbridgingeffectatjointsbetweenpanels.
Asintheresearchresultsabove,metalmaterialsthatareinstalledintheinsulation
layerinthecurtainwallarehighlylikelytobecomethermalbridges.Thesteelpipeframe
installedinthebackframetypecurtainwallasshowninFigure1calsobecomesathermal
bridgethatpenetratestheinsulationlayer,anditbecomesamajorfactorthatreducesthe
insulationperformanceoftheentirecurtainwall.Toresolvethisproblem,Shin,D.‐I.[13]
proposedatruss‐shapedinsulationframe(TIF)composedofgalvanizedsteelplates,stain‐
lesssteelwires,andinsulation,andanalyzedtheimprovementeffectofinsulationperfor‐
mance(refertoSection2fordetailsonTIF).Later,Song,J.‐H.etal.[14,15]testedthestruc‐
turaldeflectionandmovement,airleakage,waterpenetration,fireresistanceandthermal
resistancefortheexternalwallsystemtowhichtheTIFproposedbyShin,D.‐I.[13]was
applied,andreportedthatitsatisfiedtheperformancecriteriarequiredbybuildingcodes
andimprovedinsulationperformancethroughareductioninthethermalbridgingeffect.
Inthisstudy,athree‐dimensionalheattransferanalysismodelof1minsizewassetup
tocalculatetheheatingandcoolingenergydemand,andthelinearthermaltransmittance
ofthethermalbridgeontheverticalmemberwascalculated.
ItappearsthattheTIFhashighpotentialforimprovinginsulationperformanceby
reducingthethermalbridgingeffectwhenappliedtothebackframetypecurtainwall.Its
applicationrangeisexpectedtobewideasitisalsoapplicabletoexteriorinsulationand
finishsystemsofconcretewallsandnewconstructionaswellasremodeling.SincetheTIF
systemisintheearlystagesofapplication,itsperformanceevaluation,verificationand
sufficientdataarerequiredundervariousbuildingconditions.Therefore,inthisstudy,
insulationperformancewascomparedbetweencasesthatappliedtheexistingsteelpipe
frameandTIF,byreflectingtheactualmoduledimensionsandmembersofthebackframe
typecurtainwallforofficebuildings.Foranoverallinsulationperformancecomparison
oftheentirecurtainwallsystem,theeffectiveU‐factorthatintegratestheeffectsoflinear
andpointthermalbridgeelementswasderived,andtheimprovementsoninsulationper‐
formancethroughapplicationofTIFwasanalyzedandverified.

Energies2021,14,46824of17
1.2.MethodsandProcedures
AninvestigationintotheTIF,whichwasdevelopedtoreducethethermalbridging
effectcausedbytheexistingsteelpipeframe,wasperformed.UsingPhysibelTriscosoft‐
ware,three‐dimensionalsteady‐stateheattransfersimulationswereperformedtocom‐
paretheinsulationperformancebetweenthebasecasethatappliedthesteelpipeframe
andthealternativecasethatappliedtheTIF.Themoduledimensionsappliedtoactual
officebuildingswereapplied,andmodelingwasperformedincludingthermalbridgeel‐
ementssuchasverticalandhorizontalframesandmetalfasteners.Windowswereex‐
cludedfromthemodelingrangebecauseseparateframesareinstalledandthereisno
significantdifferencebetweenthebasecaseandalternativecase.Theinsulationperfor‐
manceofthealternativecasewascomparedwiththatofthebasecasebasedontheheat
lossthroughtheentireanalysisareaandtheresultingeffectiveU‐factoraswellasthe
lowestindoorsurfacetemperatureandtheresultingtemperaturedifferenceratio(TDR).
Inaddition,mock‐upsofthebasecaseandalternativecaseweremanufacturedforthe
sameanalysisarea,andtheU‐factortestwasconductedinaccordancewithKoreanStand‐
ard(KS)F2278[16]toverifytheinsulationperformanceofthealternativecase.Figure2
showsthemethodologicaloutlineofthisstudy.
Figure2.Methodologyoutlineofthestudy.
2.OverviewofTruss‐ShapedInsulationFrame
TheTIFwasdevelopedtoreducethethermalbridgingeffectcausedbythevertical
framethatactsasthestructuralframeforfixingexteriorfinishmaterialsincurtainwall
systemsorexteriorinsulationandfinishsystemsofconcretewalls[13].AsshowninFig‐
ure3,theTIFconsistsoftopandbottomplateswhicharemanufacturedbyprocessing
galvanizedsteelplates,stainlesssteelwiresthatconnectthetopandbottomplatesina
trussshape,andinsulationthatfillstheemptyspaceinsideframes.Thetopplatehasat‐
bolthole,andthebottomplatehasboltholes.Thismakesiteasytofixtheframetothe
buildingstructureandexteriormaterialstotheframe,andenablesdryconstructionwith‐
outadditionalwelding.Insidetheframe,inorganicinsulationisfilledtosecurefirere‐
sistanceandinsulationperformance.Sinceinsulationisinstalledcontinuously,asshown
inFigure4b,itispossibletoreducethethermalbridgingeffectthatinevitablyoccurswhen
theexistingsteelpipeframeisapplied.
Energies2021,14,46825of17
Figure3.Configurationofthetruss‐shapedinsulationframe(TIF).
(a)(b)
Figure4.InstallationexampleoftheTIFandinsulation:(a)installationoftheTIF;(b)installationof
insulationbetweentheTIFs.
3.SetupoftheBaseCaseandAlternativeCase
Thebuildingtypewassettoanofficebuildingforwhichtherearemanycasesof
applyingcurtainwallsystems,andtheexteriorfinishmaterialwassettothecommonly
usedaluminumsheet.Recently,inKorea,thefiresafetystandards[17]havebeen
strengthenedsignificantly,andnon‐flammablebuildingmaterialsarestronglyrecom‐
mendedforbuildingswiththreeormorefloors.Accordingly,glasswool,whichprovides
fireresistanceandinsulationperformance,wasappliedtotheTIF.Inaddition,mineral
woolwasinstalledintheinterlayerfirepartitionbetweentheinsulationandfloorslab
basedontheactualdesigndrawing.ThedesignU‐factor(U‐factorintheDesignStandard
thatassumesone‐dimensionalheattransferanddoesnotconsiderthethermalbridging
effect)ofthecurtainwallsystemwassettolessthan0.150W/m2K,whichistherequired
U‐factorforSeouldeterminedbytheDesignStandard[4].Seoulcorrespondstozone4in
theclimatezoneclassificationofASHRAEStandard90.1[18].ThedesignU‐factorthat
reflectedtheactualthicknessofeachcomponentofthecurtainwallsystemwas0.145
W/m2K.
Forthebasecase,rectangularsteelpipeframeswithcross‐sectionaldimensionsof
125mm×75mmand100mm×50mmareinstalledverticallyandhorizontally,respec‐
tively,andtwolayersofglasswoolwithatotalthicknessof220mmareinsertedbetween
frames.Ontheoutdoorsideoftheframe,arectangularsteelpipetrackwithcross‐sec‐
tionaldimensionsof50mm×50mmisinstalledtofixtheexteriorfinishmaterial,andthe
aluminumsheetwithathicknessof4mmisinstalledforanexteriorfinishmaterial.There
areaircavitiesbetweenthegypsumboard(aninteriorfinishmaterial)andtheglasswool,
andbetweenthealuminumsheet(anexteriorfinishmaterial)andtheglasswool.
Forthealternativecase,theinteriorandexteriorfinishmaterials,aswellasthetrack
forfixingtheexteriormaterial,arethesameasthoseofthebasecase,andTIFsofthesame
size(125mm×75mm)areinstalledat1mintervalsinsteadofverticalrectangularsteel
pipeframes.Asinthebasecase,twolayersofglasswoolwithatotalthicknessof220mm
areinstalledbetweenTIFs.OntheindoorsideoftheTIFs,anL‐shapedstainless‐steeltrack
Energies2021,14,46826of17
isinstalledhorizontally.TheL‐shapedstainless‐steeltracksupportsinsulationaswiththe
horizontalrectangularsteelpipeframeofthebasecase.SincetheL‐shapedstainless‐steel
trackcanbeathermalbridgeasitisconnectedtotheTIFsandinstalledinsidetheinsula‐
tionlayer,aT‐shapedpolyamideinsulationcapisappliedtooneendoftheL‐shaped
stainless‐steeltrack,asshowninFigure5.
Figure5.L‐shapedstainless‐steeltrackwithpolyamideT‐cap.
4.Three‐DimensionalSteady‐StateHeatTransferSimulation
4.1.OverviewofSimulation
4.1.1.SimulationModel
Thesimulationareaisthenon‐visionpartsofthecurtainwallsystem,anditwas
modeledinthreedimensions,includingtheplenum,ceiling,andfloorslabtowhichthe
frameisfixed.Inthecurtainwallsystem,detailedconditions,suchasspacingofvertical
andhorizontalframeandthelocationofmetalfasteners,mayvarydependingonthe
buildingsituation,butconditionsthataregenerallyappliedtoofficebuildingswereused
inthisstudy.Allelementsthatcanbethermalbridges,suchasframesandmetalfasteners,
wereincludedformodeling,andthinmembraneswithnegligibleinfluenceonheattrans‐
fer,suchasvaporbarriers,wereexcludedfrommodelinginaccordancewithISO10211
[19].
Inthree‐dimensionalmodelingthelocationofcut‐offplaneswassetbyreferringto
ISO10211[19],andthelocationofcut‐offplanesinthex‐axisdirectionwassettobethe
middlepointbetweentheverticalframesasshowninFigure6.Thelocationofcut‐off
planesinthey‐axisdirectionwassettobethepointatleast1mfromtheinteriorfinishing
surfaceofthewall.Thelocationofcut‐offplanesinthez‐axisdirectionwassettothe
middlepointofthealuminumsheet,whichwasatleast1mfromthefinishingsurfaceof
thefloorandtheceiling.
Thethree‐dimensionalmodelsofthebasecaseandalternativecasemodeledthrough
theaboveprocesshadthesamesize(1000mm,1850mm,and4000mminthex‐,y‐,and
z‐axisdirections,respectively).Inaddition,thetwomodelshadthesameoutdoorand
indoorsurfaceareassothattheevaluationresultscouldbecompared.Figures7and8
showthehorizontalandverticalsectionsofthebasecaseandalternativecase.Figure9
showsthethree‐dimensionalgeometricmodelsofthebasecaseandalternativecase.
Energies2021,14,46827of17
Figure6.Locationofcut‐offplanesforsimulation.
(a)
(b)
Figure7.Horizontalsectionsofthesimulationmodels:(a)basecase;(b)alternativecase.
(a)
Energies2021,14,46828of17
(b)
Figure8.Verticalsectionsofthesimulationmodels:(a)basecase;(b)alternativecase.
(a)(b)
Figure9.Three‐dimensionalgeometricmodelsinPhysibelTrisco:(a)basecase;(b)alternativecase.
4.1.2.SimulationMethod
Theinsulationperformanceofthebaseandalternativecaseswasevaluatedthrough
three‐dimensionalsteady‐stateheattransfersimulation.PhysibelTrisco14.0wwasused
astheheattransfersimulationsoftware.PhysibelTriscoiscommercialandmulti‐purpose
Energies2021,14,46829of17
heattransfersimulationsoftwaremadebyPhysibel,whichenablescalculationsprecisely
forthecomplexbuildingelements[20].
Intheheattransfersimulation,boundaryconditionswereappliedinaccordancewith
theDesignStandardforEnergy‐EfficientBuildings[4]andmaterialpropertiesinaccord‐
ancewiththeGuidetoDesignStandardforEnergy‐EfficientBuildings[21]andISO10077‐
2[22].Theoutdoortemperatureintheboundaryconditionsisthedesignoutdoortemper‐
atureofSeoulforheatingequipmentcapacitycalculation.Theheattransferintheaircav‐
itywascalculatedusingtheequivalentthermalconductivity,whichwasobtainedaccord‐
ingtotheconvectiveandradiativeheattransfercoefficients,aircavitygeometry,andheat
flowdirection.Tables1and2showtheboundaryconditionsandmaterialpropertiesfor
thesimulation,respectively.
Table1.Boundaryconditionsforsimulation.
BoundaryTemperature(°C)SurfaceHeatTransferCoefficient(W
/
m2K)
Outdoor−11.323.26
Indoor209.09
Table2.Materialpropertiesforsimulation.
MaterialThermalConduc‐
tivity(W/mK)MaterialThermalConduc‐
tivity(W/mK)
Concrete1.6Steel44
Cementmortar1.4Galvanizedsteel53
Gypsumboard0.18Stainlesssteel15
Glasswool0.034Polyamide 0.25
Mineralwool0.036Siliconesealant0.35
Aluminumsheet200‐ ‐
4.1.3.EvaluationIndicesofInsulationPerformance
Theinsulationperformanceofthealternativecasewascomparedwiththatofthe
basecasebasedontheheatlossthroughtheentireanalysisareaandtheresultingeffective
U‐factoraswellasthelowestindoorsurfacetemperatureandtheresultingTDR.Equa‐
tions(1)and(2)representcalculationformulasfortheeffectiveU‐factorandTDR,respec‐
tively.TDRisthecondensationpreventionperformanceindexdeterminedbytheDesign
StandardforPreventingCondensation[23]inSouthKorea.AlowerTDRvalueismore
favorableforpreventingsurfacecondensation,andthevalueisobtainedbyrounding
downthethirddecimalplace.
Uef
f
𝑞
𝐴󰇛𝑇𝑖𝑇𝑜󰇜
(1)
whereUeff:effectiveU‐factor(W/m2K),q:heatloss(W),A:outdoorsurfacearea(m2),Ti:
indoortemperature(K),To:outdoortemperature(K).
TDR 𝑇𝑖 𝑇𝑠𝑖
𝑇𝑖 𝑇𝑜 (2)
whereTi:indoortemperature(°C),Tsi:indoorsurfacetemperature(°C),To:outdoortem‐
perature(°C).
Energies2021,14,468210of17
4.2.SimulationResults
Table3showstheevaluationresultsofinsulationperformanceofthebasecaseand
alternativecasebysimulation.Forthebasecaseinwhichtherectangularsteelpipeframe
wasapplied,theheatlosswas33.1WandtheresultingeffectiveU‐factorwasfoundtobe
0.264W/m
2
K.ForthealternativecasethatappliedTIF,however,theheatlosswas21.1W
andtheresultingeffectiveU‐factorwasreducedby36%comparedtothebasecaseasit
was0.169W/m
2
K,indicatingasignificantreductioninheatlossbydecreasingthethermal
bridgingeffect.
Forthebasecase,thelowestindoorsurfacetemperaturewas17.7°C,andtheresult‐
ingTDRwas0.07.Forthealternativecase,thelowestindoorsurfacetemperaturewas18.2
°C,whichwas0.5°Chigherthanthatofthebasecase,andtheresultingTDRwas0.05,
indicatingthatthesurfacecondensationriskwasalsoreduced.Thelowestindoorsurface
temperatureoccurredonthesurfaceadjacenttothefastenerinstalledtofixthevertical
membertothestructureforboththebaseandalternativecase.Thisisduetothedropin
surfacetemperatureinneighboringareasaroundthemetalfasteners.
Table3.Evaluationresultsofinsulationperformancebysimulation.
PerformanceIndexBaseCaseAlternativeCase
Temperaturedistribu‐
tion
Outdoor
surface
Indoor
surface
Outdoor
surface
Indoor
surface
Horizontalsection
(atmetalfasteners)
Horizontalsection
(atmetalfasteners)
q(W)33.121.1
U
eff
(W/m
2
K)0.2640.169
Thelowestindoor
surfacetemperature
(°C)
17.718.2
TDR0.070.05
Energies2021,14,468211of17
5.PerformanceVerificationthroughMock‐UpTest
5.1.OverviewofMock‐UpTest
5.1.1.Mock‐UpModel
FortheU‐factortestofthebasecaseandalternativecase,mock‐upsweremanufac‐
turedforthesameanalysisareaasthesimulation.TheU‐factortestmethodforcurtain
wallsystemsisdealtwithinKSF2278[16].ThesizeofthetestspecimensspecifiedinKS
F2278is2m×2m.Accordingly,mock‐upsweremanufacturedbyincludingtwovertical
andhorizontalframes,aswellastheverticalandhorizontalmidpointsofthealuminum
sheet,anexteriorfinishingmaterial,asshowninFigures10–12.Themaximumthickness
oftheinsulationpanelsurroundingthetestspecimen(seeFigure11)installedinagov‐
ernment‐certifiedtestinglaboratory(KoreaLaboratoryAccreditationScheme)inSouth
Koreais400mm,andthemaximumallowedthicknessofthetestspecimensis350mm.
Sincethethicknessofthecurtainwallinsimulationmodelswas419mm,mock‐upswere
manufacturedbyexcludingtheaircavity(100mmthick)betweenthegypsumboardand
glasswoolaswellasthemetalstudinstalledinsidetheaircavityforfixingthegypsum
board.OwingtotheconstraintsofthetestmethodspecifiedbyKSF2278,thefloorstruc‐
turesandprimaryconnectorsincludedinthesimulationmodelswereexcludedforthe
mock‐ups.Mock‐upswerepreparedbyincludingthinmembranesthatwereexcluded
fromthesimulationmodels,suchasvaporretarders.Exceptforthese,theconfiguration
ofthemock‐upswasthesameasthatofthesimulationmodels.Figures11and12show
thehorizontalandverticalsectionsofthemock‐upsforthebasecaseandalternativecase.
Figure10.Locationofcut‐offplanesformock‐uptest.
(a)
Energies2021,14,468212of17
(b)
Figure11.Horizontalsectionsofmock‐ups:(a)basecase;(b)alternativecase.
(a)
Energies2021,14,468213of17
(b)
Figure12.Verticalsectionsofmock‐ups:(a)basecase;(b)alternativecase.
5.1.2.Mock‐UpTestMethod
TheU‐factortestdeviceconsistsofacoldboxcorrespondingtotheoutdoorenviron‐
mentandahotboxcorrespondingtotheindoorenvironment.Thereisaninsulationpanel
intowhichatestspecimenisinsertedbetweenthecoldandhotboxes,andthehotboxis
locatedinsidetheconstanttemperaturechamber.Figure13showsthemock‐upsinstalled
fortheU‐factortest.Thegapbetweentheinsulationpanelandeachmock‐upwasfilled
withpolyurethanefoam.
InKSF2278,thesurfacethermalresistanceisfirstsetbymeasuringtheairtempera‐
tureinthecoldboxandhotboxandthesurfacetemperature(seeFigure14)ofthestand‐
ardinsulationboardinstalledinplaceofthetestspecimenundersteady‐stateconditions,
andthentheU‐factorofthetestspecimenisobtainedbymeasuringtheairtemperature
inthecoldbox,hotbox,andconstanttemperaturechamberaswellastheamountsofheat
suppliedtothecoldandhotboxesundersteady‐stateconditions.TheU‐factortestforthe
basecaseandalternativecasewasconductedinagovernment‐certifiedtestinglaboratory
inSouthKorea.Table4showstheboundaryconditionsappliedtothecoldandhotboxes.
Table4.Boundaryconditionsformock‐uptest.
BoundaryTemperature(°C)SurfaceThermalResistance(m2K/W)
Coldbox0±1.00.05±0.02
Hotbox20.0±1.00.11±0.02
Energies2021,14,468214of17
(a)
(b)
Figure13.Installationofmock‐up:(a)coldboxside;(b)hotboxside.
Figure14.Surfacetemperaturemeasurementpointsonthecoldboxandhotboxsidetopre‐setthe
surfacethermalresistance.
5.2.Mock‐UpTestResults
TheU‐factorbythemock‐uptestwasfoundtobe0.220W/m2Kforthebasecaseand
0.147W/m2Kforthealternativecase.SimilartotheeffectiveU‐factorreductionlevelin
thesimulationresults,theU‐factorofthealternativecasewasreducedby33%compared
tothatofthebasecase,confirmingthesignificantheatlossreductioneffectofthealterna‐
tivecase.
Forboththebasecaseandalternativecase,theU‐factorbythemock‐uptestwas
foundtobeslightlylowerthantheeffectiveU‐factorbysimulation.Thisresultcanbe
attributedtosomedifferencesinconfigurationbetweenthesimulationmodelsandmock‐
ups(refertoSection5.1.1).TheeffectiveU‐factorreductiondegreeofthealternativecase
comparedtothebasecaseinthesimulationresults;however,itwasalmostthesameas
theU‐factorreductiondegreeinthemock‐uptestresults.
Figure15showstheeffectiveU‐factorsbysimulation,U‐factorsbythemock‐uptest,
anddesignU‐factorsaccordingtotheDesignStandard[4],whichdoesnotconsiderthe
influenceofthethermalbridgeundertheassumptionofone‐dimensionalheattransfer,
forthebasecaseandalternativecase.Forthebasecase,boththeeffectiveU‐factorby
simulationandtheU‐factorbythemock‐uptestweremuchhigherthanthedesignU‐
factorof0.145W/m2K,indicatingasignificantincreaseinheatlossduetothethermal
Energies2021,14,468215of17
bridge.ThisissimilartotheresultsofSong,J.‐H.etal.[14,15],whichtestedforinsulation
performanceoftheTIF‐appliedexternalwallsystem.AlthoughitwasdesignedasaU‐
factorthatsatisfiestheDesignStandard,theactualinsulationperformanceintermsofthe
effectiveU‐factorconsideringthethermalbridgeswerefoundtobemuchweaker.So,it
isnecessarytoimproveandsupplementthecurrentDesignStandardsothattheeffectof
thermalbridgescanbereflected.Forthealternativecase,however,bothwerefoundtobe
similartothedesignU‐factor.
Figure15.U‐factorevaluationresults.
6.Conclusions
Thepurposeofthisstudyistocompareinsulationperformancebetweenthebase
case,whichappliedtherectangularsteelpipeframe,andthealternativecase,whichre‐
ducedthethermalbridgingeffectbyapplyingthetruss‐shapedinsulationframe(TIF),by
reflectingtheactualmoduledimensionsandmembersofthebackframetypecurtainwall
forofficebuildings.Theinsulationperformanceofthealternativecasewascomparedwith
thatofthebasecasebyobtainingtheheatlossandtheresultingeffectiveU‐factoraswell
asthelowestindoorsurfacetemperatureandtheresultingtemperaturedifferenceratio
(TDR)throughthree‐dimensionalsteady‐stateheattransfersimulation.Inaddition,
mock‐uptestswereperformedtocomparetheU‐factorsofthebasecaseandalternative
case.Furthermore,mock‐upsweremanufacturedfortestingU‐factorsofthebasecaseand
alternativecase,andtheimprovementsoninsulationperformancethroughapplicationof
TIFwasverified.Themainresultsofthisstudyareasfollows.
(1) ThesimulationresultsshowedthattheeffectiveU‐factorofthealternativecasewas
36%lowerthanthatofthebasecase,indicatingasignificantreductioninheatlossby
decreasingthethermalbridgingeffect.Thelowestindoorsurfacetemperatureofthe
alternativecasewas0.5°Chigherthanthatofthebasecase,showingthatthesurface
condensationriskwasalsoreduced.
(2) Inthemock‐uptestresults,theU‐factorofthealternativecasewas33%lowerthan
thatofthebasecaseinasimilartothedegreeofreductionoftheeffectiveU‐factorin
thesimulationresults,confirmingthelargeheatlossreductioneffectofthealterna‐
tivecase.
(3) Forthebasecase,boththeeffectiveU‐factorbysimulationandtheU‐factorbythe
mock‐uptestweremuchhigherthanthedesignU‐factoraccordingtotheDesign
Standard,whichdoesnotconsidertheinfluenceofthethermalbridge,indicatinga
significantincreaseinheatlosscausedbythethermalbridge.Forthealternativecase,
however,bothwerefoundtobesimilartothedesignU‐factor.
Energies2021,14,468216of17
Inthisstudy,amongtheinorganicinsulationmaterialswidelyusedintheKorean
insulationmarket,glasswoolwithhighinsulationperformancewasapplied,andforfur‐
therstudies,high‐performanceinsulationmaterial,otherthanglasswool,thathavefire
resistanceandinsulationperformancewillbereviewed.Additionally,furtherresearch
willbeconductedtosupplementtheinsulationperformanceandconstructabilityofcur‐
tainwallsystemsthatappliedTIFbyimprovingtheinsulationperformanceofprimary
andsecondaryconnectorsforfixingframesandexteriorfinishmaterials,applyingsuch
exteriormaterialsasmetalpanelscontaininginsulationbesidestheexitingmetalsheetor
stone,andintroducingunitcurtainwallconstructionmethodsinadditiontotheexisting
stickcurtainwallconstructionmethod.
AuthorContributions:Methodology,software,validation,andwriting—originaldraftpreparation,
B.‐H.C.;projectadministration,fundingacquisition,writing—reviewandediting,andsupervision,
S.‐Y.S.Bothauthorshavereadandagreedtothepublishedversionofthemanuscript.
Funding:ThisworkwassupportedbyaKoreaInstituteofEnergyTechnologyEvaluationandPlan‐
ning(KETEP)grantfundedbytheKoreangovernment(MOTIE)(grantnumber20202020800360,
InnovativeEnergyRemodelingTotalTechnologiesfortheAgingPublicBuildings).
InstitutionalReviewBoardStatement:Notapplicable.
InformedConsentStatement:Notapplicable.
DataAvailabilityStatement:Notapplicable.
Acknowledgments:TheauthorsarespeciallygratefultoD.‐I.Shinforprovidingthedetailsofcur‐
tainwallsystemsapplyingTIFandmakingmock‐ups.
ConflictsofInterest:Theauthorsdeclarenoconflictofinterest.
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... In order to calculate the overall heat transfer coefficient (U) for our experimental system, we used the following equation (Equation (2)): here hi represents the convective heat transfer coefficient on the inside of the system, Li and Ki represent the thickness and thermal conductivity of the i-th layer, and h 0 represents the convective heat transfer coefficient on the outside of the system [23]. ...
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Green buildings model: Impact of rigid polyurethane foam on indoor environment and sustainable development in energy sector
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Analysis of Building Energy Savings Potential for Metal Panel Curtain Wall Building by Reducing Thermal Bridges at Joints Between Panels
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Evaluation of Alternatives for Reducing Thermal Bridges in Metal Panel Curtain Wall Systems
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Thermal bridging analysis on cladding systems for building facades
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Joint Ministries of Korean Government. Revision of the Basic Roadmap for Achieving the National Greenhouse Gas Reduction Target in 2030
  • Jan 2018
Joint Ministries of Korean Government. Revision of the Basic Roadmap for Achieving the National Greenhouse Gas Reduction Target in 2030; Joint Ministries of Korean Government: Sejong, Korea, 2018.
Infrastructure and Transport. Press Release, Zero Energy Building, Beyond Buildings to Cities; Korean Ministry of Land
  • Jan 2019
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Ministry of Land, Infrastructure and Transport. Press Release, Zero Energy Building, Beyond Buildings to Cities; Korean Ministry of Land, Infrastructure and Transport: Sejong, Korea, 2019.
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  • Jan 2017
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Ministry of Land, Infrastructure and Transport. Design Standard for Energy-Efficient Buildings; Notification No. 2017-881; Korean Ministry of Land, Infrastructure and Transport: Sejong, Korea, 2017.