The commercial building “cool roofing” trend is fairly recent, but it’s been understood for centuries that white and light-colored surfaces are cooler than dark surfaces. Those eye-catching, ancient, all-white buildings in Mediterranean and Middle Eastern cities are not only attractive, they are also quite cool and comfortable even on the hottest days of summer. Economic and environmental pressures have inspired a renewed interest both in the heat-reflective properties of white surfaces and the dynamics of urban heat islands (UHIs) – the environmental phenomenon where even small cities are typically three to ten degrees warmer than nearby suburbs and countrysides.
A study by Lawrence Berkeley National Laboratory (LBNL) identified the UHI chain of cause and effect: As temperatures increase, more electric power is needed for air conditioning and more fossil fuel is consumed, which leads to higher levels of air pollution. The probability of smog generation rises five percent for each one-half degree temperature increase above 70 degrees F. At this rate, the level of smog doubles with every temperature increase of 10 degrees during summer months.
Oak Ridge National Laboratory (ORNL) has conducted several studies to evaluate various materials for solar reflectance and emittance, defined as follows:
- Reflectance, also known as albedo or reflectivity, is the percentage of solar energy reflected by a surface. The higher the percentage of reflectance, the more heat energy will be initially reflected from the surface.
- Emittance, or emissivity, is the percentage of heat energy a material can absorb and then shed in the form of infrared radiation. Materials with low emittance tend to heat up more easily and stay hot because they collect and trap heat. It is interesting that while many black materials have very low reflectance, they can exhibit very high emittance.
Cool roofs exhibit both high reflectance and high emittance. By immediately reflecting solar radiation back into the atmosphere and re-emitting part of it as infrared light, cool roofs help create cooler air temperatures for urban environments during hot summer months. Cool roofs help mitigate the urban heat island effect and reduce ambient air temperatures, in turn improving air quality.
The EPA’s ENERGY STAR® Roof Products Program has established a minimum standard for products to qualify as “cool roofing”: low-sloped roof products must have an initial reflectance of at least 65 percent, and a reflectance of at least 50 percent after three years of weathering. The ENERGY STAR Program also requires products to carry warranties similar to or better than those offered by the same manufacturer for similar non-reflective roof products. ENERGY STAR ratings can be found on their web site, www.energystar.gov.