Back to top of the page

Back to the List of Participants with Research Papers

Back to Global Dialogue

Back to the Portal of the Global Community organization

Adapting Urban Areas to Climate Change with Vertical Gardens

Brad Bass
Adaptation & Impacts Research Group
Institute for Environmental Studies
University of Toronto

Roger Hansell
Institute for Environmental Studies
University of Toronto

Glenda Poole
Division of the Environment
University of Toronto
An abbreviated version of this paper was submitted to the Alternatives Journal on July 28, 1999.

Abstract

Increasing vegetation in urban areas will reduce the urban heat island, and the impacts of other urban environmental problems, which will be exacerbated under climate change. Reducing the urban heat island will also reduce the energy demand for space conditioning, and hence greenhouse gas emissions. Plants directly reduce the urban heat island through evaporative cooling but further reduce energy consumption through shading. The most common strategy to increase urban vegetation is to plant trees at ground level. However, where space is not available for trees, vegetation can be grown on building roofs, but walls offer far more space, hence vertical gardening is a viable alternative.

Data were collected on the evaporative cooling capacity of different vertical garden designs at three different sites at the University of Toronto. An infrared pyrometer was used to measure the surface temperature of a vertical garden, a light-coloured wall and a black surface. The data were compared using an analysis of variance, and in all cases, the vertical garden was significantly cooler than the other two surfaces. This reduction in surface temperature could reduce the urban heat island, providing a strategy for mitigation and adaptation, and achieve other environmental and social benefits.

Introduction

Climate change is expected to result in warmer summer temperatures and more frequent heat waves.

For Canada, climate change models predict an increase in both winter and summer temperatures which is expected to reduce the demand for space heating while increasing the demand for space cooling.

The effect at night could be exacerbated further under climate change as, since 1950, the observed increases in minimum land surface temperatures has been about double those in the maximum temperatures.

Climate change is also expected to impact the supply of energy. Water supplies that are used for hydroelectricity and cooling of thermal and nuclear plants, and water bodies that are used for shipping fuel, such as the Great Lakes, are sensitive to changes in many climatic variables.

These impacts will be exacerbated in urban areas, due to the lack of vegetation and the large amount of concrete and asphalt. Vegetation is important because it makes moisture available for evapotranspiration, the combination of evaporation of water from plant tissue and the evaporation of water from the soil. Evapotranspiration utilizes a significant amount of incoming solar energy cooling both the leaf surface as well as the air. The energy used for evapotranspiration is embodied in the water vapour, which is removed from the surface.

A lower fraction of vegetative cover in the city reduces the available moisture to direct incoming solar radiation towards evapotranspiration. The non-vegetated surfaces absorb the incoming solar radiation and reradiate it as heat. This heat artificially elevates urban temperatures, a phenomenon known as the urban heat island.

The higher temperatures increase the demand for air conditioning which pumps more waste heat into the environment, increasing the heat island. Air conditioning requires electricity that is often generated by the burning of fossil fuels leading to increased greenhouse gas (GHG) emissions. Without air conditioning, the higher temperatures will increase the incidents of morbidity and mortality due to heat stress.

The urban heat island can also exacerbate other atmospheric problems associated with health. Warmer surfaces heat the overlying air, causing it to rise and carry pollutants higher into the atmosphere. As it rises it cools and disperses over a wider area, dispersing any pollutants that may have been present, over a wider area. With increased use of air conditioning comes the increased risk of releasing additional chlorofluorcarbons into the atmosphere, chemical compounds that are responsible for the reduction of the stratospheric ozone that is necessary to protect us from harmful amounts of ultra-violet radiation. The combustion of fossil fuels to generate electricity contributes other pollutants into the atmosphere, such as sulfur dioxide, nitrous oxides and particulate matter, which have been linked to respiratory health problems.

The replacement of vegetation by hard surfaces in urban areas has other environmental impacts. Rainwater and snowmelt cannot penetrate these surfaces, thus they runoff, eventually taxing the capacity of the sewage system. As water runs off over these surfaces, it also absorbs a range of chemicals that either enter the sewage system or local groundwater supplies. Plants can filter some of the pollutants out of water. In addition, leaves filter some pollutants out of the atmosphere. Thus the urban heat island is a serious environmental issue, and increasing vegetation in urban areas is a desirable strategy for mitigation of GHG emissions and adaptation to a broad range of atmospheric and other environmental issues.

Vegetation can reduce the urban heat island through evapotranspiration and shading a building surface. With a larger fraction of vegetated surface, more of the incoming solar energy is consumed by evapotranspiration, and less is absorbed and reradiated as heat. Shading reduces both the heat flow as well as the incoming solar energy. Light-coloured surfaces can also reduce the urban heat island by reflecting a high percentage of the incoming solar energy so that it is not absorbed and reradiated as heat. However, removing dark colours from the urban environment will not address the other environmental problems that have arisen due the removal of vegetation.

Planting trees at ground level is the most common strategy for restoring vegetation. In addition to reducing the urban heat island and GHG emissions through reducing energy consumption, shade trees can further offset GHG emissions through the sequestration of carbon in their woody mass. However, higher land use densities or space restrictions in some parts of the city restrict the space required to allow trees to reach their full potential. In these cases, rooftop gardens could provide many of the same benefits, and they would take advantage of the unused roof space that is available in most urban areas. In addition, depending on its colour, the heat from bare rooftops can exacerbate the urban summer temperatures and the associated air quality problems.

Reducing the rooftop temperatures would further reduce GHG emissions in both the summer and the winter. In the summer, a typical insulated, gravel-covered rooftop temperature can vary between 60oC and 80oC.

These temperatures increase the cooling load on a building in two ways. Since the internal temperature underneath the roof is lower, the heat will always flow from the roof into the building, thus the need for air conditioning. In addition, modern high-rise buildings are constantly exchanging the internal and external air. Because of the high roof temperatures, the temperature of this external air that is brought into the building’s ventilation system is very high, requiring additional energy for cooling.

Evapotranspiration from rooftop vegetation could reduce temperature of the roof, maintaining it at 25oC on a hot, sunny day.

This would reduce the amount of heat flow into the building through the roof. The lower rooftop temperature would also reduce the temperature of the external air that is exchanged with the building’s air. The external air temperature could also be reduced if the rooftop garden is designed so as to shade the intake valves.

These lower rooftop temperatures could reduce the KWh used for air conditioning by 10%, and this is a very conservative estimate. The actual value would depend on building size, design, orientation, insulation, window type and other internal factors that contribute to heat gain or dissipation. To maintain the cooler roof temperatures it is important to install enough rooftop gardens so as to minimize the potential horizontal movement of hot air from adjacent rooftops that are still bare. During the winter, the rooftop garden would provide additional insulation, which would reduce the flow of heat through the roof. One typical type of installation in Europe, consisting of 20 cm of growth substrate and 20-40 cm of grass would provide the equivalent of 15 cm of rockwool, or and insulation factor of R2011, which is enough insulation to provide a significant reduction in heating. These reductions in summer cooling and winter heating suggest that rooftop gardens could play a significant role in reducing GHG emissions in addition to their role as an adaptation strategy.

An even greater amount of space is available for vegetation on the exterior walls of the buildings in urban areas, and we propose growing vegetation on walls to create a vertical garden. A vertical garden refers to vines growing adjacent to a wall, window shades that utilize plants, or a vertical hydroponic system. Vertical gardens increase the amount of vegetative surface in urban areas, increasing evapotranspiration, and can be used for direct shading as well. In areas that are suitable for trees, they can also to be used to cover windows that cannot be shaded by trees due to the height or specific design features of a building.

Despite the large amount of available space, there are very few studies on vertical gardening. Rooftop gardens are receiving increasing coverage, particularly as a green roof industry has been developing in Germany, Austria and France, yet no similar industry has emerged to promote vertical gardening. The lack of attention to vertical gardens is also evident in a recent report on status of rooftop and vertical gardening in Canada. Most of the examples dealt with roofs, as the requisite information on vertical examples was not available.

To address this gap, three different sites and different types of vertical gardens were evaluated at the University of Toronto between 1996 and 1998. The vertical gardens were cooler than a black surface, but they were also cooler than the light-coloured walls at each site. Before discussing our results, we will define vertical gardens, provide some simple examples of how they might work, and review previous studies on the cooling effect of vertical gardens.

Defining Vertical Gardens

Vertical gardening is a comprehensive term referring to any manner in which plants can be grown on, up, or against the wall of a building such as a vine, as part of a window shade, as a balcony garden, or in a vertical hydroponic system.

As a window shade, plants can be grown in a planter box installed below a window, or hanging plants can be suspended above a window and used as a part of an awning. Since a building's façade typically encompasses four times the area of the roof, vertical gardening may offer a greater potential impact than rooftop gardening, without the additional concerns about weight. For a high-rise building, the area of the walls is typically twenty times the area of the roof. To allow some natural light into the room, the vertical garden could be installed on a moveable louvre, or installed as part of adjustable awning, so that it could be maneuvered to intercept only direct sunlight. Additional design considerations are also required to cope with high wind speeds for plants and planter boxes located above eight stories.

More specific engineering considerations are beyond the scope of this article.

How Vertical Gardens Reduce Air Conditioning and the Urban Heat Island

There have been very few studies on the potential effect of vertical gardening on reducing either energy consumption or the urban heat island. In the summer, air conditioning is reduced through shading of windows and evapotranspiration. Since many buildings are already insulated, the largest impact of shading is on reducing the solar energy transmitted through the windows as this energy can be absorbed by objects in the room and reradiated as heat.

The effect of shading a window can be estimated by computing the monthly avoidance of cooling load with the following formula:

From Heated Rockpiles to Forest Ecosystems: Redesigning Cities in a Restorative Economy

Brad Bass
Adaptation and Impacts Research Group
Institute for Environmental Studies
University of Toronto

Roger Hansell
Institute for Environmental Studies
University of Toronto

What should our cities look like in a restorative economy? This question is not often asked in discussions on industrial ecology or biomimicry (an attempt to mimic biological processes in energy production, materials production, information storage, etc). A focus on the factory or office without considering the larger locational context in which these activities take place will not fully address problems of water quality, water supply and air quality, not to mention climate change.

In hindsight. there were several decades of research to support our contention concerning cities, but it was a 1989 aerial view of landscapes, ranging from rock quarry’s to forests, that was our initial stimulus. Using an infrared camera, Jeff Luvall and H. Holbo found that rock quarries were absorbing the incoming solar energy and reradiating it as thermodynamic waste, i.e. heat.

However, further analysis revealed that the vegetated landscapes, especially the forest, were up to 25oC cooler than the rock quarry, which meant that the same amount of incoming energy was not reradiated back into the atmosphere. In fact, almost 50% of the energy was being used for evapotranspiration, the process in which a plant moves water from the soil, through the roots, to the surface of its leaves from where it can be evaporated. When energy is used for evaporation, it gets bound up into molecules of water vapour and is only released when the water vapour becomes some form of precipitation. In other words, approximately half of the suns energy is converted by a rock quarry into heat, while evaporation from plants stores and removes the energy from the surface.

Much of any city is a negation of the forest, at least at the surface, but this is where many of the problems begin. Those urban surfaces that are devoid of vegetation behave like a rock quarry, creating islands of heat in the landscape. Northern cities are already using more energy for air conditioning than winter heating, and as the climate changes, these cities may see even warmer summers and more frequent heatwaves. In addition, there are impacts on air quality, as rising columns of hot air serve to disperse pollutants to other parts of the city. This phenomenon, known as the urban heat island, has been well documented in journals of climatology.

The centralized systems also contribute additional waste heat to the urban environment as heat that is extracted from the building’s internal air is expelled through the air exchange with the atmosphere. Of course, the air brought in from the outside is even warmer due to the urban heat island, requiring additional energy for cooling. Although not as prevalent as a few years ago, there are still many examples of buildings offering glimpses of that most unsightly of urban adaptation to summer heat, the window air conditioner. Ignoring the aesthetics, any air conditioning system not only uses energy, but is a possible contributor to the expansion of the ozone hole. The impermeable surfaces of cities also result in more stormwater runoff with the attendant problems for combined sewer overflow and water quality.

An obvious solution is to plant more trees, particularly in the suburbs given the availability of space. In the urban core, space at ground level is more limited, unless we are prepared to sacrifice roads and sidewalks. As roads and parking lots crack, they could be converted to hollowform construction which permits significant amounts of vegetation. Unfortunately, this is three times as expensive as replacing asphalt with asphalt, which reflects the thermodynamic as much as it does the economic cost of restoration.

A simpler solution might be to remember that cities exist in three not two dimensions. If space is needed for vegetation, the largest amount is on the walls and roofs of the buildings, a solution that is often found in many natural environments, such as cliffs and ravines. Placing vegetation on roofs and walls, or rooftop and vertical gardening, is not a new idea, but was widely practiced throughout history for many of the reasons that they have attracted advocates today — lack of green space, altering the indoor temperature of a building and aesthetics.

The bulk of the weight of this living system is due to the soil, especially when it is wet, but there are much lighter alternatives that cut this weight significantly. Vertical gardens are not limited strictly to vines. Many plants can be incorporated into designs for windowshades or vertical hydroponic systems, and the Austrian artist Hundredwasser has redesigned the window to allow for trees, placed in pots within the building, to become part of a vertical garden. In addition, a vine will prolong the life of good quality masonry, but it could also be trained to climb up a trellis adjacent to the building so as to completely separate it from the wall. In 1995, Roger Hansell designed the first window shade with scarlett runner beans, and the following summer Brad Bass, Roger Hansell and two students, Monica Mucka and Glenda Poole, designed two more using a wider variety of plants.

Would a city transformed in this manner exhibit the thermodynamic behaviour of a forest? Data from Europe and some simulations of urban climates suggest that vegetated roofs would reduce the urban heat island. Our own infrared data, collected at the University of Toronto with plants incorporated into window shades and a vine covered wall, confirms these findings. In addition, vegetated cities would confer some of the other benefits of forested landscapes such as improving water quality and reducing runoff, providing habitat to support biodiversity and providing sink for organic waste and closing that particular waste loop. Primarily, rooftop and vertical gardening would begin to restore the green space which many ecologists have argued would limit our impacts on the surrounding ecosystems that help sustain our economy.

Notes

Notes

1. D. Etkin, "Climate Change and Extreme Events: Canada," National Cross-Cutting Issues, Vol. VIII of the Canada Country Study (Toronto, Environment Canada 1998), pp. 31-80.

2. G. Mercier, "Energy Sector," National Sectoral Issues, Vol VII of the Canada Country Study: (Toronto, Environment Canada 1998), pp. 384-403.

3. E.B. Horton, "Geographic Distribution of Changes in Maximum and Minimum Temperatures," Atmos. Res, 37:2 (1982), pp. 102-17.

4. G. Mercier ibid.

5. Two other factors also exacerbate, albeit to a lesser degree, the urban heat island: the release of waste heat into the atmosphere from power consumption, air conditioning and the air exchange of buildings with the atmosphere and the low albedo of urban areas. D.J. Sailor, "Simulations of annual degree day impacts of urban vegetative augmentation," Atmospheric Environment, 32:1 (1988), pp. 43-52.

6. L.S. Kalkstein and K.E. Smoyer, "The Impact of Climate Change on Human Health: Some International Implications," Experientia, 49:11 (1993), pp. 969-79.

7. W. H. Terjung and P. A. O’Rourke, "Energy Input and Resultant Surface Temperatures for Individual Urban Interfaces, Selected Latitudes and Seasons," Arch. Met. Geoph. Biokl, Ser B, 29:1 (1981), pp.1-22.

8. W. H. Terjung and P. A. O’Rourke, "Relative Influence of Vegetation on Urban Energy Budgets and Surface Temperatures," Boundary Layer Meteorology, 21:2 (1981), pp. 255-63.

9. S. Peck, C. Callahan, B. Bass and M. Kuhn, Greenbacks from Green Roofs: Forging a New Industry in Canada. Status Report on Benefits, Barriers and Opportunities to Green Roof and Vertical Garden Technology Diffusion, (Ottawa, Canada, Canada Mortgage and Housing Corporation, 1999), p. 24.

10. S. Peck, C. Callahan, B. Bass and M. Kuhn ibid, p. 24.

11. S. Peck, C. Callahan, B. Bass and M. Kuhn ibid, p. 23.

12. S. Peck, C. Callahan, B. Bass and M. Kuhn ibid, Appendix 2.

13. S. Peck, C. Callahan, B. Bass and M. Kuhn ibid, p. 15.

14. Ibid.

15. Internal shades reduce transmission from 62 to 30% for single paned windows, and for double paned windows the transmitted radiation varies between 56 and 33%. Only external shades are able to cut the amount of transmitted radiation to 20%, or even 10% in some cases. T.A. Markus and E.N. Morris, Buildings, Climate and Energy (London, Pitman Publishing Ltd, 1980), p. 330.

16. R.D. Brown and T.J. Gillespie, Microclimatic Landscape Design (New York, John Wiley & Sons, 1995), p. 115.

17. The calculations are based on the method of R.D. Brown and T.J. Gillespie, ibid, p. 53.

18. R.D. Brown and T.J. Gillespie, ibid, p. 52.

19. S. Peck, C. Callahan, B. Bass and M. Kuhn ibid, p. 24.

20. A. Hoyano, "Climatological Uses of Plants for Solar Control on the Effects on the Thermal Environment of a Building," Energy and Buildings, 11:1-3 (1988), pp. 181-89.

21. The Japanese study compared the surface maximum temperature of a concrete wall with a Japanese ivy. The maximum ivy temperature was 1oC below the outdoor temperature while the concrete wall was 10oC above the outdoor temperature. However, the average surface temperature of the ivy and the bare wall were respectively 1oC and 3oC above the outdoor temperature.

22. F. Wilmers, "Green Melioration of Urban Climate," Energy and Buildings, 11:1-3 (1988), pp. 289-99.

23. D. Holm, "Thermal Improvement by Means of Leaf Cover on External Walls - a Simulation Model," Energy and Buildings, 14:1 (1989), pp. 19-30.

24. F. Wilmers, ibid.

25. D. Etkin, ibid.

26. S. Peck, C. Callahan, B. Bass and M. Kuhn ibid, p. 30, 38.

For a journal:

A.B. Author, "Article Title," Journal Name, 2:3 (1982), pp. 123-25.

For a book:

A.B. Author, Book Title (Place of publication: Publisher, 1982), p. 101.