Heat Island Effect
Dark paved surfaces on our home sites and dark roofing materials absorb solar energy, heating the microclimate around our homes. This is referred to as the heat island effect. The temperature in urban areas can be as great as ten degrees warmer than the surrounding countryside. This excess heat is transferred into our homes by convection and conduction. For our homes to be comfortable they must be cooled by using larger HVAC equipment and more electricity.
There are two significant ways to reduce the heat island effect.
1) Reroof our homes with light-colored roofing and use light-colored pavers and light-colored concrete that reflects the heat. This reflective ability is measured by material’s solar reflectance index (SRI). The higher the SRI the greater the rejection of solar heat.
2) Plant shade trees and reduce the amount of paved surfaces.
For paving materials to be considered “cool” the solar reflectance index should be greater than 29. Pavers, shingles and metal roofing materials are marked with their SRI. The SRI isn’t always apparent for other building materials, so here are examples of the SRI levels for various concrete and asphalt materials:
New grey concrete 35
Weathered grey concrete 19
New white concrete 86
Weathered white concrete 45
New asphalt 0
Weathered asphalt 6
The placement of shade trees in relation to paved areas, roofing and windows affects how effective they are in reducing the heat island effect. The shade pattern should cover critical areas around midday in mid-summer.
Shingles as well as manufactured paving material have SRI ratings. A note of caution must be sounded, however. Unless the manufacturer is specifically promoting “green” material, it may take a bit of searching to find the SRI rating since manufacturers don’t want sales of their dark colored products to be put at a disadvantage.
|This diagram shows solar radiation on a roof surface.
Image courtesy of Cool Roof Rating Council
The DOE Cool Roof Calculator was developed by the U.S. Department of Energy’s Oak Ridge Laboratory. This calculator provides an estimate of cooling and heating savings for small to medium size facilities that purchase electricity with a demand charge and an alternative version for larger facilities. http://www.ornl.gov/sci/roofs%2Bwalls/facts/CoolCalcEnergy.htm