By Jim Lindell
If we could view a snapshot of every building in a typical American city, we would likely notice that virtually all of the buildings have dark roofs. In fact, the U.S. Environmental Protection Agency reports that more than 90 percent of all roofs in the United States are dark colored and comprised of low-reflective surfaces, with little or no vegetation.
These conventional rooftops have served us well. But, with today's increased concerns about energy conservation and the environment, as well as new local, state, and federal regulations, they are fast becoming dinosaurs of another era.
The largest problem with these roofs is that they can reach temperatures of 150 to 190 degrees F. during warm summer months. When this happens, cooling and electrical needs jump dramatically, which increases costs and power usage. Central business corridors heat up and create the urban heat island effect, causing personal discomfort and, again, contributing to rising costs and usage. Finally, rooftop deterioration increases significantly because the sun's rays and heat scorch the roof's surface.
To alleviate the problem, more and more facilities are now installing "cool" and green roofing systems (pictured at right). Often, both are installed on the same rooftop.
Conventional dark roofs absorb 70 percent or more of the sun's solar energy, while a cool roof, which is usually white or light colored, absorbs less than 35 percent of this solar energy. This helps keep the roof closer to 70 degrees F. Additionally, cool roofs reflect the sun's rays and keep solar energy away from buildings, helping to reduce the amount of heat transferred into buildings. The result: Cool roofs can help lower peak cooling demands by 10 to 15 percent.
Cool roofing materials are available in a variety of coatings, membranes, coated metal roof products, and cool roof tiles - and, they are not always white. The Lawrence Berkeley National Laboratory's Environmental Energy Technologies Division has worked with the roofing industry and found that non-white, cool roofs can be manufactured using colorants that can effectively reflect the sun's radiation as well.
In some studies, even the darker colors produce reflectivity rates of 25 to 35 percent, which fall within the ENERGY STAR® requirements of a compliant cool roofing system. The ENERGY STAR program requires that roofs meet specific criteria and standards, including solar reflectivity.
While the primary objective of cool roofs is to reflect solar energy away from buildings, green roofs absorb the sun's solar energy; however, in the process, green roofs reduce heat transfer through the roof by means of biochemical processes and added mass. Essentially, they act like additional building insulation.
Although cool roofs can save energy, green roofing systems may have added cumulative benefits. For instance, since green roofs are installed over existing roofs, they help protect the roofs and extend their lifespan. Some studies report that green roofs can double the life expectancy of the existing roofs. Additionally, stormwater runoff, which can quickly overburden an urban sewer system, is reduced considerably.