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Building and Environment 45 (2010) 663–672

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Building and Environment

journal homepage: www.elsevier .com/locate/bui ldenv

Thermal evaluation of vertical greenery systems for building walls

Nyuk Hien Wong a, Alex Yong Kwang Tan a,*, Yu Chen a, Kannagi Sekar a,b, Puay Yok Tan b,Derek Chan b, Kelly Chiang b, Ngian Chung Wong c

a Department of Building, School of Design and Environment, National University of Singapore, 4 Architecture Drive, Singapore 117566, Singaporeb National Parks Board, Singapore Botanic Garden, 1 Cluny Road, Singapore 259569, Singaporec Building and Construction Authority, 5 Maxwell Road, Singapore 069110, Singapore

a r t i c l e i n f o

Article history:Received 27 April 2009Received in revised form3 August 2009Accepted 5 August 2009

Keywords:Vertical greenery systemsThermal environmentsSurface temperatureAmbient temperature

* Corresponding author. Tel.: þ65 6516 5845; fax: þE-mail address: alextanyk@alumni.nus.edu.sg (Ale

0360-1323/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.buildenv.2009.08.005

a b s t r a c t

This research involves the study of 8 different vertical greenery systems (VGSs) installed in HortPark toevaluate the thermal impacts on the performance of buildings and their immediate environment basedon the surface and ambient temperatures. VGSs 3 and 4 have the best cooling efficiency according to themaximum temperature reduction of the wall and substrate surfaces. These results point to the potentialthermal benefits of vertical greenery systems in reducing the surface temperature of buildings facades inthe tropical climate, leading to a reduction in the cooling load and energy cost. In terms of the lowestdiurnal range of average wall surface temperature fluctuation, VGSs 4 and 1 show the highest capacities.No vertical greenery system performs well in term of the diurnal range of average substrate temperaturefluctuation. By limiting the diurnal fluctuation of wall surface temperatures, the lifespan of buildingfacades is prolonged, slowing down wear and tear as well as savings in maintenance cost and thereplacement of façade parts. The effects of vertical greenery systems on ambient temperature are foundto depend on specific vertical greenery systems. VGS 2 has hardly any effect on the ambient temperaturewhile the effects of VGS 4 are felt as far as 0.60 m away. Given the preponderance of wall facades in thebuilt environment, the use of vertical greenery systems to cool the ambient temperature in buildingcanyons is promising. Furthermore, air intakes of air-conditioning at a cooler ambient temperaturetranslate into saving in energy cooling load.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The unstoppable force of urbanization is consuming vastquantities of natural vegetation, replacing them with concretebuildings and low albedo surfaces. These resulting changes in thethermal properties of surface materials and the lack of evapo-transpiration in urban areas lead to a phenomenon known as theurban heat island (UHI) effect.

With the idea of introducing nature back into the urban land-scape, a partnership is strengthening between nature and the citywith the aim to create a new sustainable urban lifestyle. Greenery isthe key element of this transformation. Since the outer surfaces ofbuilding offer a great amount of space for vegetation in urban cities,planting on roofs and walls has became one of the most innovativeand rapidly developing fields in the worlds of ecology, horticultureand the built environment.

65 6775 5502.x Y.K. Tan).

All rights reserved.

The greening of the façade of building walls, known as verticalgreenery systems (VGSs), has yet to be fully explored and exploited.Simply due to the sheer amount of building walls, the widespreaduse of vertical greenery systems not only represents a greatpotential in mitigating the UHI effect through evapotranspirationand shading, it is also a highly impactful way of transforming theurban landscape.

Vertical greenery systems have been the features of architec-tures for centuries where it is a common practice to grow climberson the exterior walls of buildings. The technical idea of verticalgreenery systems was based on the fact that certain plant likeorchids do not depend on soil and can be applied to urban settings.Nevertheless, vertical greenery systems are still a relatively newdiscipline. In Germany, a good research base has been developedover the last twenty years on the environmental performance ofvertical greenery systems where regulations and guidelines havebeen published [1]. However, much of the technical and researchliterature are mostly unknown or published in German.

This research involves the study of 8 different vertical greenerysystems installed in HortPark, Singapore with the objective of

N.H. Wong et al. / Building and Environment 45 (2010) 663–672664

evaluating the thermal impacts of various vertical greenery systemson the performance of buildings and their immediate environmentbased on the surface and ambient temperatures. This is a projectinitiated by the Centre for Urban Greenery and Ecology (CUGE) ofthe National Parks Board (NParks), in collaboration with theBuilding and Construction Authority (BCA) and the NationalUniversity of Singapore (NUS).

2. Literature review

Although there are significant published articles on urbangreenery, most of them generally focus on the benefits and effectsof vegetation on the urban climate and buildings, with dominantemphasis on rooftop gardens rather than vertical greenery systems.

Vegetation can play an important role in the topoclimate oftowns and the microclimate of buildings. With buildings, somevegetative climatic effects could be made by combining green coveron walls, roofs and open spaces in the vicinity of buildings [2].Although there are many benefits in reintroducing vegetation tothe surfaces of urban buildings and their related spaces, manytechnical problems are faced during implementation [3].

In a research report by the Canadian Mortgage and HousingCorporation, a comprehensive review of the quantitative andqualitative benefits of vertical greenery systems is discussed. Themajor barriers to the more rapid diffusion of these ‘‘sustainabledevelopment’’ technologies were also described and a number ofinitiatives were proposed [4]. In another Canadian research, Bass[5] studied the potential of rooftop gardens and vertical greenerysystems in an urban environment. Both technologies reducedsurface temperatures sufficiently to suggest that considerablereductions of the UHI effects would be possible if they wereemployed on a widespread basis.

In a research project aimed at defining the thermal performanceof double skin façade covered with plants, a simulation model wasdeveloped to analyze the influence of plants on the performance ofthe double skin façade. Further simulations of the entire buildingproved that plants can contribute to a comfortable indoor climateand energy savings [6].

In the University of Brighton, research on the shading perfor-mance of climbing plant canopies, measurement of area coverageand solar transmittances of different leaf layers as well as comingup with a novel technique to provide an assessment of the shadingperformance of a climbing plant canopy were carried out [7].

Lambertini [8] presented a pictorial collection of the mostimportant architectural projects that embraced the emerging trendof designing and cultivating once inconceivable greenery ona vertical plane while Dunnett [9] cited the associated benefits andreasons for integrating green techniques of organic architectureinto our built environment as well as provided a massive collectionof appropriate plant information and extensive plant directories forboth rooftop gardens and vertical greenery systems.

The Green Walls Group, a sub-committee of the Green Roofs forHealthy Cities in North America had produced an introduction ofvertical greenery systems, citing the benefits, factors for successfulimplementation, maintenance issues, policies and LEED certifica-tion [10].

Centre for Subtropical Design in the Queensland University ofTechnology in Brisbane, Australia is currently embarking on anextensive research programme on living wall systems. Theirresearch project aims to identify the benefits of living wall systemand to comprehend the challenges in successfully introducing themin the built environment of subtropical Queensland [11].

Lastly, Van Bohemen [12] showed within an ecological engi-neering context the impact of the greening of outdoor walls andquestioned the hesitation to implement vertical greenery systems

as outer layer of buildings with special emphasis on the relation-ship between particulate matter and aerosol deposition withvegetation.

2.1. Thermal benefits – temperature reduction

Research showed that the humid climates of Hong Kong canachieve substantial benefits of a maximum temperature decrease of8.4 �C with vertical greenery systems in an urban canyon [13]. Thisis significant as the distribution of ambient air in a canyon influ-ences the energy consumption of buildings as higher temperaturesin canyon increase heat convection to a building and correspond-ingly increases the cooling load [14].

It was also noted that vegetation can alleviate UHI directly byshading heat-absorbing surfaces and through evapotranspirationcooling [15]. Vegetation can dramatically reduce the maximumtemperatures of a building by shading walls from the sun, withdaily temperature fluctuation being reduced by as much as 50% [9].Through evapotranspiration, large amounts of solar radiation canbe converted into latent heat which does not cause temperature torise. In addition, a façade fully covered by greenery is protectedfrom intense solar radiation in summer and can reflect or absorb inits leaf cover between 40% and 80% of the received radiation,depending on the amount and type of greenery [16].

Moreover, surface temperatures of vertical greenery systemshave been observed in different settings at the University of Tor-onto since 1996 [5]. These results have consistently demonstratedthat areas with vertical vegetation are cooler than light-colouredbricks, walls and black surfaces that are typically found in urbanareas. Lastly, in Japan, experiments show that vines can reduce thetemperature of a veranda with south-western exposure [17]. InAfrica, a temperature reduction of 2.6 �C was observed behindvegetated panels of vines [18].

Therefore, together with the insulation effect of vegetation,temperature fluctuations at the wall surface can be reduced frombetween 10 �C and 60 �C to between 5 �C and 30 �C [4].

2.2. Thermal benefits – shading and insulation

In an experimental investigation of the effect of shading build-ings walls with plants, it is suggested that more thermal energyflows into the non-shaded walls due to direct exposure to the sunand resulted in higher surface wall temperature. The energyabsorbed will advance into the inner wall surfaces, resulting inelevation of the interior temperature. Consequently, when an air-conditioning system is used to cool the room, more energy will beconsumed [19].

In another study, the shading effect of vertical greenery systemsreduces the energy used for cooling by approximately 23% and theenergy used by fans by 20%, resulting in an 8% reduction in annualenergy consumption [5]. In addition, vertical greenery systems canreduce air-conditioning load by shading walls and windows fromincoming solar energy as a 5.5 �C reduction in the temperatureimmediately outside of a building can reduce the amount of energyneeded for air-conditioning by 50% to 70% [4].

Furthermore, projected energy savings ranging from 90% to 35%for various cities when all possible façades are implemented withvertical greenery systems highlighted the potential for producingsignificant improvements in thermal comfort in the built environ-ment and reductions in the cooling load demands [13].

Since insulation applied to the exterior of buildings is muchmore effective than interior insulation, especially during thesummer months, vertical greenery systems would have the two-fold effect of reducing incoming solar energy into the interior

Fig. 1. Control wall and the 8 vertical greenery systems in HortPark.

N.H. Wong et al. / Building and Environment 45 (2010) 663–672 665

through shading and reducing heat flow into the building throughevaporative cooling, both translating into energy savings.

3. Methodology

In Singapore, vertical greenery systems are still at its infancystage. However, with the government of Singapore championingmore innovative ways to integrate greenery into built-forms in thecity, the future of vertical greenery systems in Singapore seemsencouraging. In fact, several buildings in Singapore have actuallyadopted the use of vertical greenery systems. They are the BotanyCentre in Singapore Botanic Gardens, Shangri-La Hotel, SingaporeManagement University, the carpark in Republic Polytechnic and anextensive green facade within Terminal 3 of Changi InternationalAirport.

Research on the thermal effects of plants on vertical façades oftall buildings in Singapore was rare and only undertaken by Ong[20]. Results of the study showed that differences in surface

Table 1Description of vertical greenery systems in HortPark.

VGS System typology Description

1 Living wall – Modular panel, vertical interface,mixed substrate

Combination of 2 sysplanter system. Versimixture of green rooftextile membrane whcages system.

2 Green façade – Modular trellis Climber plants in planpanels on the wall.

3 Living wall – Grid and modular, verticalinterface, mixed substrate

Plant panels embeddeinto fitting frames.

4 Living wall – Modular panel, verticalinterface, inorganic substrate

Employed the Parabie(composite peat mosspanel encased in a stawith integrated irriga

5 Living wall – Planter panel, angled interface,green roof substrate

This system uses a UVintegrated horizontal

6 Living wall – Framed mini planters, horizontalinterface, soil substrate

Individual mini plant

7 Living wall – Vertical moss-tile, verticalinterface, inorganic substrate

Patented ceramic tilecreating tiling design

7a Living wall – Flexible mat tapestry, horizontalinterface, soil substrate

Lightweight panel coonto a supporting grabetween mats. Suitab

8 Living wall – Plant cassette, horizontal interface,soil substrate

Use of planters to holPlanters are secured omedium is used.

temperatures between surfaces with and without vegetation can beas high as 11 �C, attesting to the potential of vertical greenerysystems to ameliorate thermal conditions in a high-riseenvironment.

With an intention to promote vertical greenery, CUGE of NParksinstalled 8 different vertical greenery systems sourced fromdifferent parts of the world at the recently opened HortPark, as seenin Fig. 1. The 8 vertical greenery systems are selected to cover thewide spectrum of systems ranging from the simple Green Façadesystem to the complex Living Wall system with vertical, angled orhorizontal interfaces as shown in Table 1, hoping to serve as a guidefor other vertical greenery systems with similar characteristics.Furthermore, the HortPark study is one part of the overall attemptto fill the gaps and voids in the knowledge of vertical greenerysystems in the tropical context.

Dimensions of the concrete walls are similar among all the 8vertical greenery systems and the control wall, measuring 4 m wideby 8 m high. The thickness of the concrete walls is 0.300 m thick.

Plant size

tems: the versicell-based and ‘plug-in’ slotcell planters have drainage cells with selected

and soil planting media wrapped in geo-ile the slotted planters are mainly planter

Small to medium

ters forming green screens across mesh Climber plants

d within stainless steel mesh panels inserted Small

nta system with a patented growing medium) as a planting media inlay. The peat mossinless steel cage is hung onto supports linedtion.

Small

-treated plastic as a molded base panel withplanting bays.

Small

ers placed and secured onto stainless steel frame. Small

s shipped with pre-grown moss species. Suitable fors

Small, custom-grownon tiles

mprising 2 layers of moisture retention mats securedting or mesh. Plants slotted and pre-grown inle for flat and curved surfaces. Allows ease of change.

Small to medium

d wider variety of plant types and of larger sizes.nto the wall through hinges. Lightweight growing

Small to medium-large

Table 2Thickness of substrates and plants of vertical greenery systems in HortPark.

Vertical greenery system Average thickness (m)

Substrate Plants Total

Green façade Mesh system 2 0.080 0.010 0.090Living wall Vertical interface 1 0.250 0.100 0.350

3 0.230 0.120 0.3504 0.080 0.120 0.2007 – – –

Angled interface 5 0.070 0.110 0.180Horizontal interface 6 0.065 0.055 0.120

7a 0.060 0.120 0.1808 0.280 0.200 0.480

N.H. Wong et al. / Building and Environment 45 (2010) 663–672666

All the 8 vertical greenery systems are on average 1 m above theground. However, the thickness of the substrates and plants ofeach vertical greenery systems vary and their values are shown inTable 2. The substrate of VGS 2 is located at the bottom of the walland consists of soil inside pots that are 0.610 m thick while VGS 5has an air space of 0.085 m between the wall and substrate.

All the 9 walls are intended to simulate building walls. Theplanted side is comparable to the external wall while the other sideis the interior. It is important to note that the ‘‘interior wall’’ is alsoexposed to the sun and the corresponding heat gain can cause anincrease in the surface temperature of the ‘‘exterior wall’’.Furthermore, an interior space of a building will normally be ata lower temperature compared to the exposed exterior although itwill have similar diurnal temperature fluctuation if the space isnaturally ventilated. Hence, the observed surface temperatures onthe 8 ‘‘external’’ walls with vertical greenery systems may be higherthan that of an actual building’s external wall with vertical greenerysystems, underestimating the influences of vertical greenerysystems on surface temperatures.

Therefore, although the 9 walls are different from building walls,this experiment formed the foundational step before proceeding toanalyze actual building walls with vertical greenery systems,currently under construction in the zero-energy building inSingapore. In addition, this experiment is useful in determining theinput boundary conditions for implementing vertical greenerysystems into building simulation model.

In addition, throughout the period of measurement, externalfactors that can possibly have influenced the temperature readingsare encountered. This included trimming of plants and thereplacement of dead plants. These events which can affect thetemperature readings are noted in consultation with NParks offi-cers who are responsible for the maintenance and upkeep of the

Fig. 2. Positions of thermocouple wires for measuring wall and substrate surfacetemperatures.

vertical greenery systems. Possible changes in temperature read-ings as a result of these inconsistencies are kept track of.

3.1. Instrumentation and parameters

44 sets of single channel Hobo U12T type thermocouple dataloggers with an accuracy of �1.5 �C are used for the measurementof surface temperature of the 9 walls, including the control wall.Measurements of surface temperatures are taken at 2 layers, thetemperature of the wall and substrate surfaces, as shown in Fig. 2.For the control wall, only the temperature of the wall surface ismeasured.

3 clear days, 24 February, 28 April and 21 June 2008, are selectedfor analysis of the surface temperature profiles of the 8 verticalgreenery systems. On 24 February 2008, the surface temperature ofthe control wall reaches a maximum of 33 �C while 39 �C is expe-rienced on both 28 April and 21 June 2008. This difference incontrol wall surface temperatures is expected to influence the walland substrate surface temperatures. However, the trend should besimilar among the 3 days. In addition, the growth or death ofvarious plant species around the thermocouple data loggers willaffect the localized surface temperatures and are taken into accountduring analysis.

For the ambient temperature, 16 sets of Hobo H8 Pro tempera-ture/relative humidity data loggers with an accuracy of �0.5 �C areused. They are placed in front of the control wall as well as verticalgreenery systems 1, 2 and 4. The data loggers are secured oncustomized stands and are placed at intervals of 0.15 m, 0.30 m,0.60 m and 1.00 m away from the substrate surface, as seen in Fig. 3and 1 December 2008 is selected for analyzing the ambienttemperature profiles.

Lastly, a Hobo weather station is set up nearby to collect themeteorological parameters which include the ambient airtemperature, relative humidity, solar radiation, wind speed, winddirection and rainfall.

4. Discussion and analysis

4.1. Surface temperatures

The average wall and substrate surface temperatures withrespect to the control wall for the 8 vertical greenery systems arediscussed and shown in Fig. 4. The values are the average of theseveral thermocouples readings placed within each verticalgreenery systems.

Fig. 3. Positions of temperature/relative humidity data loggers for measuring ambienttemperatures.

Fig. 4. Average wall and substrate surface temperatures for all 8 vertical greenery systems on 21 June 08.

N.H. Wong et al. / Building and Environment 45 (2010) 663–672 667

In VGS 1, the average temperature of the wall surface is lowercompared to the control wall, with temperature difference reaching10.03 �C. The highest temperature reduction is observed where thefoliage density is highest. In addition, the diurnal temperaturefluctuation of the wall surface is more stable than the control wall.

The average temperature on the substrate surface is lower thanthe wall surface in the evening and night but tends to be vice versain the daytime. These point to the potential of vertical greenerysystems in reducing the UHI effects through evapotranspiration asthe latent heat used in evapotranspiration reduces the amount oflong-wave radiation radiating back to the environment in the night.Furthermore, the diurnal temperature fluctuation on the substratesurface on 21 June 2008 is fairly high and comparable with thecontrol wall. This can be due to the reduction in the foliage densityas the leaves are observed to have dried out over time.

VGS 2 has no substrate attached on the surface wall. Instead,plants creep along a system of steel mesh attached to the wall.There is no significant difference in the temperature reduction,especially in the night. However, there is still a 4.36 �C reduction inthe average temperature of the wall surface on 21 June 2008.Overall, the presence of climbing plants does have a decreasingeffect on the overall wall surface average temperature even withoutthe insulating presence of substrate.

The temperature profiles at the various locations on the wallsurfaces show a tendency to follow the temperature profile of thecontrol wall, as seen in Fig. 5. The extent of temperature reductionappears to depend on the density of the foliage cover and theconsequent shading effect of the leaves. The temperature of ther-mocouples located below the steel mesh planters is higher thanthose located behind the climbing plants.

Generally among the 3 observed days and various temperaturechannels, the temperature of the wall surface of VGS 3 is 4–12 �C and4 �C lower than that of the control wall in the daytime and at nightrespectively. Furthermore, the diurnal temperature fluctuation of

the wall surface follows that of the control wall. Similarly, thetemperature of the substrate surface is lower than the control wallby about 2–C. In addition, the substrate temperature profile behindHemigraphis repanda, a red-leaved plant species, is lower than theother channels. Besides having a higher foliage density, the effect ofvegetation colour on temperature reduction can be a possiblereason.

The average temperature reduction of the wall surface of VGS 4is substantial especially during the daytime when solar radiation ishigh, with a maximum reduction of about 10.94 �C observed in theafternoon on 28 April 2008. Furthermore, the diurnal temperaturefluctuation of the wall surface is minimal. This high temperaturestability is observed despite having thinner substrate.

The temperatures of the substrate surface are lower than thecontrol wall by about 3–6 �C at night and 9 �C in the day as well aslower than the wall surface by about 1 �C at night. However, thetemperature of the substrate surface approaches the control wallsurface in the daytime. Although the insulating effect of thesubstrate results in a lower surface wall temperature, the substrateitself gets heated up directly from the high solar radiation.

For VGS 5, the average temperature of the wall surface is lowerthan the control wall by a maximum of 10.03 �C in the afternoon at1240 h on 28 April 2008, as seen in Fig. 6. The diurnal temperaturefluctuation is also slighter higher. The temperature reductionbetween the substrate surface and the control wall is substantialwith a difference of about 4 �C at night. The cooling effect fromevapotranspiration accounts for this temperature difference.

Whilst the temperature of the substrate surface is generallylower than the wall surface during the night, the temperatureapproaches that of the wall surface in the day. The insulating effectof the substrate at high temperatures may account for the lowerwall surface temperature whilst the substrate itself with its highheat capacity tends to register a higher temperature during periodsof high solar radiation.

Fig. 5. Wall and substrate surface temperatures of VGS 2 on 24 Feb 08.

N.H. Wong et al. / Building and Environment 45 (2010) 663–672668

The thermocouple on the wall surface of VGS 6 behind Phyl-lanthus myrtifolius shows high temperature fluctuation withtemperatures as low as the substrate in the night and soaring totemperatures equaling to the control wall in the day. On the otherhand, another wall surface thermocouple behind the large leavesTradescantia spathacea ‘Compacta’ has minimal temperature fluc-tuation of only about 1 �C. Furthermore, the wall surface below therelatively sparse P. myrtifolius has a lower temperature at nightcompared to the wall surface temperature beneath the thicker anddenser T. spathacea ‘Compacta’.

Whilst the P. myrtifolius leaves are sparse, the T. spathacea‘Compacta’ has thick, fleshy leaves which can serve as a bufferagainst temperature fluctuations and contribute to the stabletemperature of the wall surface beneath. Hence, the interactionsbetween leaf area, geometry, colour and other microclimaticparameters such as solar radiation are complex and result indifferences in the cooling efficiency at night and daytime.

In addition, the maximum average temperature reduction of thesubstrate surface compared to the control wall is 6.11 �C in thedaytime, with several occasions where the temperatures are higherthan the control wall.

VGS 7, the moss-tile system, was changed midway in April 2008,as the moss was not able to adapt due to unknown conditions,possibly due to a combination of high temperature and waterquality. The earlier moss-tile system is referred as VGS 7 and thesubsequent geo-textile membrane system with plants incorporated

Fig. 6. Wall and substrate surface tem

within pockets in the geo-textile membrane is referred as VGS 7a.In the later installation, the average temperature reduction is now3 �C in the night and 6 �C in the day. In addition, the averagetemperature profile of the substrate surface achieves lowertemperature at night but is subjected to high temperature regimesin the daytime when there is high solar radiation.

Lastly, in VGS 8, the temperature reduction of the wall surfacecompared to the control wall ranges from about 2 �C at night andup to about 9 �C in the afternoon. The temperature of the substratesurface is lower than the control wall by up to about 4 �C in thenight and up to about 8 �C in the afternoon.

4.2. Overall trends

For most of the vertical greenery systems, the temperature onthe substrate surface is lower compared to the wall surface in theevening and night but with a reversal in the day. This is explainedby the higher temperature of the substrate surface due to directexposure to solar radiation in the day whilst the wall surface iscovered by the plant panels, substrate and plants, resulting ina lower temperature. At night, the substrate surface with its highheat capacity tends to lose heat faster than the wall surface which iscovered behind the substrate and tends to retain heat, thuscontributing to the higher temperature of the wall surface ascompared to the substrate surface.

peratures of VGS 5 on 28 Apr 08.

Fig. 7. Ambient temperatures at a distance of 0.15 m (top), 0.30 m (middle) and 0.60 m (bottom) away from wall on 1 Dec 08.

N.H. Wong et al. / Building and Environment 45 (2010) 663–672 669

The only exception is observed in vertical greenery system 2which has no substrate on the wall surface and the cooling effect isdirectly due to the shading effect and evapotranspiration from theleaves of the climbing plants. There is no substrate to contribute tothe cooling effect due to evaporation of moisture from the substrateas in the other vertical greenery systems.

There is a distinct reduction of the temperatures of the wall andthe substrate surfaces as compared to the control wall for all the 8vertical greenery systems although the extent of temperaturereduction differs between various vertical greenery systems. Thetemperature reduction is most prominent around noon when it isthe hottest, attesting to the benefits of vertical greenery systems.

In terms of the average wall surface temperature reduction,VGSs 4 and 3 appear to have the best cooling efficiency in the day,reaching a maximum temperature reduction of more than 10 �C.

This is followed by VGSs 1, 5 and 8 where the maximum averagewall surface temperature reduction ranges from 8 �C to 10 �C. VGSs6 and 7a both achieve slightly lower maximum wall surfacetemperature reduction of around 6 �C.

VGS 2 consists of spare climbers and hence does not benefitfrom the insulation and cooling effect from evaporation of moistureas it has no substrate. Hence the maximum wall surface tempera-ture reduction is 4.36 �C.

VGSs 3, 4 and 5 show the best capacity for substrate surfacetemperature reduction, reaching beyond 8 �C, followed by VGSs 8, 1and 2, ranging between 6 �C and 8 �C. It is interesting to note thatalthough VGS 2 has no substrate, there seem to be an overallcooling effect especially in the afternoon when the temperaturereduction is the highest. Lastly, VGSs 7 and 6 have the leastperformance where reductions are below 6 �C and there are several

Table 3Summary of average wall surface temperatures from 24 Feb 08, 28 Apr 08 and 21 Jun 08.

VGS Maximum reduction of average wall surfacetemperature (�C) and corresponding time

Diurnal range of average wall surfacetemperature (�C)

Type Date 24/2 28/4 21/6 24/2 28/4 21/6

Mesh system 2 1.101010 h

3.331335 h

4.360955 h

4.55 7.02 7.37

Vertical interface 1 4.851435 h

10.031240 h

8.601245 h

1.45 1.21 1.43

3 4.701520 h

11.581235 h

8.401225 h

3.60 4.69 2.68

4 5.601430 h

10.941240 h

9.061245 h

1.00 1.64 1.43

7 3.051400 h

– – 2.75 – –

Angled interface 5 4.051335 h

10.031240 h

7.341235 h

2.60 2.26 3.48

Horizontal interface 6 2.300250 h

6.851245 h

5.061435 h

5.05 5.76 7.13

7a – 6.581120 h

7.131350 h

– 10.28 3.40

8 4.001335 h

9.271240 h

8.431350 h

2.70 2.13 2.09

N.H. Wong et al. / Building and Environment 45 (2010) 663–672670

occasions where the average substrate surface temperatures actu-ally exceed that of the control wall.

Within individual vertical greenery systems, changes in thepatterns of temperature reduction are observed with correspond-ing changes in the foliage density over time. For example, inspecific areas where the plants have less dense foliage or wherethe leaves have dried or died out over time, corresponding changesin the temperature profile are observed. This bears out theimportance of foliage density and the need for healthy growth ofplants for the effective thermal performance of vertical greenerysystems. In this regard, careful selection of plants which are suitedto the peculiar conditions of the particular types of verticalgreenery systems is imperative for the successful use of verticalgreenery systems.

4.3. Ambient temperature

From Fig. 7, trends in the ambient temperature of selected VGSs1, 2 and 4 are compared with the control wall at various distances to

Table 4Summary of average substrate surface temperatures from 24 Feb 08, 28 Apr 08 and 21 J

VGS Maximum reduction of average substrate surftemperature (�C) and corresponding time

Type Date 24/2 28/4

Mesh system 2 2.451335 h

7.321305 h

Vertical interface 1 5.231720 h

7.931505 h

3 4.921855 h

9.211300 h

4 5.301720 h

8.951305 h

7 4.252235 h

–

Angled interface 5 4.481830 h

8.481300 h

Horizontal interface 6 3.251355 h

6.111250 h

7a – 6.121250 h

8 3.721355 h

7.841240 h

determine the impact of vertical greenery systems on ambienttemperature and their potential in producing a cooling effect on theimmediate external environment. The ambient temperature read-ings obtained at 1.00 m away are found to be corrupted and hencenot analyzed.

For distance of 0.15 m away from the vertical greenery systems,the ambient temperature of the control wall shows the highesttemperature throughout the day. This is followed by VGSs 2,1 and 4.All 3 vertical greenery systems showed similar temperature profilesand peak hours. During the daytime from 1000 h to 1800 h, theambient temperature difference among the vertical greenerysystems is more obvious.

Ambient temperature may be affected by air circulation. Thoughboth VGSs 1 and 4 are covered by well-distributed greenery, VGS 1has thicker greenery near the temperature data logger which mayblock the air circulation and trap heat. Hence, it can be inferred thatat a distance of 0.15 m away from the substrate, the ambienttemperature is most affected by the presence of the verticalgreenery systems.

un 08.

ace Diurnal range of average substrate surfacetemperature (�C)

21/6 24/2 28/4 21/6

6.351110 h

3.20 5.86 5.10

5.331815 h

1.97 5.69 9.75

5.691425 h

4.10 3.63 6.25

6.341420 h

2.10 5.42 7.29

– 5.20 – –

6.531420 h

4.20 5.52 6.05

4.040840 h

3.73 7.80 11.96

4.971415 h

– 10.24 10.33

6.611000 h

3.90 5.07 4.46

Table 5Summary of ambient temperatures.

VGS Temperature (�C)

0.15 m away 0.30 m away 0.60 m away

Lowest Highest Lowest Highest Lowest Highest

Control Wall 26.34 34.85 25.17 33.59 25.17 33.591 24.79 31.93 26.34 34.01 25.17 32.342 25.56 32.76 25.56 32.76 25.56 32.764 25.17 31.52 25.17 31.93 25.95 32.76

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At a distance of 0.30 m away from the substrate, VGS 1 has thehighest ambient temperature throughout the whole day, even higherthan the control wall. This may be caused by the thick greenery whichis very near to the temperature data logger, restricting air circulationwhich helps to dissipate heat. The control wall and VGS 2 showa similar ambient temperature profile and only differed slightlybetween 0800 h and 1600 h. Hence, it can be inferred that VGS 2 is nolonger influencing the ambient temperature. Lastly, VGS 4 shows thelowest ambient temperature throughout the day, showing that it isstill influencing the ambient temperature around the environment.

At a distance of 0.60 m away, the 3 vertical greenery systemsand the control wall showed a similar ambient temperature. In thedaytime between 0800 h and 1600 h, the control wall showeda slighter higher ambient temperature which is not very significant.Hence, it can be concluded that all 3 vertical greenery systems nolonger influence the ambient temperature.

5. Conclusion

5.1. Surface temperatures

The comparison of the effects of the 8 vertical greenery systemsin HortPark on the reduction of wall and substrate surfacetemperatures is shown in Tables 3 and 4. In terms of the maximumreduction of average wall surface temperature as compared to thecontrol wall, VGSs 4 and 3 show the best thermal performance.

In terms of the diurnal range of average wall surface tempera-ture (difference between the highest and lowest values), VGSs 4and 1 show the highest capacities. The capacity of the verticalgreenery systems to limit the fluctuation of wall surface tempera-tures of building facades is valuable in prolonging the lifespan ofbuilding facades and slowing down wear and tear as well as costsavings in maintenance and replacement of façade parts.

The reason for the differences in the thermal performance ofthese vertical greenery systems can be a combination of variousfactors including substrate type, insulation from the system struc-ture, substrate moisture content as well as the shade and insulationfrom greenery coverage. At the same time, the interactions betweenleaf area, geometry, colour and other microclimatic parameters suchas solar radiation are complex and may result in different coolingefficiency during the day and night.

These results point to the potential thermal benefits of verticalgreenery systems in reducing the surface temperature of buildingsfacades in the tropical climate. Maximum reductions of 11.58 �C in thewall surface temperatures on clear days are observed respectively.

This is a significant reduction in wall temperature that will leadto a corresponding reduction in the energy cooling load andconsequent saving in energy cost.

On the other hand, vertical greenery systems 4, 3 and 5 show thebest thermal performance for the maximum average substratesurface temperature reduction. For the least diurnal fluctuation inaverage substrate surface temperature, no vertical greenery systemperforms relatively well, having a mixed range of values. VGSs 6 and7a perform the worst, reaching a diurnal fluctuation beyond 10 �C.

5.2. Ambient temperature

The effects of vertical greenery systems on ambient temperatureare found to depend on specific vertical greenery systems. VGS 2has hardly any effect on the ambient temperature while the effectsof VGS 4 are felt as far as 0.60 m away.

From Table 5, reductions in the ambient temperature of up to3.33 �C are observed from VGS 4 at a distance of 0.15 m away. Giventhe preponderance of vertical surfaces and wall facades in the builtenvironment, the use of vertical greenery systems to cool theambient temperature in building canyons is promising. Further-more, a cooler ambient temperature means that the air intakes ofair-conditioning are at a lower temperature, translating into savingin energy cooling load.

5.3. Recommendations

Results highlight that the various benefits of vertical greenerysystems in the tropical environment are promising. To furtherestablish these results, studies of vertical greenery systems shouldmove on and be analyzed on actual building facades. By doing so,the performance of various thermal parameters may possiblyreveal more insight.

Furthermore, many factors such as the physical structure,materials and dimensions of the panels holding the substrate andplants species, substrate type, composition, depth and moisturecontent have an impact on the various performance of verticalgreenery systems. However, these factors are not analyzed sepa-rately while keeping the rest of the other factors constant. There-fore, future experiments can be tailored to study the impact ofthese factors individually as well as to formulate a plant paletteoptimal for different greenery systems in Singapore.

In all, with all the encouraging thermal results of verticalgreenery systems, it is with anticipation that vertical greenerysystems will gradually become one of the driving forces realizingSingapore’s vision of attaining the status of ‘‘City in the Garden’’.

Acknowledgements

This research was supported by the National University ofSingapore, National Parks Board and Building and ConstructionAuthority of Singapore under the collaborative research projecttitled ‘‘Evaluation of Vertical Greenery Systems for Building Walls’’.

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