If mention of urban heat islands has you scratching your head, the confusion ends now. The U.S. Environmental Protection Agency (EPA) offers some basic information and helpful tips on how you can help mitigate the problem of urban heat islands. According to the EPA, “Heat islands are characterized by urban air and surface temperatures that are higher than nearby rural areas.” The cause of this rise in urban temperatures is a result of the following:
- A reduction in the amount of trees and vegetation in urban environments, which minimizes cooling from shade and evaporation of water from soil and leaves.
- High-rise buildings combined with narrow streets trap heat between them and reduce wind flow.
- Vehicles, factories, and air-conditioners produce waste heat that adds warmth to the air.
Aside from elevated summertime temperatures, the negative effects of urban heat islands range from increased peak energy demand and air-conditioning costs, to higher air pollution levels and heat-related illness and fatalities.
Communities can reduce the effects of urban heat islands in four ways - cool roofs, trees and vegetation, green (or garden) roofs, and cool pavement.
According to the EPA, “On a hot, sunny day, traditional roofing materials can reach peak temperatures of 190 degrees F. By comparison, cool roofs reach maximum temperatures of 120 degrees F.” Reducing the surface temperature of roofing materials by installing systems with high solar reflectance can result in lower energy bills, peak energy demand, and, consequently, reduced air pollution and greenhouse gas emissions.
Trees and Vegetation
The increase in a city’s vegetative cover is beneficial because in addition to cooling from direct shading, trees and plants cool the air through a process called evapotranspiration. The EPA reports that scientists at Lawrence Berkeley National Laboratory estimate planting trees and vegetation for shade can result in a 25-percent reduction in a building’s cooling energy consumption annually.
Green (or Garden) Roofs
Rooftop gardens or “green roofs” replace heat absorbing surfaces on the roof with vegetation that work to counteract the effects of urban heat islands via evapotranspiration. There are two types of green roofs - intensive and extensive - that are distinguished by the amount of soil and plant cover installed on the roof. “This reduces summertime AC demand by lowering heat gain to the building,” reports the EPA.
While the EPA reports that cool paving research is still in its infancy, early studies show that large parking areas, terminal facilities, airfields, and urban roadways with large expanses of paved surface may be practical applications. With a higher solar reflectance, cool paving materials (although not mandated by any standard or labeling program) can lower surface temperature and even allow stormwater to pass through.
This information was excerpted from the U.S. Environmental Protection Agency’s informative brochure “Cooling Summertime Temperatures: Strategies to Reduce Urban Heat Islands.” For more information, visit (www.epa.gov/heatisland).