IOP Conference Series: Earth and
Environmental Science
PAPER • OPEN ACCESS
The Effect of Plants on Extensive Green Roofs in
Urban Heat Island Mitigation Efforts in Humid
Tropical Cities
To cite this article: Irfandi et al 2021 IOP Conf. Ser.: Earth Environ. Sci. 881 012043
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Published under licence by IOP Publishing Ltd The 5th International Conference on Rebuilding Place
IOP Conf. Series: Earth and Environmental Science881 (2021) 012043
IOP Publishing
doi:10.1088/1755-1315/881/1/012043
1
TheEffectofPlantsonExtensiveGreenRoofsinUrbanHeat
IslandMitigationEffortsinHumidTropicalCities
Irfandi*,AbdulMunir,Muslimsyah,KhairulHuda
ArchitectureandPlanningDepartmentEngineeringFaculty,UniversitasSyiahKuala,
JalanTeukuNyakArief,Darussalam-BandaAceh23111,Indonesia
*Email:[email protected]
Abstract.Oneoftheurbanheatislandmitigationstrategiesinreducingurbantemperaturesin
tropicalcitiesistheapplicationofagreenroofsystem.Thisstudycomparesthereductionin
temperatureandheatflowrateprovidedbythreetypesofplantsonextensivegreenroofs
(EGR).WedemonstratedthataEGRconstructedwiththreetypesofplants(groundcover,and
shrubs)couldresultinadecreaseintemperaturerelativetothenormalroof(NR).Theresults
showedthatthebasetemperatureoftheEGRofthebushandgroundcoverwaslowerthanthe
basetemperatureoftheNRwhichwas10.2ºConindoorair,17.8ºContheinsideand19.1ºC
ontheoutside.Thepeakindoortemperaturewasover50ºCfortheNRprototype.Inthemodel
withpennisetumpurpureumschamachastheEGR,themaximumtemperaturewas40.1ºC,
whileforportulacagrandifloraandtradescantiaspathaceathepeakswere37.6ºCand37.5ºC,
respectively.Thisshowsthatplantswithlargeleafwidthsareabletoreduceheatgreaterthan
plantswithsmallleafwidths.
1.Introduction
Theprocessofurbanizationthatoccursinurbanareashasaninfluenceontheincreaseinpopulation.
Thisurbanizationprocessresultsintheconversionofundevelopedlandintobuilt-uplandwhichhas
animpactonmicroclimatechangeswheretheairtemperatureinurbanareasishigherthanthe
surroundingairtemperature[1].ThisphenomenonisoftencalledUrbanHeatIsland(UHI),whichis
thehighconcentrationofheatinurbanareas,sothetemperatureincitiestendstobehotterthanrural
areas.SeveralfactorsthatcantriggertheoccurrenceofUHIinurbanareasincludethedegreeofheat
ofbuildingmaterials,theheightanddistancebetweenbuildings,andthelevelofairpollution.During
thedaythesefactorscancausemoresolarenergytobecaptured,absorbed,andstoredonurban
surfacesthanonruralsurfaces.Meanwhile,atnight,lessenergyisreleased,resultinginhigherair
temperaturesinurbanareas.Coupledwithlessevaporationthatcausesurbanareastobecomedry,
withreducedcoolness.
Aroundtheworld,differentstrategiesareformulatedtoreduceUHIwithtwoapproachesnamely
increasinggreenareacoverageandthermophysicalproperties,albedo,andemissivityofbuilding
envelopematerialsusedinurbanareas[2]-[3].Heatmitigationstrategiesimplementedintropical
climateareasincludegreeninfrastructuredevelopment,useofshade,modificationofurbangeometric
structures,increaseinalbedoanduseofwaterbodies.Thisbecomesastudyinanefforttoreduceheat
intropicalclimatesandidentifythemosteffectiveheatreductionmitigationmeasures[4].Thecooling
The 5th International Conference on Rebuilding Place
IOP Conf. Series: Earth and Environmental Science881 (2021) 012043
IOP Publishing
doi:10.1088/1755-1315/881/1/012043
2
impactoftheapplicationofgreeninfrastructureinurbanareas,bothintheformoflandscapesinthe
formoftreesinroadcorridors,grass,andcityparksaswellastheapplicationofgreeninfrastructure
onfacadesandroofsofbuildingshasbeenwidelycarriedoutinpreviousstudies[5]-[6]-[7]-[8].The
applicationofagreenroofisoneofthestrategiesintheurbanheatislandmitigationeffortbecauseit
isabletoreduceurbantemperaturesandbuildingtemperatures.Placementofplantsontheroofof
buildingsprovidesmanyecologicalandeconomicbenefits,includingstormwatermanagement,energy
conservation,mitigatingUHIeffectsandextendingthelifeofroofingmaterials,aswellasproviding
anaestheticallybetterenvironment[9].Theapplicationofgreenroofscanreducetheurbanheatisland
effectcausedbyvariouspollutants,improvelivingconditions,beautifytheurbanenvironment,and
improvethequalityofurbanecology[10]-[11]-[12].
Greenroofscanbedividedintothreecategories,namelyextensive(extensive),semi-intensive
(semi-intensive)andintensive(intensive).Thiscategoryisbasedonweight,substratelayerthickness,
maintenance,cost,plantspeciesandirrigationsystem[13].Extensivegreenroof(EGR)requiresa
shallowplantingmedium(soil),theplantsusedarelightornamentalplantsandrelativelylow
maintenancecosts.EGRiswidelyusedinresidentialbuildingsbecausethisgreenroofsystemmodel
hasathinstructurallayerandthesurvivalofvegetationisanimportantaspectthatneedstobe
consideredinrelationtotheselectionofgreenroofs[14].1Researchongreenroofvegetationlayers
hasbecomeahotissueinrecentyearsbyusingsedumplantsintheextensivegreenroofsystem(EGR)
vegetationlayerbecauseofitsefficientuseofwater,toleranceforextremeheatanddrought
conditionscarriedoutondrytropicalandsubtropicalclimatecharacteristics[15]-[16]-[17],butnotall
typesofsedumplantsusedongreenroofsaresuitableforclimaticcharacteristicsinallregions[15].
Selectionofplantspeciesaccordingtoclimaticcharacteristicscanoptimizethegreenroofecosystem
andincreaseenvironmentalbenefits[18].
ThisexperimentalstudywasconductedinBandaAceh,whichhasahumidtropicalclimateby
evaluatingtheextensivegreenroofofthreetypesofvegetationwithdifferentgrowthformsand
biomassstructures.ThecoolingeffectoftheEGRwasinvestigatedwithrespecttotemperature
variationsacrosstheEGRlayer.Thisstudycanprovideinformationaboutthethermalperformanceof
EGRusingvarioustypesofvegetationandalsothethermalperformanceofNR.Thisfindingcanalso
provideascientificbasistosupportthemovementtoimplementextensivegreenroofsinhumid
tropicalcitiesasoneoftheurbanheatislandmitigationstrategies.
2.Materialsandmethods
2.1.Experimentsite
ThisExtensiveGreenRoof(EGP)experimentwasconductedattheSyiahKualaUniversityCampus,
BandaAcehCity,whichislocatedinahumidtropicalclimatewithlatitudeandlongitudezonesof
5°33'27.72"N,95°19'19.92"E.Theweatherisdominatedbyamonsoonmacroclimatewithmonthly
averagetemperaturesforthewetseasonmonthsrangingfrom25°to29°C,andforsummermonths
rangingfrom30°to39°C.AnnualrainfallintheBandaAcehcityarearangesfrom1039to1907
millimeters.Thehumiditylevelinthisareaisbetween70%to80%[19].
2.2.Prototypemodelbuilding
Thetestwascarriedoutbymaking3unitsoftheEGRexperimentalmodelintheformofacubical
testroomwithasizeof600x600x600mmand1unitofanexperimentalmodelintheformofa
concreteroof(normalroof/NR)withasizeof600x600x600mm[20].AllEGRmodelswiththe
samegreenroofsystem,butdifferenttypesofplants.TheEGRmodelismadeofconcretewitha1%
slopeofthefloorslabtowardsthedrainagepipewithadiameterof3/4'(Figure1).
The 5th International Conference on Rebuilding Place
IOP Conf. Series: Earth and Environmental Science881 (2021) 012043
IOP Publishing
doi:10.1088/1755-1315/881/1/012043
3
Figure1.CubicalModelExtensivegreenroofandnormalroof.
2.3.Plantparameter(plantmaterialsandgrowthcondition)
ThetypesofplantsusedinEGRaregroundcoverplantsandlocalshrubsinhumidtropicalclimate
conditionsthatareeasytogrowanddevelop,resistanttoheatandwind,resistanttopollution,require
littlewaterandfertilizer.Threetypesofplantsareusedasagreenroofvegetationlayerconsistingof
Pennisetumpurpureumschamach,Portulacagrandiflora,andRhoeodiscolor.Thesethreetypesof
plantswerechosenwiththeconsiderationthatthesetypesofplantsareeasytogrow,developfast
growth,areresistanttoheat,andareeasilyavailableinthestudyarea.Thethreetypesofplantscanbe
seeninFigure2.
(a) (b) (c)
Figure2.Vegetationofextensivegreenroof,(a)Pennisetumpurpureumschamach;
(b)Portulacagrandiflora;(c)Rhoeodiscolor.
2.4.Experimentaldesign
MeasurementsweremadebyplacingthisEGRandNRtestbuildingprototypeindirectsunlightand
openair.Themeasurementvariableistheresponseofeachprototypemodelofthegreenroofbuilding
toairtemperature,surfacetemperature,plantingmediatemperatureandheatflow.Temperatureis
measuredusingathermocouplesensorconnectedtoadataloggertoreadandrecorddataduringthe
measurementprocessandheatflowisalsomeasuredusingaheatfluxsensorattachedtothebottomof
theconcreteplateofthetestobject.Datarecordingwascarriedoutin10secondintervalswhichlasted
for3daysfrom9.00amto16.30pm.Measurementsweremadeinsunnyweatherconditionswith
sunlightnotcoveredbyclouds.Figure3andTable1detailsofmeasurementitemsandmeasurement
points
The 5th International Conference on Rebuilding Place
IOP Conf. Series: Earth and Environmental Science881 (2021) 012043
IOP Publishing
doi:10.1088/1755-1315/881/1/012043
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Figure3.Extensivegreenrooflayerstructureandmeasurementpointsfor
temperature(thermocouple)andheatflow(heatfluxsensor).
Table1.Measurementitemsandinstrument.
Instrumen MeasurementVariable
Thermocouple(Ttype,0.3mm
indiameter)
Ambientairtemperature
Surfacemediumtemperature,buriedabout0.5cminmedium
surface
Mediumtemperature,buriedinmidofmedium
Surfacetemperatureconcretepanel(insideandoutside),pastedthe
sensorsonthesurfaceusingaluminum
Indoorairtemperature
HeatSensor(P01e,FluxTreq
Instrument)
HeatFlux.,pastedthesensorsonthesurfaceusingaluminum
Datalogger(GL800,Graphtec
Instrument)
DataAcquisition
3.Resultsanddiscussion
3.1.Extensivegreenrooftemperatureprofilecomparison
Basedonthemeasurementresults,itcanbeseenthatthemaximumindoorairtemperatureattheEGR
forallmeasurementdaysis37.9ºC,36.3ºC,and35.2ºC,whileatNRitreaches50.4ºC,47.3ºC.and
42.2ºC.ThereisatemperaturedifferencebetweenEGRandNRof10.2ºC.Themaximumexternal
surfaceNRtemperatureforallmeasurementdaysrespectivelyreached58.4ºC,54.8ºC,and47.0ºC.
Theconcretesurfaceabsorbssolarenergyfromdirectsunlightandistransferredtothespaceunderthe
roof.Thereisnosignificantdifferenceintemperaturebetweentheexternalsurfaceandinternal
surfaceontheconcreteroofwherethevariationislessthan1ºC.Theinternalsurfacetemperaturesare
57.7ºC,53.9ºC,and46.4ºC.Ontheotherhand,thelowestpeaktemperatureswereforexternal
concretepanels(undergrowingmedia)ofthegreenroofmodel,being35.4ºC,34.1ºC,and33.4ºCfor
alldaysrespectively(Table2).Thegreenrooflayersignificantlyreducestheheatgainedbythe
concretepanels.
The 5th International Conference on Rebuilding Place
IOP Conf. Series: Earth and Environmental Science881 (2021) 012043
IOP Publishing
doi:10.1088/1755-1315/881/1/012043
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Table2.Maximumtemperatureonextensivegreenroofandnormalroof.
Temperature
NormalRoof
(ºC)
ExtensiveGreenRoof
(ºC)
Different
(ºC)
Day1Day2Day3Day1Day2Day3
Indoorair 50,4 47,3 42,2 37,9 36,3 35,2 10,2
Internalsurface 57,7 53,9 46,7 36,3 34,7 34,0 17,8
Ekternalsurface 58,4 54,8 47,0 35,4 34,1 33,4 19,1
ComparedtotheNRmodel,theinternalsurfacetemperatureoftheEGRmodelismuchsmallerat
itspeakwithanaveragetemperaturedifferenceof17.8ºC.Ontheotherhand,theindoortemperature
ishigherthanthesurfacetemperatureoftheconcreteslabduringthedayandloweratnight.This
indicatesthatthegreenroofdoesnottransmitheattotheroomduringthedayandreleasesheatinto
theroomatnight.Theseresultsindicatethatmostofthesolarenergyisabsorbedandreleasedback
intotheenvironmentbythegreenroofsystemandonlyasmallamountofsolarenergyisstoredand
transmittedintotheroom.
3.2.Thermalperformanceofextensivegreenroof
Thefirststageinvolvesinvestigatingtheroomtemperatureandroofsurfacetemperaturewithand
withoutvegetation.ThemechanismofsolarradiationheatreductionbyEGRisinfluencedbyplant
characteristicsrelatedtotheabilitytoabsorbandtransmitheatintotheroom.Inthisstudy,threetypes
ofplantswereused,namelyPennisetumpurpureum,Portulacagrandiflora,andTradescantia
spathacea,asshowninFigure4,eachplanthasdifferentcharacteristics.
Figure4.RoomtemperatureontheEGRprototypeforvarioustypesofplants
androomtemperatureontheNRprototype.
Themaximumindoortemperaturewasover50ºCfortheNRprototype.IntheEGRmodelwith
Pennisetumpurpureumthemaximumtemperatureis40.1ºC,whileforPortulacagrandifloraand
Tradescantiaspathaceathepeakpointsare37.6ºCand37.5ºC,respectively.Asimilartrendwasalso
shownbythesurfacetemperatureoftheinnerpanelwiththehottestorderbeingNR,EGRwith
Pennisetumpurpureum,EGRwithPortulacagrandiflora,andEGRwithTradescantiaspathacea.
Thissequenceisconsistentlyshownbyothertemperatureparameters,namelythetemperatureofthe
The 5th International Conference on Rebuilding Place
IOP Conf. Series: Earth and Environmental Science881 (2021) 012043
IOP Publishing
doi:10.1088/1755-1315/881/1/012043
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soilsurface(surfaceofthegrowingmedia)andthetemperatureintheplantmedia.Thisshowsthatthe
amountofheatradiationreceivedbytheroofsystemisinfluencedbythetypeofplantwiththeir
respectivecharacteristics.Ingeneral,itcanbedescribedthattheplantlayerssequentiallyfromthinto
thickarePennisetumpurpureum,Portulacagrandiflora,andTradescantiaspathaceaplants.Thisisin
linewiththerateofheatgainofeachofthesemodels.Itcanbeconcludedthatthethicknessofthe
plantlayeraffectstheradiationheatresistanceofthegreenroofsystem.Theperformanceofgreen
roofssignificantlyreducesheatgaininprototypebuildingswhichisinlinewiththeadvantagesof
usinggreenroofsputforwardbyvariousliteratures,especiallyincreasingthethermalperformanceof
buildingsintropicalclimates.
Figure5.SurfacetemperatureoftheinnerpanelonEGRandNRprototype.
Figure6.TherateofheatflowthroughtheroofpanelsintotheEGRandNRprototypechambers.
Figure6showsthemechanismofheattransferfromthepanelsurfaceintotheroomforeach
buildingprototypebasedonthedifferencebetweenthesurfacetemperatureandtheindoor
temperature.Thetemperaturedifferenceisdirectlyproportionaltotheamountofheattransferthat
occursinaunitoftime.Thedifferenceinpanelsurfacetemperatureandroomtemperaturewherethe
positivedifferenceduringthedayonlyoccursfortheNRprototype.IntheEGRprototype,the
temperaturedifferenceisnegativeduringtheday.Thiscanbeinterpretedthatthereisheattransfer
The 5th International Conference on Rebuilding Place
IOP Conf. Series: Earth and Environmental Science881 (2021) 012043
IOP Publishing
doi:10.1088/1755-1315/881/1/012043
7
fromtheroomtotheroofpanelbecausethesurfacetemperatureoftheinnerroofpanelislowerthan
theroomtemperature.Atnight,theroofpanelwithoutplants,thetemperaturedifferencedecreases
rapidlyuntilitreachesanegativevalue.Thisindicatesthattheconcreteroofpanelhastheabilityto
storelittleheat.Ontheotherhand,inthegreenroofsystem,atnightthereisapositivetrendwherethe
surfacetemperatureofthepanelsishigherthantheroomtemperature.Thisisbecausetheplanting
mediumhasabetterheatstoragecapacitythanconcrete.Theheatfluxissignificantlyreduceddueto
theevapotranspiration,shading,protective,photosyntheticandreflectiveeffectsofthevegetation
layeraswellastheinsulation,absorptionandevaporationeffectsofthegreenroofgrowingmedia
layer.
4.Conclusions
Thermaltestresultsshowthattheextensivegreenroofsignificantlyreducestheheatloadfromsolar
radiationduringtheday.extensivegreenroofshaveagreaterheatcapacityandwillreleaseheat
slowlyatnight,whileduringthedayonlyasmallamountofheatistransferredfromtheroofintothe
room,whichisfavorableconditionsfortropicalareaswithhighsolarintensitythroughouttheyear.
Furtherstudiesarestillneededforthepracticalapplicationofroofsystemssuchasinter-panel
connectionsystems,watersupplysystems,greenroofdrainagesystems,andvariousothertechnical
aspects.
Acknowledgment
TheauthorwouldliketothanktheDirectorateGeneralHigherEducation,MinistryofNational
Education,IndonesiaforitsfinancialsupportthroughtheSyiahKualaUniversityResearchand
ServiceInstitute.
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