The Thermal Effects of Green Roofs and Green Façades on an Urban Canyon

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1 September 24 Page 1 of 6 The Thermal Effects of Green Roofs and Green Façades on an Urban Canyon Eleftheria Alexandri and Prof. Phil Jones Welsh School of Architecture, Cardiff University, Cardiff, Wales, UK ABSTRACT: This paper investigates quantitatively the thermal effect of plants in the formation of the urban canyon microclimate. Through this theoretic work an attempt is done to indicate whether covering building surfaces (roofs and walls) in existing urban environments with vegetation could be a realistic proposal for the mitigation of raised urban temperatures. A two-dimensional, dynamic heat and mass transfer model has been developed, describing the thermal exchanges in an urban canyon, due to both temperature and humidity concentration changes. A parametric analysis is done for diverse climates (dry, humid, and arid) and for different urban canyon geometries. The surface materials of walls and roofs are changed from construction ones to plants (grass and creepers) parametrically. Conference Topic: 3 Comfort and well-be ing in urban spaces Keywords: green roofs, green walls, urban canyon, microclimate, heat and mass transfer models INTRODUCTION Existing cities tend to have raised temperatures, compared to their surrounding rural areas [1]. This is mainly due to the use of construction materials with high thermal capacities, small porosity, low albedos, and the lack of vegetation in urban spaces. These raised temperatures can be uncomfortably high for the urban inhabitants, especially during hot seasons. In conjunction with urban pollution they can even be responsible for the loss of lives [2]. A way of lowering these raised temperatures in the whole urban scale is by covering urban surfaces (such as building walls and roofs ) with vegetation. Thus urban temperatures can be decreased by the evapotranspiration of plants, the change of surface albedo and the shading of construction materials. The extend of this reduction depends on the climatic conditions of the area (air temperature and humidity, irradiation, wind speed and direction), the characteristics of the urban materials (including their albedos) and the urban geometry. This paper presents the thermal effects which vegetation has on the built environment, when placed on building surfaces. An analysis is carried out for diverse geometries and climates. This calculation is done with a dynamical, two-dimensional heat and mass transfer micro-scale model, developed in Cardiff University, which describes the temperature and humidity distributions in an urban canyon. 2. METHODOLOGY 2.1 Method of Calculating Urban Temperatures Urban temperatures and humidities (both air and surface ones) are calculated dynamically with the use of heat and mass transfer equations. The impact of mass transfer on heat transfer has been considered to be crucial in the rational estimation of the impact of vegetation on temperatures for diverse humidity concentrations. This set of algorithms for building materials, soil, vegetation and air has been presented in [3]. The model formed by these algorithms has been programmed in C++. It is a two-dimensional one, describing the typical geometry of an urban canyon. Its outputs are temperature and humidity for each of its nodes, for both nodes in the air and nodes in the materials (including building materials, soil asphalt, sand and loam (underneath the asphalt layer), and vegetation). Wind distributions within and over the canyon are calculated with the CFD code winair4 [4]. Shading in the canyon is calculated with the software Ecotect [5]. A visual representation of the model is given in figure 1. Turban RHurban Turban RHurban Tin RHin Tground Hground Tin RHin asphalt sand loam Turban RHurban Figure 1: Representation of the two-dimensional urban canyon model.

2 September 24 Page 2 of 6 The air temperature and humidity at 1.m height above the roofs and at the boundaries of the buildings is assumed to have a that are the input of the meteorological data of an existing city (Turban, RHurban, as can be seen in figure 1). Air temperature and humidity inside the buildings (Tin, RHin) are assumed to be constant (2 o C, 5%, respectively). Temperature and humidity are also considered to be constant in 8.m depth in the ground (Tground, Hground, 23 o C and.41m 3 /m 3, respectively). 2.2 Canyon Geometries and Characteristics Two geometries of urban canyons are chosen, a "shaded" and an "exposed to the sun" one. Their dimensions are chosen according to the flow field which is created in the canyon; building heights and street widths are chosen in respect to whether their combination creates isolated roughness or skimming flow, when the wind flow is vertical to the canyon axis, according to [6]. With a constant building length of 4m, building heights and road widths are the following ones: Table I: Case studies of urban geometries and their characteristic dimensions. Case No Building Height Street width Type of flow A 1 m 5 m Skimming B 5 m 15 m Isolated roughness Three cases of vegetation on buildings are examined; Case i, where no vegetation is put, case ii, where both roofs are green and case iii, where both walls are green. The canyon s axis is the East-West one. Wind speed is constantly vertical to the canyon s axis. Walls and roofs are made of typical construction urban materials; for reasons of comparableness both of them are concrete, with 2cm width, for both buildings. The street is covered with1cm of asphalt. A 4cm layer of sand is underneath the asphalt and the ground is considered to have the characteristics of loam. Plants for roofs and for walls are considered to have the same characteristics, again for reasons of comparableness (grass for roofs and creepers for both canyon walls). The thermal and vapour characteristics of urban materials, soils and plants are summarised in tables II-IV. Table II: Thermal and vapour characteristics of constructive materials. Characteristic Concrete Asphalt Specific thermal capacity (MJ/m3K) Thermal Conductivity (W/mK) Vapour diffusivity (1-6 m 2 /s) Ratio of vapour.2.1 diffusion coefficient to total moisture diffusion coefficient Emissivity Albedo.23.1 Table III: Thermal and vapour characteristics of soils. Characteristic Soil Sand Loam Specific thermal capacity of solid soil (MJ/m 3 K) Hydraulic Conductivity (1-4 m/s) Moisture Potential, when soil is saturated (cm) Maximum volumetric water content (m 3 /m 3 ) Coefficient b Table IV: Thermal and vapour characteristics of plants. Characteristic Plant (grass and creepers) Convective heat 2 resistance (s/m ) Resistance 1 expressing the plant type (s/m) Level of soil moisture.25 below which permanent wilting of the plant occurs (m 3 /m 3 ) Emissivity.94 Albedo.23 Three cities, which are considered to be hot, but with diverse humidity concentrations are chosen as case studies; Athens (hot and dry), Riyadh (hot and arid) and Mumbai (hot and humid). Their thermal behaviour is examined on a diurnal scale. Their meteorological data are taken from Meteonorm [7], as averaged hourly data for a typical hot day. Athens and Riyadh are examined for a typical day in mid-july and Mumbai for a typical day in mid-may. Their climatic characteristics for these days are given in figures 2-3. Solar Radiation (W/m 2 ) Time (hours) Global Irradiation on a horizontal plane, Athens (July) Global Irradiation on a vertical plane with South orientation, Athens (July) Global Irradiation on a horizontal plane, Riyadh (July) Global Irradiation on a vertical plane with South orientation, Riyadh (July) Global Irradiation on a horizontal plane, Mumbai (May) Global Irradiation on a vertical plane with South orientation, Mumbai (May) Figure 2: Global irradiation on a horizontal and on a vertical plane with South Orientation, for Athens and Riyadh for a typical day in July and for Mumbai for a typical day in May.

3 September 24 Page 3 of Time (hours) Air Temperature, Athens (July) Air Temperature, Riyadh (July) Air Temperature, Mumbai (May) Relative Humidity, Athens (July) Relative Humidity, Riyadh (July) Relative Humidity, Mumbai (May) Figure 3: Air temperature and relative humidity for Athens and Riyadh for a typical day in July and for Mumbai for a typical day in Ma y Relative Humidity (%) Ts,South,Riyadh Ta,South,Riyadh Ta1,Riyadh Ta2,Riyadh Ta,North,Riyadh Ts,North,Riyadh Turban, Riyadh Figure 5: Temperature distributions for Riyadh, across the centre of the canyon (2.5m height) in case Bi (buildings height 5m, canyon width 15m, no vegetation). In case B, Ta1 is located at a distance of 3m from the South oriented wall and Ta2 is at 3m from the North oriented wall. 3. RESULTS AND DISCUSSION 3.1 Case i, No Vegetation on the the Buildings For all three climates, in case i, where no vegetation is input, the vicinity of the canyon has higher temperatures than its surrounding urban temperature. Case B has higher surface and air temperatures than case A (figures 4 and 5). This is mainly due to the fact that both the asphalt and the walls of case B are hardly shaded, due to its geometry (lower buildings height and bigger street width), although its wind velocities within the canyon are higher than in case A. In general surface roof temperatures (figure 6) tend to be higher than surface temperatures of walls with South orientation, due to the highest amounts of irradiation that the horizontal plain receives at the time examined (figure 2). Nevertheless, air temperatures above the roof are affected more by the distribution of the urban temperature (figure 6) than the temperatures within the canyon, which follow their own distribution (figure 5), affected by the surface temperatures (walls and asphalt) and the wind distributions within the canyon Ts,South,Riyadh Ta,South,Riyadh Ta1,Riyadh Ta2,Riyadh Ta,North,Riyadh Ts,North,Riyadh Turban, Riyadh Figure 4: Temperature distributions for Riyadh, across the centre of the canyon (5m height) in case Ai (buildings height 1m, canyon width 5m, no vegetation). Ts,South symbolises the surface temperature of the South-oriented wall, Ta,South, the boundary air layer temperature (3cm from the South wall), Ta1, the air temperature at 1m distance from the South wall, Ta2, the air temperature at 1m distance from the North wall, Ta,North, the boundary air layer temperature (3cm from the North wall) and Ts,North, the surface temperature of the North wall. This symbolism is used in all graphs Ts,r,Riyadh Ta,r,Riyadh Ta1,r,Riyadh Ta2,r,Riyadh Turban, Riyadh Figure 6: Temperature distributions for Riyadh, at the roof of case Bi. Ts,r symbolizes the roof s surface temperature, Ta,r, the boundary air layer temperature (3cm over the roof), Ta1,r and Ta2,r the air temperature at 2m and 4m height, respectively, over the roof. This symbolism is used in all graphs. 3.2 Case ii, Green Roofs Vegetation on the roof lowers temperatures at the vicinity of the roof (both air and surface ones ) quite significantly (figure 7). When vegetation is placed on a surface, its diurnal temperature fluctuations are much milder, with a lower discrepancy, of the magnitude of 5. o C. The temperature of the air above the green roof is decreased, reaching a maximum difference with the urban temperature of the magnitude of 6. o C at 2m height above the roof and 3. o C at 4m height. Aerial nodes at both levels follow the distribution of the urban temperature instead of the surface one, decreasing much faster in the evening hours than the vegetation s surface temperature (figure 7). When compared with case i, case ii results in high temperature decreases. The temperature decrease reaches at midday a maximum decrease of the magnitude of 3. o C between the surface temperature of a green and a concrete roof to the magnitude of 1. o C for the air layer at 4m height above the roof (figure 8). These figures (3. o C, 1. o C) varry up to 5. o C for the different climates, examined in this paper, as is discussed in paragraph 3.4. The impact of vegetation on the roof is not insignificant for the temperatures within canyon, as well. Although surface wall and asphalt temperatures are not influenced, a difference of the magnitude of

4 September 24 Page 4 of 6.5 o C to 4. o C is noted in the air in the canyon (figure 12). Generally, the highest differences are observed in case B. This is due to its smaller buildings height and to its broader canyon width. Its geometry allows the formation of higher wind velocities within the canyon, transferring cooler air masses from the roof to the canyon Tl,r,Mumbai Ta,r,Mumbai Ta1,r,Mumbai Ta2,r,Mumbai Turban,Mumbai Figure 7: Temperature distributions at a green roof for Mumbai, case A DTl,r,Mumbai DTa,r,Mumbai DTa1,r,Mumbai DTa2,r,Mumbai Figure 8: Temperature differences between a green and a concrete roof for Mumbai, case A. 3.3 Case iii, Green Walls The impact of green walls on the canyon temperatures is of a high magnitude; Air temperature within the canyon is lower than the urban air temperature (figure 1), reaching a maximum decrease of the magnitude of 8. o C for Mumbai, 9.8 o C for Athens and 14. o C for Riyadh. As the canyon s microclimate is affected less by the urban temperature than the roof s microclimate, the surface materials of its walls are very crucial for the fluctuations of its air temperatures. As can be seen in figure 1, the temperature fluctuations of the air nodes in the canyon follow the distribution of surface canyon temperatures than the distribution of the temperature of the urban air node. In general surface temperatures (of both South and North oriented walls) have a smaller discrepancy, of the magnitude of 5. o C for case B and 4. o C for case A. This difference is due to the fact that the South-oriented wall gets less amounts of direct solar radiation in case A than in case B. When compared with case i, green walls result in big temperature decreases within the canyon. Vegetation on the South-oriented wall plays an important role in the formation of the canyon s microclimate, as it lowers temperature to the magnitude of 33.5 o C (figure 11). North-oriented wall surface temperatures are decreased too, but to a much lower level than the South-oriented ones, of the magnitude of 8. o C. The effect of green walls on the roofs (which are plain concrete in case iii) is almost negligible. This is due to the wind distribution within the canyon; lower air velocities and vortices do not allow the air in the canyon to circulate and mix with the air on the roofs. Apart from that, cooler air masses move to lower levels. In this way they cool the street level, but not the roof one Tl,South,Athens Ta1,Athens Ta,North,Athens Turban, Athens Ta,South,Athens Ta2,Athens Tl,North,Athens Figure 1: Temperature distributions within the canyon for Athens, case A DTl,South,Riyadh DTa,South,Riyadh DTa1,Riyadh DTa2,Riyadh DTa,North,Riyadh DTl,North,Riyadh Figure 11: Temperature differences between the surface and air temperatures in the canyon for green and concrete walls for Riyadh, case B. 3.4 Differences in Temperature Distributions due to Humidity Concentrations In [3], with the use of a one-dimensional heat and mass transfer model, it had been concluded that the magnitude of the effect of vegetation on the built environment strongly depends on the humidity concentrations of the area. This paper, with a twodimensional model again reaches the same conclusions. As is obvious from figure 12, the impact of green roofs on the canyon is more notable in arid Riyadh, reaching 4. o C, for case A. The effect is less noticeable in humid Mumbai, due to its high humidity concentrations (figure 2), reaching a difference of the magnitude of 1.8 o C. Dry Athens, with its mild humidity concentrations is in between, reaching a 3.4 o C difference. Similar results are drawn for the effect of green roofs on the air above them (reaching a difference from 2.4 o C for Riyadh to 19.5 o C for Mumbai, at 2m height) and for the effect of green walls on canyon temperatures (reaching a difference from 12.6 o C for Riyadh to 9.3 o C for Mumbai, at the air layer at 1m

5 September 24 Page 5 of 6 distance from the South oriented wall). As is obvious, humidity concentrations have an important impact on the mitigation of urban temperatures due to vegetation. The arider the climate is, the higher the impact of the evapotranspiration of plants on the formation of its urban temperatures DTa1,Mumbai DTa2,Mumbai DTgrrf,a1,Athens DT,grrf,a2,Athens DTa1,Riyadh DTa2,Riyadh Figure 12: Temperature differences between the air temperatures in the canyon for a green and a concrete roof for Mumbai, Athens and Riyadh, case A. 3.5 Green Walls Versus Green Roofs It has been shown that green roofs have a significant effect on the mitigation of both surface and air temperatures in the vicinity of roofs. Apart from that, they can have an impact on temperature distributions in the air in the canyon as well. Their effect within the canyon is not as high as that of green walls, as they do not lower the surface temperatures of the canyon, which play an important role in the formation of the canyon s microclimate. When compared with a canyon without any vegetation, green roofs can lower canyon air temperatures up to the magnitude of 4. o C for the case of Athens (figure 12). On the other hand, green walls can have a much stronger impact on both air and surface canyon temperatures. Canyon air temperatures can decrease up to 1. o C (figure 12). Surface temperatures of walls decrease up to the magnitude of 3. o C. The combination of decreased surface and air temperatures, which green walls offer within the canyon, can lead to improved conditions of outdoors thermal comfort. Users of both street and buildings can enjoy more pleasant and healthier thermal conditions in hot climates, thanks to green walls. On the critical side, green walls do not influence at all temperatures at the roof vicinity. If they are implemented on an urban scale it is dubious whether they would influence temperatures at an urban scale; due to the wind circulation within the canyon, they can only have local effects, though significant ones. Apart from the thermal perspective, there might be cases where vegetation cannot be implemented on existing urban façades, due to the buildings use and/or architecture. On the contrary, vegetation can be applied quite easily on existing roofs. In most cases urban roofs are empty spaces, which are not in use. By placing vegetation on them, they could become a space of its own and not merely a layer covering the construction, approaching LeCorbusier s vision of roofs, in the same way the constructivists had tried to put green roofs in practice in the early Soviet Union [8]. From a thermal point of view, green roofs can affect the whole of the urban temperature much more effectively than green walls, although they do not improve the street level conditions as much as green walls. What should be implemented in existing cities, depends on the desired effect; local, with a strong impact on the street level, or for the whole urban scale, with a strong effect on the roof level. A combination of both green roofs and walls could be the ideal solution to the mitigation of raised urban temperatures DTgrwl,a1,Athens DT,grwl,a2,Athens DTgrrf,a1,Athens DT,grrf,a2,Athens Figure 13: Temperature differences with case i in air temperatures within the canyon due to green walls (case iii, suffix grwl) and green roofs (case ii, suffix grrf) for Athens, case A. CONCLUSIONS It was shown in this paper that vegetation can mitigate the climate of the built environment quite efficiently. The scale of its effect depends on where it is implemented (roofs or walls) and which part of the built environment is examined (inner canyon or roof vicinity). Temperature differences of up to the order of 3. o C were calculated, when vegetated surfaces were compared with concrete ones, in a day with high irradiation levels. Generally, it was found out that green walls have a stronger effect within the canyon than green roofs, but they do not effect the temperature of the air masses above the canyon. Green roofs, on the other hand, do have a strong effect on the air layer above them, of the magnitude of up to 1. o C at 4m height. Urban geometry plays an important role on the formation of surface and air temperatures, due to its impact on wind velocity distributions and the shading of urban surfaces. In general, the wider a street is, the stronger the effect is, especially from green walls, due to the shading of the construction materials from the plants and to the hogher wind speeds that occur within the canyon. The differences that are observed for the equivalent nodes within the canyon reach the magnitude of 1. o C for the surface temperature of the South-oriented wall (comparison between the difference of caseai-caseaiii to casebi-casebiii). For the green roofs (comparison of caseai-caseaii with casebi-casebii) these differences are lower on the

6 September 24 Page 6 of 6 roof, reaching a maximum of 5.5 o C and they are negligible within the canyon. The climate of the area is a crucial factor for the magnitude of the thermal effect of vegetation on the built microclimate. Arider climates tend to benefit more from the existence of vegetation; due to the lower humidity levels of the climate, the evapotranspiration of plants, which changes humidity concentrations has a stronger effect on temperatures. ACKNOWLEDGEMENTS This research has been funded by the National Scholarship Foundation of Greece (IKY). The authors are extremely grateful to the computer programmer, Panagiotis Doussis, for his contribution and invaluable support with computer programming. REFERENCES [1] T. R. Oke, Boundary Layer Climates. Methuen & Co LTD, London, [2] S. Kovats, T. Wolf and B. Menne, Heatwave of August 23 in Europe: Provisional Estimates of the Impact on Mortality. Eurosurveillance Weekly vol. 8, no. 11 (24). [3] E. Alexandri and P. Jones, Micro-Scale Modelling of the Thermal Effects of Green Roofs, Proc. PLEA23, Santiago - Chile (23). [4] Welsh School of Architecture, Cardiff University, WinAir4, Cardiff, 23. [5] A. Marsh, Ecotect, Version 5.2(b), Square One Research PTY LTD, 23. [6] T. R. Oke, Street Design and Urban Canopy Layer Climate, Energy and Buildings, Vol. 11 (1988). [7] J. Remund, R. Lang, and S. Kunz, Meteonorm, Version 3.. Meteotest, Bern, [8] W. J. R. Curtis, Modern Architecture, Since 19. Phaidon, Oxford, 1982.