The Urban Heat Island and Roof Garden

The Urban Heat Island Effect is the difference you feel when you go from an asphalt parking lot to a tree-shaded park on a hot summer day. It is also the difference in temperature between a city and the rural area that surrounds it. 
The key to reducing the Urban Heat Island Effect is to reduce the total area of dark, heat-absorbing surfaces such as rooftops and pavement.

The use of vegetation on a roof is an excellent option. Plants reflect heat, provide shade, and help cool the surrounding air through evapotranspiration. A rooftop garden cuts the energy use within the building, especially for cooling. The insulation a garden provides helps conserve both heating and cooling energy.
Rooftop gardens absorb rainfall and reduce urban runoff that otherwise would collect pollutants and empty into sewers. A rooftop garden filters and moderates the temperature of any water that is released to the sewer. In addition, plants actually filter the air. Plants improve air quality by using excess carbon dioxide to produce oxygen. 

On a neighborhood or regional level, temperatures are lowered and air pollution is reduced when the overall area of dark surfaces is reduced and the area of reflective and shaded surfaces is increased.  The layers of a rooftop garden protect the constructed roof from damage and can extend both the warranty and the useful life of the roof. A rooftop garden can add usable leisure space to a property that is attractive not just to people, but to wildlife such as birds and butterflies.

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