According to the White Coatings Council of the Washington, D.C.-based Roof Coatings Manufacturers Association (RCMA), white coatings can extend the life expectancy of many different types of commercial roofing systems simply by avoiding the high roof temperatures associated with exposure to the sun.
The technical term for exposure is "insolation," and it is measured in terms of the rate of solar radiation received per unit area (typically expressed in watts per square meter). Under very clear skies, up to 1220 W/m2 of solar radiation reaches the rooftop. Imagine ordinary household hair-dryers every 10 feet in both directions of a two-dimensional array: that is the intensity of the sun’s energy incident upon a rooftop. Heavily insulated roof systems block this heat from penetrating the building as heat; however, in summer months, convective heat transfer to the surrounding air and radiant heat transfer are inefficient. Consequently, the heat has nowhere to go and roofing membrane temperatures can soar.
For commercial roofing systems made from organic materials, high temperatures can shorten the life of the roofing system. Rubber, as well as other synthetic polymers (and especially asphalt), are susceptible to damage at these elevated temperatures. Roof temperatures can rise above ambient air temperatures by as much as 90 degrees F. (50 degrees C.). Meanwhile, the rate of degradation of the roofing materials begins to accelerate at modestly elevated temperatures; for example, at 140 degrees F. (60 degrees C.). Significant damage can begin to occur at these elevated temperatures, causing both short- and long-term performance issues.
As temperatures rise to the peak of this range, degradation of roofing systems increases exponentially. Lighter and hence more volatile organic compounds may evaporate into the atmosphere, and chemical reaction rates with water, oxygen, and contaminants also increase. As a result, roof life expectancies are shortened. A rule of thumb for thermal aging is that service life is cut in half for every 18 degrees F. (10 degrees C.) increase in temperature.
"Most people appreciate that white coatings result in savings on cooling costs, but enhanced roof longevity, especially in northern states, can be another major benefit that could result in at least as much savings," says Thomas L. Meyer, co-chairman of the White Coatings Council. "Without white coatings, roof temperatures skyrocket in the summer months, especially for buildings in northern climates, because they typically have a lot of insulation to help keep in the heat during the winter months."
According to Meyer, the inverse correlation between insulation thickness and roof life is well documented by roof asset management firms. "Since ripping out the insulation every summer is, of course, impractical, white coatings are one of the best means available to counteract the deleterious effects of insolation and enhance the prospects for a long lasting roofing system," he concludes.
This information was reprinted with permission from the Roof Coatings Manufacturers Association, the national trade association representing manufacturers of cold-applied protective roof coatings and cements, and the suppliers of products, equipment, and/or services to and for the industry. For additional information on white coating products and the White Coatings Council, visit (www.roofcoatings.org/wcc.html) or call RCMA at (202) 207-0919.