![[logo] US EPA](https://webarchive.library.unt.edu/eot2008/20081108210740im_/http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
What Can Be Done
- Denotes link to glossary
definition
There are a number of steps that communities can take to lessen the impacts of heat islands. These "heat island reduction strategies" include:
- Installing cool or vegetated green roofs;
- Planting trees and vegetation; and
- Switching to cool paving materials.
Workers apply a cool roof coating on a rowhouse in Baltimore, MD.
The extent to which urban areas can benefit from heat island reduction strategies depends on several factors. Some of these factors, like prevailing weather patterns, geography, and pollution transported from up-wind regions, are largely beyond the influence of local policy.
However, factors such as land-use patterns,
materials used in road and building construction, and
the coverage of urban trees and vegetation can be
directly affected by decision makers. This is where
policies and programs to reduce the impacts of heat
islands – and achieve related environmental and
energy-savings goals – can be most effective.
Heat Island Mitigation with Roofs
On a hot, sunny, summer day, traditional roofing materials may reach summertime peak temperatures of up to 190°F (88°C). By comparison, cool roofs only reach peak temperatures of 120°F (49°C).
The term "cool roof" is used to describe roofing
material that has high solar
reflectance , or albedo . These materials reflect a large portion of the sun's
energy. Cool roofs also may have a high thermal emittance , and thus release a large percentage of absorbed heat.
This keeps the material cooler and helps to reduce the
heat island effect.
There are two types of cool roofs: those used on low-slope or flat buildings (primarily commercial) and those used on steep-sloped buildings (primarily residential). Most cool roof applications for low-slope buildings have a smooth, bright white surface to reflect solar radiation, reduce heat transfer to the interior, and reduce summertime air conditioning demand. Most cool roof applications for steep-slope buildings come in various colors and can use special pigments to reflect the sun's energy.
Another alternative to traditional roofing materials is a rooftop garden or "green roof." Installed widely in a city, green roofs contribute to heat island reduction by replacing heat-absorbing surfaces with plants, shrubs, and small trees that cool the air through evapotranspiration (or evaporation of water from leaves). Planted rooftops remain significantly cooler than a rooftop constructed from traditional heat-absorbing materials. Further, green roofs reduce summertime air conditioning demand by lowering heat gain to the building.
Heat Island Mitigation with Trees and Vegetation
Increasing the cover of trees and vegetation in a city is a simple and effective way to reduce the urban heat island effect. Trees provide a wide range of other benefits, from increasing property value to reducing storm water runoff.
Shade trees also can make homes and buildings significantly more energy efficient. Scientists estimate that strategically planting trees and vegetation reduces cooling energy consumption by up to 25%. For many, this research comes as no surprise—trees have been used to cool homes for hundreds of years.
Heat Island Mitigation with Paving Materials
This pervious lot in Fair Oaks, California's Miller Park features 23 mature Olive trees, which provide shade and give the parking lot a natural look.
Cool paving materials minimize the absorption of solar heat and the subsequent transfer of this heat to the surroundings.
There are two types of cool paving materials: lighter-colored materials and porous materials. Lighter-colored materials have higher solar reflectance, so they absorb less of the sun's energy and stay cooler. Lighter-colored materials come in shades of white, beige, light gray and terra cotta.
Permeable, or porous, pavements allow water to filter into the ground, keeping the pavement cool when moist. Permeable pavements can be constructed from a number of materials including concrete, asphalt, and plastic lattice structures filled with soil, gravel, and grass.
Although there is no official standard or labeling program to designate cool paving materials, communities interested in reducing the heat island effect may consider surface reflectance and permeability – along with other costs and benefits – when selecting a paving product.