Urban Heat Islands

Rooftop prairie garden in Chicago, IL

Many cities in the United States experience temperatures up to 5.6° Celsius warmer than the surrounding countryside. This phenomenon—where air and surface temperatures become elevated and drought conditions more pronounced than in nearby rural areas—is known as the urban heat island effect.

The effect occurs because urban surfaces, such as roads and buildings, tend to absorb heat. There are also far fewer trees and other plants to provide cooling through the process known as transpiration.

Among the biggest heat absorbers in cities are rooftops. Rooftops typically make up 5 to 35 percent of the urban landscape, and the U.S. Environmental Protection Agency (EPA) reports that over 90 percent of the rooftops in the United States are dark in color. As a result, these low-reflectance, heat-absorbing surfaces can reach temperatures of 66° to 88° Celsius during the summer.

Because of these conditions, more energy is needed to cool the interior spaces within buildings, leading to higher peak electricity demands and conditions where power grids can become overburdened. The level of air pollution in a city can also be intensified by the urban heat island effect. For example, higher temperatures in cities can increase levels of ground-level ozone, a pollutant that causes respiratory problems.

Green roofs counteract the heat island effect

Green roof technology can help reduce urban heat islands and conserve energy. Green roofs have been shown to keep rooftops cooler than conventional roofs even when a green roof isn’t irrigated. Green roofs can also reduce energy use across different climates, depending on the depth of the substrate, or growing medium, used for plants. In research studies, for example, energy use has dropped by 10 to 30 percent when a building has a green roof versus a conventional one. And builders can often recoup the extra cost constructing a green roof in energy savings.