Hotter roofs have shorter lives -- the effects of degradation from thermal expansion and contraction are well documented.  Roofs that are cooler will experience less temperature differential during the day, resulting in less stress from thermal expansion and contraction.  Metal roofs exhibit better performance than most other roofing materials because the standing seam roof system utilizes a sliding clip design that allows the roof to expand and contract without a stress build-up.  Couple that ability with a cool paint color and you have a system that not only is sustainable for as much as twenty to thirty years, but is guaranteed to maintain performance for at least 20 years by many manufacturers.

Some purportedly “cool” roof systems are especially prone to dirt collection, algal or fungal growth, and staining from acid rain and other environmental effects, not to mention UV degradation.  The dirtier these roofs become, the more emittance and reflectance performance is compromised and the hotter they become.  As the roof surface becomes hotter, these contaminants actually bake into the roof surface making it even harder to clean.  Cool roofs are usually made of light-colored materials, and if they don’t remain light they won’t remain cool. 

In January 2003, the Cool Roof Rating Council (CRRC) Product Rating Program launched a labeling system to rate the initial as well as the 3-year aged performance of cool roofing products. Because the Product Rating Program was launched January 1, 2003, testing system just began, 3-year results won’t be available until 2006. Therefore, only initial radiative properties will be printed on the label until then.  The 3-year performance column will show an asterisk until the aged testing is complete for any product.  This means consumers will have to wait 3-years for the critical data regarding radiative performance to be collected for any new product that enters the marketplace.  Any new products appearing in the future will require a 3-year testing period before performance values are determined and, until then, an asterisk will appear on the CRRC label. The effects of weathering on cool roof products are clearly shown when 3-year aged test values are compared

among samples that have been exposed to the environment at weathering farms.  Materials that exhibit poor 3-year performance will not carry forward the initial good intentions for specifying a cool roof in the first place.  Metal roofs have shown little or no degradation in the harshest of environments at weathering test farms in Florida, Arizona, and in tests conducted at the U.S. Department of Energy’s Oak Ridge National Lab (ORNL) in Tennessee.

The information in the Table 3 shows that Unpainted Galvalume shows a 0.17 decrease in Reflectance and a 0.05 increase in Emittance.  Unpainted Galvanized shows a 0.40 decrease in Reflectance, but a 0.34 increase in Emittance.  Acrylic coated Galvalume shows a 0.07 decrease in Reflectance and a 0.04 increase in Emittance.  The outstanding performer in this study is White Painted GalvalumeTM, which shows a small 0.02 decrease in Reflectance over three years and a 0.02 increase in Emittance.  The results speak for themselves.  White Painted PVDF metal roofs are very sustainable cool roof options.  In areas where higher Emittance products are needed, unpainted or acrylic coated galvanized or Galvalume™ will show an increase in Emittance over time.