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Building and Environment 40 (2005) 1505–1511

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Performance evaluation of green roof and shading for thermalprotection of buildings

Rakesh Kumar�, S.C. Kaushik

Centre for Energy Studies, Indian Institute of Technology, New Delhi-110016, India

Received 29 July 2003; received in revised form 5 July 2004; accepted 22 November 2004

Abstract

The present paper describes a mathematical model for evaluating cooling potential of green roof and solar thermal shading in

buildings. A control volume approach based on finite difference methods is used to analyze the components of green roof, viz. green

canopy, soil and support layer. Further, these individual decoupled models are integrated using Newton’s iterative algorithm until

the convergence for continuity of interface state variables is achieved. The green roof model is incorporated in the building

simulation code using fast Fourier transform (FFT) techniques in Matlab. The model is validated against the experimental data

from a similar green roof-top garden in Yamuna Nagar (India), and is then used to predict variations in canopy air temperature,

entering heat flux through roof and indoor air temperature. The model is found to be very accurate in predicting green canopy-air

temperature and indoor-air temperature variations (error range 73.3%, 76.1%, respectively). These results are further used to

study thermal performance of green roof combined with solar shading. Cooling potential of green roof is found adequate (3.02 kWh

per day for LAI of 4.5) to maintain an average room air temperature of 25.7 1C. The present model can be easily coupled to different

greenhouse and building simulation codes.

r 2005 Published by Elsevier Ltd.

Keywords: Foliage; Canopy; Green rooftop; Leaf area index; Shading

1. Introduction

Enormous use of ground for various purposes has leadto disappearance of green planted surfaces. In order toprevent dangerous and uncomfortable urban heat islandeffects the indispensable need of planted surfaces is quietinevitable as is confirmed by many researchers viz. [1–5].Space constraints have further reduced the applicability ofgreen surfaces in various areas surrounding the buildingenvelope. Consequently, planted roofs become the onlypromising and stabilizing choice in the present scenario.Good thermal protection can greatly reduce the high

thermal loads that badly affect the comfort conditioning

e front matter r 2005 Published by Elsevier Ltd.

ildenv.2004.11.015

ing author. Tel.: +9111 2659 1253;

9 1266.

esses: krakesh1999@hotmail.com (R. Kumar),

ernet.iitd.in (S.C. Kaushik).

of building during summers. Eumorfopoulou andAravantinos [2] reported that planted roofs contributenot only in reducing the thermal loads on the building’sshell but also in reducing urban heat island effects indensely built areas having a little natural environment.Akbari et al. [6] have described the cooling energypotential of shade trees by reduction of the localambient temperature. For their biological functionssuch as photosynthesis, respiration, transpiration andevaporation, the foliage materials absorb a significantproportion of the solar radiation. Thermal protectiontechniques of green roof can provide a great degree ofreduction in the local air temperature near canopy, thusreducing the incoming heat flux into the building. Astudy done by Onmura [4,7] revealed that in closedspaces with the planted roofs, the air temperaturebeneath the plants is lower than that of the air above,by nearly 4–51C. Thermal performance of green roof

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Nomenclature

d average leaf thicknessDVT vapor diffusivityhaN heat transfer coefficient to airhg convective coefficient of vapor transportHc height of green canopyHs height of soil layerHR thickness of roof supporti nodal pointsL canopy layer thicknesspl leaf vapor pressurepa air vapor pressurepg vapor pressure at soil surface

re mean canopy resistance to sensible heattransfer

T c temperature of canopyTa temperature of airT sky sky temperatureTg ground temperatureT so soil TemperatureTN air temperature

Greek letters

ts ¼ shortwave transmittancett ¼ transmittance of the leaf tissueya air specific humiditycs solar radiationr densitylsu thermal conductivity of support material

Fig. 1. Schematic of green roof model.

R. Kumar, S.C. Kaushik / Building and Environment 40 (2005) 1505–15111506

have been studied in great detail recently by severalresearchers. It is worth mentioning the works onprediction of thermal performance of green roofs byDel Barrio [3] and Good [8]; and on implementation ofgreen roof in the buildings by Dominguez and Lozano[9]; Eumorfopoulou and Aravantinos [2] and Takakuraet al. [10]. Capelli et al. [11] predicted thermal behaviorand effectiveness of vegetation covers with differentaverage absorptance for solar radiation and diffusiveproperties, which shields roof-covering structures ofdifferent masses. In a study conducted by Niachou et al.[12], investigation of green roof is done in two phases; inthe first phase, extensive data measurements fortemperature, both indoor and outdoor are considered,and in the second phase, thermal properties of greenroof are studied using a mathematical approach.Hoyano [13,14] conducted an experimental study oneffect of rooftop lawn planting on thermal environmentand also described for climatological uses of plantsfor solar control and the efforts on the thermalenvironment.Many of the studies predict thermal performance of

green roof localized to experimental site or employseveral numerical techniques to evaluate thermal per-formance. This restricts the applicability of the greenroof to particular buildings and hence thermal spaceconditioning of different building cannot be predicted,as green roof model is to be coupled to the buildingsimulation code. Effect of parametric variations inthermal components of green roof on cooling potentialis also not described. This model improves upon theseaspects by incorporating thermal modeling of green roofcomponents, parametric variations in the green roofcomponents and coupling the model to the buildingsimulation code. The process of heat transfer into theplanted roof is very different from bare roof, both

qualitatively and quantitatively. The analysis of greenroof can be classified into three sub-regions, viz. greencanopy, soil, and the roof support. Dynamic perfor-mance of each of the sub-region can be evaluated andfurther coupled with each other using the boundaryconditions.The objective of studying green roof is multifold: to

determine the effect of variations in foliage character-istics, viz. leaf area index (LAI) and foliage heightthickness on thermal performance of green canopy,estimation of thermal load reduction in the building andevaluation with thermal shading on building spaceconditioning. Results obtained from the simulation ofgreen roof model are validated with experimental datafrom an existing green roof with similar foliagecharacteristics and building parameters in YamunaNagar, Haryana state of India.

2. Mathematical formulation

The green roof top modeling described is based onDel Barrio [3]. Fig. 1 shows the model of green roof

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Fig. 2. Average calculation of LAI for foliage.

R. Kumar, S.C. Kaushik / Building and Environment 40 (2005) 1505–1511 1507

constituting three elements of green canopy, soil and theroof support. A very important parameter for greencanopy is denoted by LAI. It is the ratio of area ofleaves to the area of the base occupied. Fig. 2 shows theaverage LAI for soyabean crop.The governing equation of energy balance for the

green canopy model is

ðrCpÞd � LAIdT c

dt¼ ½1� ts � ð1� tsÞr1�ð1þ tsrgÞcs

þ ð1� tlÞ½sT4sky þ sT4

g � 2sT4c �

� 2LAIrCp

re

ðTc � TaÞ

� 2LAIrCp

gðre þ riÞðpc � paÞ; ð1Þ

where pa is the vapor pressure of the canopy air, pc is thevapor pressure at the soil surface and pN is the outdoorair vapor pressure. Here, the first term denotes net solarradiation absorbed by the canopy. tsrgcs represents netsolar radiation reflected by the ground. The second termdenotes net thermal radiation in long wave as absorbedby the canopy. Tg is the ground temperature, T sky is thesky temperature, re indicates the mean canopyresistance. The third term signifies net convectiveheat transfer from green canopy to air. Finally, thefourth term denotes net transpiration losses from plantto air.Canopy air temperature can be determined from the

following equation:

ðrcÞaLdTa

dt¼ 2LAI

rCp

re

ðT c � TaÞ

þ hgðTg � TaÞ þ ha1ðT1 � TaÞ; ð2Þ

where hg is the convective coefficient of vapor transportand haN is the heat transfer coefficient between canopyair and outdoor air, re is the mean canopy resistance tothe sensible heat transfer, and TN is the outdoor airtemperature.

Energy balance for air specific humidity is given by

raLqyaqt

¼ hgðpg � paÞ � ha1ðpa � p1Þ: (3)

Soil model is governed by the following:

rCpðx; tÞqT soðy; tÞ

qt

¼qqy

lþ LDvT ðo;T soÞ½ �qT soðy; tÞ

qy

� �; ð4Þ

where l is the soil thermal conductivity, x is the localvolumetric moisture content in the porous mediumdomain, L is the latent heat of vaporization and DnT isthe non-isothermal vapor diffusivity.This equation is reduced into algebraic form using

finite difference method. Backward differencing in spaceand time has been used. The roof support is consideredas a homogenous material. The overall one-dimensionalheat conduction equation is taken as

rCp

qT suðy; tÞ

qt¼ lsu

q2T suðy; tÞ

qt2; (5)

where T suðy; tÞrepresents the temperature field, r is thedensity, and lsu is the thermal conductivity of thesupport material.Finite volume method [15] is used to discretize the

governing equation in to algebraic equation. Backwarddifferencing and explicit time-marching scheme is used.Mixed or Robin boundary condition is applied on thebottom of the roof support layer, whereas, specifiedtemperature or Dirichlet boundary condition is appliedat the top of the roof support layer to preservecontinuity.The three decoupled models are then coupled by the

interfaces where the relations between the connectedmodels are described. Newton iteration technique isemployed to determine the interface state variables thatsatisfy the constraints of flux continuity, together withcontinuity of state variables.The room temperature determined from building

simulation code is governed by one-dimensional heatconduction equation. The resultant heat flux from greenroof model is incorporated in the building simulationcode [16] in Matlab using FFT. Solar thermal shading isincorporated in the building simulation code byascertaining the fraction of solar radiation received byvarious building components based on building loca-tion, orientation and shaded portions.A room size of 6� 5� 4 with occupancy of 4 people is

considered for modeling. An air change rate of 5 h�1 istaken for a window area of 2m2. The predicted results ofgreen roof model coupled to building simulation codeare obtained for parametric values shown in Table 1.However, the model is adaptive to include any value

of the parameter specified by the user as per buildingspecification and selected green roof parameters.

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Table 1

Planted roof parameters and properties

Element Effective conductivity (lw) Thickness Specific heat (J/kgK)

Green canopy Varying (on foliage characteristics) 0.4m Variable

Soil 1.21W/mK 840

Sieve layer 0.4W/mK 0.002 1340

Wash off layer 0.035W/mK 0.03 Varying

Reinforced Concrete 1.58W/mK 0.006 880

Roof plaster 0.72 W/m-K 0.018 820

Fig. 3. Schematic of experimental site.

R. Kumar, S.C. Kaushik / Building and Environment 40 (2005) 1505–15111508

3. Results and discussions

The parametric values used in this research forvarious constants are:

Soil thermal diffusivity

2.2e(�4)m2/s Vapor diffusivity 2.e( 6)m2/s Latent heat of evaporation 2257 kJ/kg

The accuracy of the presented model is verified on thebasis of a comprehensive set of experimental data. Fivetemperature and humidity sensors (8160TFF10) sup-plied by Luftopus-200/200I were placed in each room atthe corners and the center of green canopy layer exactlyat the central height of the foliage (0.20, 0.40, 0.60 and1.0m above the soil surface). Five platinum resistancethermometers (PT-500) were used to measure the soiltemperature at a height of 0.10, and 0.20m above theroof surface. Three temperature humidity sensors wereplaced at 0.50, 1.5 and 2.5m above the floor in side theroom. Fig. 3 shows the photo of the experimental sitewith marked circles to indicate the testing points.The performed simulations extend over validation

and parametric study of green roof with cooling energypotential. Fig. 4 shows the roof top garden to which the

modeling simulation results are validated. Validation ofsimulated results with experimental observations isshown in Fig. 5 for canopy air temperature. And it isfound that the results matched experimental data withinan accuracy of73.3%. Further, validation of indoor airtemperature obtained from building simulation code(after incorporating green roof model) is shown in Fig.6. Results matched within an accuracy of 76.1% withexperimental data obtained from 8 check points tomeasure the room temperature.Effect on indoor air temperature is also shown in Fig.

6. It is found that the indoor air temperature is reducedby an average of 7.2 1C. However, the reduction oftemperature follows a pattern with a maximum reduc-tion observed in peak heating period (12:00–15:00 h) anda minimum in off sunshine period.LAI is one of the important parameter affecting the

micro-climate of the green canopy and hence theinteriors of the building. Fig. 7 shows the effect ofparametric variations in LAI on canopy air temperature.A peak reduction of 9.3 1C in canopy air temperature isobserved for a cycle of 8 days from June 1–8. Peakcanopy air temperature and temperature width are bothreduced with increased in LAI as summarized in Fig. 7.Hence, it is observed from the figure that fluctuations ofthe order of 11.6 1C are reduced to only 3.6 1C with anincrease in LAI from 0.5 to 3.5.

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24

26

27

28

29

30

31

48Time (Hrs)

Can

opy

air

tem

pera

ture

(˚C

)

72 96 120

Predicted Canopy air temperature data

10 - 14 June, 2001LAI: 1.5

Experimental measurements data

Fig. 5. Validation for green canopy air temperature.

Fig. 4. Green roof top garden (picture view from side).

100 200 300 400 500 600 700

20

25

30

35

40

Ambient air temperature

Station : Yamuna Nagar, HaryanaTime : 1May to 30 May, 2001

Indoor air temperature (Theoretical)

MeasurementsIn

door

air

tem

pera

ture

(˚C

)

Time (Hrs)

Fig. 6. Prediction and validation of indoor air temperatures for

monthly cycle.

R. Kumar, S.C. Kaushik / Building and Environment 40 (2005) 1505–1511 1509

The long-term dynamic performance of green roof forshielding heat radiations is presented with parametricvariations in foliage thickness and LAI in Figs. 8 and 9.Heating flux entering green roof with a foliage height of0.6m is 1.94Whm�2 (6984 J) and this increasedfurther by nearly 4 times (14,400 J)Whm�2 for the bareroof as seen from Fig. 8. Also, the fluctuations in theentering heat flux are very less for green roof ascompared to bare roof.It is important to determine the heat flux penetrating

the building affected by green roof for various leaf areaindices denoting the extent of coverage of the roof.Internal gains for the room as shown in Fig. 9 are anaverage 3.02 kWh (10872 kJ) (5.18 kWh for bare roof &2.16 kWh for LAI of 4.5). The results show an increasein the entering heat flux with decreased in LAI. Increase

in LAI in general leads to a reduction in coverage areaand hence the insulating effect. However, it also relatesto lessening of biological activities by foliage materials,affecting the micro-climate of the building.Finally, building space conditioning is shown in Fig.

10. Average indoor air temperature of a (6� 5� 4)room is 28.47 1C without green roof and this is reducedby 3.3 1C, leading to an average indoor air temperatureof 25.7 1C. For shading combined with green roof,temperature reduces by 2.1 1C leading to an averageindoor air temperature of 23.6 1C.Diurnal fluctuations in indoor air temperature

show reduced temperature width with green roof.Hence, it is seen from the figure that the temperaturewidth of 10.2 1C in indoor temperature withoutgreen roof is reduced to only 5.1 1C with green roof

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Time (Hrs)

Can

opy

air

tem

pera

ture

(0 C)

24 48 72 96 120 144 168 192

20

25

30

35

1-8 June, 200140 Ambient air temperature

LAI =1.5LAI = 2.5LAI = 3.5

Fig. 7. Effect of various LAI on canopy air temperature.

50 100 150

7

6.5

6

5.5

5

4.5

4

3.5

3

2.5

2

Time (Hrs)

Hea

t flu

x (W

/ m

2 )

bare roofTime : 1 to 8 June, 2001Roof Area : 30 Sq.m.

fht = 0.6fht = 0.5fht = 0.4fht = 0.3fht = 0.2

Fig. 8. Heat flux through roof for various foliage height thickness.

Time (Hrs)

Hea

tflu

x(W

/m2 )

0 24 48 72 96 120 144 168 192-2

0

2

4

6Bare roofLAI = 0.5LAI = 2.5LAI = 3.5

17 - 25 May, 2001Yamuna Nagar

Fig. 9. Total heat flux through planted roof for various leaf area

indices.

50 100 150Time (˚C)

Indo

or a

ir te

mpe

ratu

re (

˚C)

20

22

24

26

28

30

32

34

36

38

40

Ambient air temperatureStation : Yamuna Nagar, HaryanaTime : 1 to 8 June, 2001

Indoor air temperature With bare roof

Green roof

Green roof + Shading

Fig. 10. Indoor air temperature variations (from building simulation

code).

R. Kumar, S.C. Kaushik / Building and Environment 40 (2005) 1505–15111510

which has further dropped to only 2.1 1C while shadingis included.

4. Conclusions

1.

Thermal performance of green roof model is pre-sented with extensive set of validation with experi-mental data for canopy air temperature and indoorair temperature.

2.

Parametric variations in LAI and foliage heightthickness are carried out to determine the modulationof canopy air temperature, and reduction in tem-perature width and to estimate the penetrating heatflux. Results determine that larger LAI reduces thecanopy air temperature, stabilized the fluctuatingvalues and reduced the penetrating flux by nearly4W/m2.

3.

Indoor air conditioning of the building is predicted.Green roof combined with solar thermalshading reduced averaged indoor air temperature by5.1 1C, from the average indoor air temperature forthe bare roof. Green roof provided a coolingpotential of 3.02 kWh per day which is found to beadequate.

Earth covering methods for thermal protection likegreen roofs and solar thermal shading provide a veryeffective solution in building habituation for presentstate of affairs being very energy efficient and ecoresponsive. Results of this investigation are applicable tothe entire building geometrical realm.

ARTICLE IN PRESSR. Kumar, S.C. Kaushik / Building and Environment 40 (2005) 1505–1511 1511

Acknowledgements

The authors are thankful to Ministry of Non-Conventional Energy Sources (MNES) New Delhi, forits financial help in carrying out this research work. Weare also thankful to Indian Meteorological Department(IMD) New Delhi, for providing the Weather Data.

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