Proposal for green roofing in a city to combat urban heat effect.

Photo Courtesy of Diane Cook and Len Jenshel National Geographic

Using Green Roofing to Combat the Urban Heat Effect

By Caitlin Connelly          December 10, 2017

In March 2017, France passed a law that requires the roofs of all new commercial buildings to be at least partially covered by a renewable energy system or a green roof. The new law is, of course, focused partially on energy generation, which is a topic of interest in many discussions of climate impacts around the world, but also includes green roofing as a measure to assist with building efficiency and local climate regulation. When urban areas expand and convert natural areas to less permeable man-made material, a distinct warming in the “microclimate” is observed. This is known as an urban heat island, or the urban heat effect. Policies are starting to incorporate green roofing as a way to both regulate urban climates and mitigate global changes.

Known as the “urban heat effect,” cities tend to experience warmer temperatures than more rural areas located in similar regions, caused by these surface modifications. The microclimates observed in urban areas are due to modifications in surface features from natural to man-made which change the typical energy balance of the system. As cities are built up and population expands, manufactured materials like asphalt and concrete replace the original vegetation. Shifting the surface from vegetation to man made materials leads to higher rates of solar radiation absorption, decreased rates of convective cooling, and lower rates of evaporative cooling. Man made materials like asphalt and concrete store much more energy than natural vegetation because of their low albedo and high heat capacity, which leads to higher surface temperatures. Policies like those in France are beginning to mandate green roofing as a way to mitigate these impacts on local climate.

In rural ecosystems, vegetation is responsible for capturing much of the available energy. In addition, plants also drive the flow of water through a combination of evaporation and transpiration. This process, also known as evapotranspiration, regulates the local climate and decreases surface temperatures. In cities evapotranspiration has been less able to assist with temperature regulation for a few reasons. First, typical city plans leave few green spaces for vegetation to actually exist. Second, urban areas replace existing vegetation with impermeable surfaces that increase runoff and decrease soil absorption. Since there are both fewer plants and less water in the soil for them to use, the rate of evapotranspiration in urban areas are much smaller than in rural, vegetated areas. To reverse the negative impacts of reducing A city utilizing its roof surface area with vegetation would have the potential to reverse this process.

Green roofs have the potential to both mitigate local heat effects and global climate changes by reducing the amount of energy needed for heating and cooling buildings. Compared to a typical black roof, green roofs are able to keep the building below cooler by isolating it from solar energy. A typical black roof made of tar, bitumen, or gravel will have an albedo of between 0.1 to 0.2, meaning that it reflects 10-20% of the incoming energy. In comparison, green roofs covered in vegetation are able to reflect up to 85% of the incoming solar energy. In addition to reflecting energy, green roofs also acts as insulation which can reduce energy consumption, especially in buildings that were built before modern insulation standards. Studies have shown that buildings with green roofs experience around 70-90% less heat gain in the summer and 10-30% less heat loss in the winter, making them an advantageous feature with the potential to reduce both costs and emissions.

The release of anthropogenic energy and emissions create a positive net energy balance in the urban climate system, further adding the the urban heat island effect. The urban heat effect can raise peak electricity load, increase pollutant emissions, increase heat related illness, and decrease water quality. Today, scientists and engineers alike study the urban heat effect to understand these impacts, especially because of the energy use implications of the effect on an already changing climate. Green roofs have the potential to mitigate some of the major impacts of the urban heat effect by decreasing the absorption of solar energy, increasing evaporative cooling, and decreasing the energy consumption of buildings. Although adding small amounts of green space have not appeared to make a large impact, policies like those in France requiring green roofing in all new buildings may be a good way to start to mitigate the processes driving the urban heat effect.



Green roofs; building energy savings and the potential for retrofit


U.S. Environmental Protection Agency. 2008. Reducing urban heat islands: Compendium of strategies. Draft.


The urban heat island effect, its causes, and mitigation, with reference to the thermal properties of asphalt concrete


Utilising green and bluespace to mitigate urban heat island intensity



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