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1 Building and Environment 45 (2010) Contents lists available at ScienceDirect Building and Environment journal homepage: 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 , Singapore b National Parks Board, Singapore Botanic Garden, 1 Cluny Road, Singapore , Singapore c Building and Construction Authority, 5 Maxwell Road, Singapore , Singapore article info abstract Article history: Received 27 April 2009 Received in revised form 3 August 2009 Accepted 5 August 2009 Keywords: Vertical greenery systems Thermal environments Surface temperature Ambient temperature This research involves the study of 8 different vertical greenery systems (VGSs) installed in HortPark to evaluate the thermal impacts on the performance of buildings and their immediate environment based on the surface and ambient temperatures. VGSs 3 and 4 have the best cooling efficiency according to the maximum temperature reduction of the wall and substrate surfaces. These results point to the potential thermal benefits of vertical greenery systems in reducing the surface temperature of buildings facades in the tropical climate, leading to a reduction in the cooling load and energy cost. In terms of the lowest diurnal 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 temperature fluctuation. By limiting the diurnal fluctuation of wall surface temperatures, the lifespan of building facades is prolonged, slowing down wear and tear as well as savings in maintenance cost and the replacement of façade parts. The effects of vertical greenery systems on ambient temperature are found to depend on specific vertical greenery systems. VGS 2 has hardly any effect on the ambient temperature while the effects of VGS 4 are felt as far as 0.60 m away. Given the preponderance of wall facades in the built environment, the use of vertical greenery systems to cool the ambient temperature in building canyons is promising. Furthermore, air intakes of air-conditioning at a cooler ambient temperature translate into saving in energy cooling load. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The unstoppable force of urbanization is consuming vast quantities of natural vegetation, replacing them with concrete buildings and low albedo surfaces. These resulting changes in the thermal properties of surface materials and the lack of evapotranspiration in urban areas lead to a phenomenon known as the urban heat island (UHI) effect. With the idea of introducing nature back into the urban landscape, a partnership is strengthening between nature and the city with the aim to create a new sustainable urban lifestyle. Greenery is the key element of this transformation. Since the outer surfaces of building offer a great amount of space for vegetation in urban cities, planting on roofs and walls has became one of the most innovative and rapidly developing fields in the worlds of ecology, horticulture and the built environment. * Corresponding author. Tel.: þ ; fax: þ address: alextanyk@alumni.nus.edu.sg (Alex Y.K. Tan). The greening of the façade of building walls, known as vertical greenery systems (VGSs), has yet to be fully explored and exploited. Simply due to the sheer amount of building walls, the widespread use of vertical greenery systems not only represents a great potential in mitigating the UHI effect through evapotranspiration and shading, it is also a highly impactful way of transforming the urban landscape. Vertical greenery systems have been the features of architectures for centuries where it is a common practice to grow climbers on the exterior walls of buildings. The technical idea of vertical greenery systems was based on the fact that certain plant like orchids do not depend on soil and can be applied to urban settings. Nevertheless, vertical greenery systems are still a relatively new discipline. In Germany, a good research base has been developed over the last twenty years on the environmental performance of vertical greenery systems where regulations and guidelines have been published [1]. However, much of the technical and research literature are mostly unknown or published in German. This research involves the study of 8 different vertical greenery systems installed in HortPark, Singapore with the objective of /$ see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi: /j.buildenv

2 664 N.H. Wong et al. / Building and Environment 45 (2010) evaluating the thermal impacts of various vertical greenery systems on the performance of buildings and their immediate environment based on the surface and ambient temperatures. This is a project initiated by the Centre for Urban Greenery and Ecology (CUGE) of the National Parks Board (NParks), in collaboration with the Building and Construction Authority (BCA) and the National University of Singapore (NUS). 2. Literature review Although there are significant published articles on urban greenery, most of them generally focus on the benefits and effects of vegetation on the urban climate and buildings, with dominant emphasis on rooftop gardens rather than vertical greenery systems. Vegetation can play an important role in the topoclimate of towns and the microclimate of buildings. With buildings, some vegetative climatic effects could be made by combining green cover on walls, roofs and open spaces in the vicinity of buildings [2]. Although there are many benefits in reintroducing vegetation to the surfaces of urban buildings and their related spaces, many technical problems are faced during implementation [3]. In a research report by the Canadian Mortgage and Housing Corporation, a comprehensive review of the quantitative and qualitative benefits of vertical greenery systems is discussed. The major barriers to the more rapid diffusion of these sustainable development technologies were also described and a number of initiatives were proposed [4]. In another Canadian research, Bass [5] studied the potential of rooftop gardens and vertical greenery systems in an urban environment. Both technologies reduced surface temperatures sufficiently to suggest that considerable reductions of the UHI effects would be possible if they were employed on a widespread basis. In a research project aimed at defining the thermal performance of double skin façade covered with plants, a simulation model was developed to analyze the influence of plants on the performance of the double skin façade. Further simulations of the entire building proved that plants can contribute to a comfortable indoor climate and energy savings [6]. In the University of Brighton, research on the shading performance of climbing plant canopies, measurement of area coverage and solar transmittances of different leaf layers as well as coming up with a novel technique to provide an assessment of the shading performance of a climbing plant canopy were carried out [7]. Lambertini [8] presented a pictorial collection of the most important architectural projects that embraced the emerging trend of designing and cultivating once inconceivable greenery on a vertical plane while Dunnett [9] cited the associated benefits and reasons for integrating green techniques of organic architecture into our built environment as well as provided a massive collection of appropriate plant information and extensive plant directories for both rooftop gardens and vertical greenery systems. The Green Walls Group, a sub-committee of the Green Roofs for Healthy Cities in North America had produced an introduction of vertical greenery systems, citing the benefits, factors for successful implementation, maintenance issues, policies and LEED certification [10]. Centre for Subtropical Design in the Queensland University of Technology in Brisbane, Australia is currently embarking on an extensive research programme on living wall systems. Their research project aims to identify the benefits of living wall system and to comprehend the challenges in successfully introducing them in the built environment of subtropical Queensland [11]. Lastly, Van Bohemen [12] showed within an ecological engineering context the impact of the greening of outdoor walls and questioned the hesitation to implement vertical greenery systems as outer layer of buildings with special emphasis on the relationship between particulate matter and aerosol deposition with vegetation Thermal benefits temperature reduction Research showed that the humid climates of Hong Kong can achieve substantial benefits of a maximum temperature decrease of 8.4 C with vertical greenery systems in an urban canyon [13]. This is significant as the distribution of ambient air in a canyon influences the energy consumption of buildings as higher temperatures in canyon increase heat convection to a building and correspondingly increases the cooling load [14]. It was also noted that vegetation can alleviate UHI directly by shading heat-absorbing surfaces and through evapotranspiration cooling [15]. Vegetation can dramatically reduce the maximum temperatures of a building by shading walls from the sun, with daily temperature fluctuation being reduced by as much as 50% [9]. Through evapotranspiration, large amounts of solar radiation can be converted into latent heat which does not cause temperature to rise. In addition, a façade fully covered by greenery is protected from intense solar radiation in summer and can reflect or absorb in its 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 systems have been observed in different settings at the University of Toronto since 1996 [5]. These results have consistently demonstrated that areas with vertical vegetation are cooler than light-coloured bricks, walls and black surfaces that are typically found in urban areas. Lastly, in Japan, experiments show that vines can reduce the temperature of a veranda with south-western exposure [17]. In Africa, a temperature reduction of 2.6 C was observed behind vegetated panels of vines [18]. Therefore, together with the insulation effect of vegetation, temperature fluctuations at the wall surface can be reduced from between 10 C and 60 C to between 5 C and 30 C [4] Thermal benefits shading and insulation In an experimental investigation of the effect of shading buildings walls with plants, it is suggested that more thermal energy flows into the non-shaded walls due to direct exposure to the sun and resulted in higher surface wall temperature. The energy absorbed will advance into the inner wall surfaces, resulting in elevation of the interior temperature. Consequently, when an airconditioning system is used to cool the room, more energy will be consumed [19]. In another study, the shading effect of vertical greenery systems reduces the energy used for cooling by approximately 23% and the energy used by fans by 20%, resulting in an 8% reduction in annual energy consumption [5]. In addition, vertical greenery systems can reduce air-conditioning load by shading walls and windows from incoming solar energy as a 5.5 C reduction in the temperature immediately outside of a building can reduce the amount of energy needed 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 with vertical greenery systems highlighted the potential for producing significant improvements in thermal comfort in the built environment and reductions in the cooling load demands [13]. Since insulation applied to the exterior of buildings is much more effective than interior insulation, especially during the summer months, vertical greenery systems would have the twofold effect of reducing incoming solar energy into the interior

3 N.H. Wong et al. / Building and Environment 45 (2010) Fig. 1. Control wall and the 8 vertical greenery systems in HortPark. through shading and reducing heat flow into the building through evaporative cooling, both translating into energy savings. 3. Methodology In Singapore, vertical greenery systems are still at its infancy stage. However, with the government of Singapore championing more innovative ways to integrate greenery into built-forms in the city, the future of vertical greenery systems in Singapore seems encouraging. In fact, several buildings in Singapore have actually adopted the use of vertical greenery systems. They are the Botany Centre in Singapore Botanic Gardens, Shangri-La Hotel, Singapore Management University, the carpark in Republic Polytechnic and an extensive green facade within Terminal 3 of Changi International Airport. Research on the thermal effects of plants on vertical façades of tall buildings in Singapore was rare and only undertaken by Ong [20]. Results of the study showed that differences in surface temperatures between surfaces with and without vegetation can be as high as 11 C, attesting to the potential of vertical greenery systems to ameliorate thermal conditions in a high-rise environment. With an intention to promote vertical greenery, CUGE of NParks installed 8 different vertical greenery systems sourced from different parts of the world at the recently opened HortPark, as seen in Fig. 1. The 8 vertical greenery systems are selected to cover the wide spectrum of systems ranging from the simple Green Façade system to the complex Living Wall system with vertical, angled or horizontal interfaces as shown in Table 1, hoping to serve as a guide for other vertical greenery systems with similar characteristics. Furthermore, the HortPark study is one part of the overall attempt to fill the gaps and voids in the knowledge of vertical greenery systems in the tropical context. Dimensions of the concrete walls are similar among all the 8 vertical greenery systems and the control wall, measuring 4 m wide by 8 m high. The thickness of the concrete walls is m thick. Table 1 Description of vertical greenery systems in HortPark. VGS System typology Description Plant size 1 Living wall Modular panel, vertical interface, Combination of 2 systems: the versicell-based and plug-in slot Small to medium mixed substrate planter system. Versicell planters have drainage cells with selected mixture of green roof and soil planting media wrapped in geotextile membrane while the slotted planters are mainly planter cages system. 2 Green façade Modular trellis Climber plants in planters forming green screens across mesh Climber plants panels on the wall. 3 Living wall Grid and modular, vertical interface, mixed substrate Plant panels embedded within stainless steel mesh panels inserted into fitting frames. Small 4 Living wall Modular panel, vertical interface, inorganic substrate 5 Living wall Planter panel, angled interface, green roof substrate 6 Living wall Framed mini planters, horizontal interface, soil substrate 7 Living wall Vertical moss-tile, vertical interface, inorganic substrate 7a Living wall Flexible mat tapestry, horizontal interface, soil substrate 8 Living wall Plant cassette, horizontal interface, soil substrate Employed the Parabienta system with a patented growing medium (composite peat moss) as a planting media inlay. The peat moss panel encased in a stainless steel cage is hung onto supports lined with integrated irrigation. This system uses a UV-treated plastic as a molded base panel with integrated horizontal planting bays. Individual mini planters placed and secured onto stainless steel frame. Patented ceramic tiles shipped with pre-grown moss species. Suitable for creating tiling designs Lightweight panel comprising 2 layers of moisture retention mats secured onto a supporting grating or mesh. Plants slotted and pre-grown in between mats. Suitable for flat and curved surfaces. Allows ease of change. Use of planters to hold wider variety of plant types and of larger sizes. Planters are secured onto the wall through hinges. Lightweight growing medium is used. Small Small Small Small, custom-grown on tiles Small to medium Small to mediumlarge

4 666 N.H. Wong et al. / Building and Environment 45 (2010) Table 2 Thickness of substrates and plants of vertical greenery systems in HortPark. Vertical greenery system Average thickness (m) Substrate Plants Total Green façade Mesh system Living wall Vertical interface Angled interface Horizontal interface a All the 8 vertical greenery systems are on average 1 m above the ground. However, the thickness of the substrates and plants of each vertical greenery systems vary and their values are shown in Table 2. The substrate of VGS 2 is located at the bottom of the wall and consists of soil inside pots that are m thick while VGS 5 has an air space of m between the wall and substrate. All the 9 walls are intended to simulate building walls. The planted side is comparable to the external wall while the other side is the interior. It is important to note that the interior wall is also exposed to the sun and the corresponding heat gain can cause an increase in the surface temperature of the exterior wall. Furthermore, an interior space of a building will normally be at a lower temperature compared to the exposed exterior although it will have similar diurnal temperature fluctuation if the space is naturally ventilated. Hence, the observed surface temperatures on the 8 external walls with vertical greenery systems may be higher than that of an actual building s external wall with vertical greenery systems, underestimating the influences of vertical greenery systems on surface temperatures. Therefore, although the 9 walls are different from building walls, this experiment formed the foundational step before proceeding to analyze actual building walls with vertical greenery systems, currently under construction in the zero-energy building in Singapore. In addition, this experiment is useful in determining the input boundary conditions for implementing vertical greenery systems into building simulation model. In addition, throughout the period of measurement, external factors that can possibly have influenced the temperature readings are encountered. This included trimming of plants and the replacement of dead plants. These events which can affect the temperature readings are noted in consultation with NParks officers who are responsible for the maintenance and upkeep of the vertical greenery systems. Possible changes in temperature readings as a result of these inconsistencies are kept track of Instrumentation and parameters 44 sets of single channel Hobo U12T type thermocouple data loggers with an accuracy of 1.5 C are used for the measurement of surface temperature of the 9 walls, including the control wall. Measurements of surface temperatures are taken at 2 layers, the temperature of the wall and substrate surfaces, as shown in Fig. 2. For the control wall, only the temperature of the wall surface is measured. 3 clear days, 24 February, 28 April and 21 June 2008, are selected for analysis of the surface temperature profiles of the 8 vertical greenery systems. On 24 February 2008, the surface temperature of the control wall reaches a maximum of 33 C while 39 C is experienced on both 28 April and 21 June This difference in control wall surface temperatures is expected to influence the wall and substrate surface temperatures. However, the trend should be similar among the 3 days. In addition, the growth or death of various plant species around the thermocouple data loggers will affect the localized surface temperatures and are taken into account during analysis. For the ambient temperature, 16 sets of Hobo H8 Pro temperature/relative humidity data loggers with an accuracy of 0.5 Care used. They are placed in front of the control wall as well as vertical greenery systems 1, 2 and 4. The data loggers are secured on customized 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. 3 and 1 December 2008 is selected for analyzing the ambient temperature profiles. Lastly, a Hobo weather station is set up nearby to collect the meteorological parameters which include the ambient air temperature, relative humidity, solar radiation, wind speed, wind direction and rainfall. 4. Discussion and analysis 4.1. Surface temperatures The average wall and substrate surface temperatures with respect to the control wall for the 8 vertical greenery systems are discussed and shown in Fig. 4. The values are the average of the several thermocouples readings placed within each vertical greenery systems. Fig. 2. Positions of thermocouple wires for measuring wall and substrate surface temperatures. Fig. 3. Positions of temperature/relative humidity data loggers for measuring ambient temperatures.

5 N.H. Wong et al. / Building and Environment 45 (2010) Fig. 4. Average wall and substrate surface temperatures for all 8 vertical greenery systems on 21 June 08. In VGS 1, the average temperature of the wall surface is lower compared to the control wall, with temperature difference reaching C. The highest temperature reduction is observed where the foliage density is highest. In addition, the diurnal temperature fluctuation of the wall surface is more stable than the control wall. The average temperature on the substrate surface is lower than the wall surface in the evening and night but tends to be vice versa in the daytime. These point to the potential of vertical greenery systems in reducing the UHI effects through evapotranspiration as the latent heat used in evapotranspiration reduces the amount of long-wave radiation radiating back to the environment in the night. Furthermore, the diurnal temperature fluctuation on the substrate surface on 21 June 2008 is fairly high and comparable with the control wall. This can be due to the reduction in the foliage density as 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 in the average temperature of the wall surface on 21 June Overall, the presence of climbing plants does have a decreasing effect on the overall wall surface average temperature even without the insulating presence of substrate. The temperature profiles at the various locations on the wall surfaces show a tendency to follow the temperature profile of the control wall, as seen in Fig. 5. The extent of temperature reduction appears to depend on the density of the foliage cover and the consequent shading effect of the leaves. The temperature of thermocouples located below the steel mesh planters is higher than those located behind the climbing plants. Generally among the 3 observed days and various temperature channels, the temperature of the wall surface of VGS 3 is 4 12 C and 4 C lower than that of the control wall in the daytime and at night respectively. Furthermore, the diurnal temperature fluctuation of the wall surface follows that of the control wall. Similarly, the temperature of the substrate surface is lower than the control wall by about 2 C. In addition, the substrate temperature profile behind Hemigraphis repanda, a red-leaved plant species, is lower than the other channels. Besides having a higher foliage density, the effect of vegetation colour on temperature reduction can be a possible reason. The average temperature reduction of the wall surface of VGS 4 is substantial especially during the daytime when solar radiation is high, with a maximum reduction of about C observed in the afternoon on 28 April Furthermore, the diurnal temperature fluctuation of the wall surface is minimal. This high temperature stability is observed despite having thinner substrate. The temperatures of the substrate surface are lower than the control wall by about 3 6 C at night and 9 C in the day as well as lower than the wall surface by about 1 C at night. However, the temperature of the substrate surface approaches the control wall surface in the daytime. Although the insulating effect of the substrate results in a lower surface wall temperature, the substrate itself gets heated up directly from the high solar radiation. For VGS 5, the average temperature of the wall surface is lower than the control wall by a maximum of C in the afternoon at 1240 h on 28 April 2008, as seen in Fig. 6. The diurnal temperature fluctuation is also slighter higher. The temperature reduction between the substrate surface and the control wall is substantial with a difference of about 4 C at night. The cooling effect from evapotranspiration accounts for this temperature difference. Whilst the temperature of the substrate surface is generally lower than the wall surface during the night, the temperature approaches that of the wall surface in the day. The insulating effect of the substrate at high temperatures may account for the lower wall surface temperature whilst the substrate itself with its high heat capacity tends to register a higher temperature during periods of high solar radiation.

6 668 N.H. Wong et al. / Building and Environment 45 (2010) Fig. 5. Wall and substrate surface temperatures of VGS 2 on 24 Feb 08. The thermocouple on the wall surface of VGS 6 behind Phyllanthus myrtifolius shows high temperature fluctuation with temperatures as low as the substrate in the night and soaring to temperatures equaling to the control wall in the day. On the other hand, another wall surface thermocouple behind the large leaves Tradescantia spathacea Compacta has minimal temperature fluctuation of only about 1 C. Furthermore, the wall surface below the relatively sparse P. myrtifolius has a lower temperature at night compared to the wall surface temperature beneath the thicker and denser T. spathacea Compacta. Whilst the P. myrtifolius leaves are sparse, the T. spathacea Compacta has thick, fleshy leaves which can serve as a buffer against temperature fluctuations and contribute to the stable temperature of the wall surface beneath. Hence, the interactions between leaf area, geometry, colour and other microclimatic parameters such as solar radiation are complex and result in differences in the cooling efficiency at night and daytime. In addition, the maximum average temperature reduction of the substrate surface compared to the control wall is 6.11 C in the daytime, with several occasions where the temperatures are higher than 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 water quality. The earlier moss-tile system is referred as VGS 7 and the subsequent geo-textile membrane system with plants incorporated within pockets in the geo-textile membrane is referred as VGS 7a. In the later installation, the average temperature reduction is now 3 C in the night and 6 C in the day. In addition, the average temperature profile of the substrate surface achieves lower temperature at night but is subjected to high temperature regimes in the daytime when there is high solar radiation. Lastly, in VGS 8, the temperature reduction of the wall surface compared to the control wall ranges from about 2 C at night and up to about 9 C in the afternoon. The temperature of the substrate surface is lower than the control wall by up to about 4 C in the night and up to about 8 C in the afternoon Overall trends For most of the vertical greenery systems, the temperature on the substrate surface is lower compared to the wall surface in the evening and night but with a reversal in the day. This is explained by the higher temperature of the substrate surface due to direct exposure to solar radiation in the day whilst the wall surface is covered by the plant panels, substrate and plants, resulting in a lower temperature. At night, the substrate surface with its high heat capacity tends to lose heat faster than the wall surface which is covered behind the substrate and tends to retain heat, thus contributing to the higher temperature of the wall surface as compared to the substrate surface. Fig. 6. Wall and substrate surface temperatures of VGS 5 on 28 Apr 08.

7 N.H. Wong et al. / Building and Environment 45 (2010) 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. The only exception is observed in vertical greenery system 2 which has no substrate on the wall surface and the cooling effect is directly due to the shading effect and evapotranspiration from the leaves of the climbing plants. There is no substrate to contribute to the cooling effect due to evaporation of moisture from the substrate as in the other vertical greenery systems. There is a distinct reduction of the temperatures of the wall and the substrate surfaces as compared to the control wall for all the 8 vertical greenery systems although the extent of temperature reduction differs between various vertical greenery systems. The temperature reduction is most prominent around noon when it is the 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 average wall surface temperature reduction ranges from 8 Cto10 C. VGSs 6 and 7a both achieve slightly lower maximum wall surface temperature reduction of around 6 C. VGS 2 consists of spare climbers and hence does not benefit from the insulation and cooling effect from evaporation of moisture as it has no substrate. Hence the maximum wall surface temperature reduction is 4.36 C. VGSs 3, 4 and 5 show the best capacity for substrate surface temperature reduction, reaching beyond 8 C, followed by VGSs 8, 1 and 2, ranging between 6 C and 8 C. It is interesting to note that although VGS 2 has no substrate, there seem to be an overall cooling effect especially in the afternoon when the temperature reduction is the highest. Lastly, VGSs 7 and 6 have the least performance where reductions are below 6 C and there are several

8 670 N.H. Wong et al. / Building and Environment 45 (2010) Table 3 Summary of average wall surface temperatures from 24 Feb 08, 28 Apr 08 and 21 Jun 08. VGS Maximum reduction of average wall surface temperature ( C) and corresponding time Diurnal range of average wall surface temperature ( C) Type Date 24/2 28/4 21/6 24/2 28/4 21/6 Mesh system h 1335 h 0955 h Vertical interface h 1240 h 1245 h h 1235 h 1225 h h 1240 h 1245 h h Angled interface h 1240 h 1235 h Horizontal interface h 1245 h 1435 h 7a h 1350 h h h h occasions where the average substrate surface temperatures actually exceed that of the control wall. Within individual vertical greenery systems, changes in the patterns of temperature reduction are observed with corresponding changes in the foliage density over time. For example, in specific areas where the plants have less dense foliage or where the leaves have dried or died out over time, corresponding changes in the temperature profile are observed. This bears out the importance of foliage density and the need for healthy growth of plants for the effective thermal performance of vertical greenery systems. In this regard, careful selection of plants which are suited to the peculiar conditions of the particular types of vertical greenery systems is imperative for the successful use of vertical greenery systems Ambient temperature From Fig. 7, trends in the ambient temperature of selected VGSs 1, 2 and 4 are compared with the control wall at various distances to determine the impact of vertical greenery systems on ambient temperature and their potential in producing a cooling effect on the immediate external environment. The ambient temperature readings obtained at 1.00 m away are found to be corrupted and hence not analyzed. For distance of 0.15 m away from the vertical greenery systems, the ambient temperature of the control wall shows the highest temperature throughout the day. This is followed by VGSs 2,1 and 4. All 3 vertical greenery systems showed similar temperature profiles and peak hours. During the daytime from 1000 h to 1800 h, the ambient temperature difference among the vertical greenery systems is more obvious. Ambient temperature may be affected by air circulation. Though both VGSs 1 and 4 are covered by well-distributed greenery, VGS 1 has thicker greenery near the temperature data logger which may block the air circulation and trap heat. Hence, it can be inferred that at a distance of 0.15 m away from the substrate, the ambient temperature is most affected by the presence of the vertical greenery systems. Table 4 Summary of average substrate surface temperatures from 24 Feb 08, 28 Apr 08 and 21 Jun 08. VGS Maximum reduction of average substrate surface temperature ( C) and corresponding time Diurnal range of average substrate surface temperature ( C) Type Date 24/2 28/4 21/6 24/2 28/4 21/6 Mesh system h 1305 h 1110 h Vertical interface h 1505 h 1815 h h 1300 h 1425 h h 1305 h 1420 h h Angled interface h 1300 h 1420 h Horizontal interface h 1250 h 0840 h 7a h 1415 h h h h

9 N.H. Wong et al. / Building and Environment 45 (2010) Table 5 Summary 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 At a distance of 0.30 m away from the substrate, VGS 1 has the highest ambient temperature throughout the whole day, even higher than the control wall. This may be caused by the thick greenery which is very near to the temperature data logger, restricting air circulation which helps to dissipate heat. The control wall and VGS 2 show a similar ambient temperature profile and only differed slightly between 0800 h and 1600 h. Hence, it can be inferred that VGS 2 is no longer influencing the ambient temperature. Lastly, VGS 4 shows the lowest ambient temperature throughout the day, showing that it is still influencing the ambient temperature around the environment. At a distance of 0.60 m away, the 3 vertical greenery systems and the control wall showed a similar ambient temperature. In the daytime between 0800 h and 1600 h, the control wall showed a slighter higher ambient temperature which is not very significant. Hence, it can be concluded that all 3 vertical greenery systems no longer influence the ambient temperature. 5. Conclusion 5.1. Surface temperatures The comparison of the effects of the 8 vertical greenery systems in HortPark on the reduction of wall and substrate surface temperatures is shown in Tables 3 and 4. In terms of the maximum reduction of average wall surface temperature as compared to the control wall, VGSs 4 and 3 show the best thermal performance. In terms of the diurnal range of average wall surface temperature (difference between the highest and lowest values), VGSs 4 and 1 show the highest capacities. The capacity of the vertical greenery systems to limit the fluctuation of wall surface temperatures of building facades is valuable in prolonging the lifespan of building facades and slowing down wear and tear as well as cost savings in maintenance and replacement of façade parts. The reason for the differences in the thermal performance of these vertical greenery systems can be a combination of various factors including substrate type, insulation from the system structure, substrate moisture content as well as the shade and insulation from greenery coverage. At the same time, the interactions between leaf area, geometry, colour and other microclimatic parameters such as solar radiation are complex and may result in different cooling efficiency during the day and night. These results point to the potential thermal benefits of vertical greenery systems in reducing the surface temperature of buildings facades in the tropical climate. Maximum reductions of Cinthe wall surface temperatures on clear days are observed respectively. This is a significant reduction in wall temperature that will lead to a corresponding reduction in the energy cooling load and consequent saving in energy cost. On the other hand, vertical greenery systems 4, 3 and 5 show the best thermal performance for the maximum average substrate surface temperature reduction. For the least diurnal fluctuation in average substrate surface temperature, no vertical greenery system performs relatively well, having a mixed range of values. VGSs 6 and 7a perform the worst, reaching a diurnal fluctuation beyond 10 C Ambient temperature The effects of vertical greenery systems on ambient temperature are found to depend on specific vertical greenery systems. VGS 2 has hardly any effect on the ambient temperature while the effects of VGS 4 are felt as far as 0.60 m away. From Table 5, reductions in the ambient temperature of up to 3.33 C are observed from VGS 4 at a distance of 0.15 m away. Given the preponderance of vertical surfaces and wall facades in the built environment, the use of vertical greenery systems to cool the ambient temperature in building canyons is promising. Furthermore, a cooler ambient temperature means that the air intakes of air-conditioning are at a lower temperature, translating into saving in energy cooling load Recommendations Results highlight that the various benefits of vertical greenery systems in the tropical environment are promising. To further establish these results, studies of vertical greenery systems should move on and be analyzed on actual building facades. By doing so, the performance of various thermal parameters may possibly reveal more insight. Furthermore, many factors such as the physical structure, materials and dimensions of the panels holding the substrate and plants species, substrate type, composition, depth and moisture content have an impact on the various performance of vertical greenery systems. However, these factors are not analyzed separately while keeping the rest of the other factors constant. Therefore, future experiments can be tailored to study the impact of these factors individually as well as to formulate a plant palette optimal for different greenery systems in Singapore. In all, with all the encouraging thermal results of vertical greenery systems, it is with anticipation that vertical greenery systems will gradually become one of the driving forces realizing Singapore s vision of attaining the status of City in the Garden. Acknowledgements This research was supported by the National University of Singapore, National Parks Board and Building and Construction Authority of Singapore under the collaborative research project titled Evaluation of Vertical Greenery Systems for Building Walls. References [1] Newton J, Gedge D, Early P, Wilson S. Building greener guidance on the use of green roofs, green walls and complementary features on buildings. London, UK: CIRIA; [2] Wilmers F. Effects of vegetation on urban climate and buildings. Energy and Buildings 1990;15: [3] Johnston J, Newton J. Building green: a guide to using plants on roofs, walls and pavements. London, UK: London Ecology Unit; [4] Peck SW, Callaghan C, Bass B, Kuhn ME. Research report: greenbacks from green roofs: forging a new industry in Canada. Ottawa, Canada: Canadian Mortgage and Housing Corporation (CMHC); [5] Bass B, Baskaran B. Evaluating rooftop and vertical gardens as an adaptation strategy for urban areas. 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