How Cool Roofing Works
The trend may be new, but mankind has understood for centuries that white or light-colored surfaces are cooler than dark surfaces. Those stunning, ancient, all-white Mediterranean and Middle Eastern cities are not only aesthetically pleasing, they are also surprisingly cool and comfortable even on the hottest days of summer. Economic and environmental pressures have inspired a renewed interest in the heat-reflective properties of white surfaces, and recent research into the dynamics of urban heat islands, or UHIs – the phenomenon where even small cities are typically three to ten degrees warmer than nearby suburbs and countrysides.
The UHI chain of cause and effect is clear: 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 rises five percent for each one-half degree increase in ambient temperature above 70°F.
Meanwhile, Oak Ridge National Laboratory (ORNL) has conducted several related studies to evaluate various materials for solar reflectance and emittance.
- 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 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 because they collect and trap heat. It is interesting that while many black materials have very low reflectance, they can exhibit very high emittance.
Although there is no industry-wide definition of a cool roof per se, the Environmental Protection Agency’s (EPA) ENTERGY STAR® Roof Products Program has established a minimum standard for products to qualify. The EPA standard requires that low-slope roof products have an initial reflectance of at least 65 percent, and a reflectance of at least 50 percent after three years of weathering. The ENGERGY 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.
In our next installment we will discuss: Cool Roofing Options and Choosing the Best Cool Roofing System.
