U.S. Department of Energy Secretary Steven Chu is a big proponent of cool roofing. In his July 2010 announcement, Chu made it clear that he was going to push for the installation of cool roofing systems on all federal buildings to help reduce energy usage. Secretary Chu is well-informed about cool roofing because he was formerly the head of Lawrence Berkeley National Laboratories (LBNL), the entity that pioneered the study of Urban Heat Islands (UHIs).
Not only will cool roofing reduce building energy usage, it will also help mitigate the UHI effect. The UHI effect is the tendency for urban areas to be hotter than surrounding areas. LBNL found that the average temperature on a hot summer day will be seven degrees warmer in North American urban areas than surrounding rural areas. During an extended heat wave the difference can be even more pronounced. Studies have shown that there are three primary factors that cause the majority of the UHI effect.
Vegetation
The first factor is that urban areas have less vegetation than rural areas. Not only do trees and shrubs provide shade, but thriving vegetation keeps itself cool through a process called evapotranspiration. Similar to how the human body sweats to keep itself cool, vegetation releases moisture to stay cool. About 56%, or almost four degrees, of the seven degree difference is due to less vegetation in urban areas than rural.
Dark Pavement
Many might think that dark pavement would account for much of the UHI effect. While walking down a city street, one can feel the heat radiating up. But dark pavement accounts for only 6%, or less than one-half degree, of the seven degree difference.
Dark Roofing
Roofing takes up a lot of surface area in urban areas, but roofing is not often considered a source of urban heat because it is “out of sight, out of mind.” Yet dark roofing accounts for 38%, or almost three degrees, of the seven degree difference associated with UHIs.
Many cities have attempted to increase green space and vegetation through civic programs and building codes, but for every tree planted or park developed there is much more green space that succumbs to urban sprawl. Green space initiatives are at best a long term means of mitigating UHIs and can entail significant expense.
Paving products made from lighter colored materials are available, but implementing these measures is capital intensive and can take years to accomplish. And considering the relatively minor role that paving plays in UHIs, there are options that provide more bang for the buck.
Installation of cool roofing during initial construction or when re-roofing offers immediate benefits, not only toward mitigation of UHIs but to the building owner in the form of energy savings. A good roofing system is essential for protecting any building from the elements. Selecting and installing a cool roofing system is easy to accomplish, inexpensive relative to other UHI mitigation efforts, and provides benefits immediately.
Even in northern geographic areas where net energy savings may be minimal, cool roofing systems offer significant benefits that may be less tangible but are essential to the long term performance and durability of the roof, insulation and HVAC equipment.
