A few months back, I was flipping through the Sunday funnies looking for some comic relief when I happened upon a Doonesbury strip addressing of all things, cool roofing-not cool metal roofing, per se, but the science behind cool roofing. In the episode, Michael Doonesbury and Bernie muse over the ultimate roofing system: a switchable roof system that would be maximally reflective during the summer but dark in the winter to absorb and retain as much heat as possible. Such a system, if produced, would result in tremendous energy savings and reduced heating/cooling loads. (The full strip, dated 8/7/11, can be found at www.doonesbury.com.)
The fact that a subject such as “cool roofing” is addressed within the plot lines of Doonesbury tells me that the topic has certainly hit the mainstream. A decade ago, I wasn’t conversant in elements of cool roofing such as solar reflectance, albedo or emissivity. Now, it’s one of the first items discussed in the development of a roofing specification. The metal roofing industry is uniquely positioned to play an important role as the nation shifts to more energy-efficient design and it’s essential that all the players in the metal roofing supply chain have an understanding of what constitutes a “cool metal roof.”
Two elements essentially work together to create a cool metal roof: solar reflectivity and emissivity. Solar reflectivity (also known as albedo) is measured on a scale from 0 to 1 in gradations of hundredths. A Matte Black-coated metal roof system (circa 1975) would have a typical reflectivity rating of 0.05. The same system in Bone White would have a rating of 0.71. An increase of 0.01 in reflectivity equates to a 1-degree cooler roof and less demand for cooling on a hot summer day.
The coatings industry has worked with organizations such as the Lawrence Berkeley and Oak Ridge National Laboratories to develop new products that would extend the color spectrum available to building designers while providing significant improvement in reflectivity ratings. Infrared reflective pigments are the result of this effort and there have been tremendous improvements in reflectivity as the industry has switched to these new products. As an example, Elk Grove Village, Ill.-based Petersen Aluminum Corp.’s original finish, Classic Bronze, had a reflectivity rating of 0.06. The reformulated color, almost identical to the eye, now has a reflectivity rating of 0.27 and qualifies as an Energy Star color for steep slope roof application.
The second element in a cool metal roof is emissivity. Emissivity is essentially a measure of a material’s ability to retain heat or shed heat. It is measured on a scale from 0 to 100 with a higher number connoting higher emissivity. The U.S. Department of Energy uses the example of a baked potato in describing emissivity (I’m not making this up!). A baked potato, left on a table in a cold room will quickly lose its heat-a potato skin has high emissivity. The same potato, wrapped in aluminum foil, will retain its heat over a longer period-lower emissivity.
Bare uncoated metal (like aluminum foil or uncoated galvanized steel) therefore has high reflectivity but low emissivity-an uncoated metal roof will get hot and stay that way, thereby raising cooling loads. The coating on a metal roof can be thought of as a potato skin, if you will. The coating features high emissivity and will shed solar heat.
Finally, each product is measured in a complex formula combining solar reflectivity and emissivity to create a Solar Reflectance Index or SRI: The higher the index number, the better the solar reflectivity performance.
The metal roofing industry enjoys a unique position within the cool roofing world. We are able to offer a broad color palette of Energy Star and Cool Roof Rating Council (CRRC) qualifying products to the design community. Across the industry, architectural metal manufacturers have changed their standard product lines to include these infrared reflective pigments within their standard product lines. This sea change in standard inventories has been accomplished at no additional cost to our customer base.
One of the basic parameters in achieving an Energy Star or CRRC-rated product is the requirement to provide a three-year, unwashed measurement of that product’s solar reflectivity to prove that the product’s reflectivity can be maintained under real world conditions. Under current criteria, in order for a low slope (2:12 or less pitch) product to qualify, Initial Solar Reflectivity (ISR) must be 0.65. The three-year rating must be 0.50 or greater. For steep slope roofing, the ISR must be 0.25 or greater. The three-year measurement must be 0.15 or greater.
Controlled testing undertaken by ORNL has shown that fluropolymer coated metal panels (e.g., Kynar/Hylar finishes) are able to retain around 95 percent of their solar reflectivity for more than 10 years and in some documented cases up to 20 years. This high level of long-term maintenance of reflectivity puts cool metal roofing well ahead of competing roofing products.
In my opinion, the metal roofing industry should push for longer-term measures of reflectivity and emissivity within the Energy Star, CRRC and LEED arena. If we are truly attempting to design sustainable buildings that will maintain their initial energy efficiency over the life of the building, why do we stop at three years and why do we allow a 40 percent decline in reflectivity performance? How about requiring a five-year, 10-year or even 20-year performance? Coated metal roofs would perform well under that criteria.
To conclude, cool roofing has emerged as an essential element in modern design. Its popularity has been driven by the improvement in energy performance intrinsic to its use. It has become more widespread by changes in building codes and the emergence of LEED design principles. Coated metal roofing that meets the requirements of cool roofing is available now. In our company’s case, 31 of our 35 standard colors meet the cool color requirement for steep slope application. Many of our competitors can make similar claims. Better yet, cool metal roofing offers longer term reflectivity/emissivity performance than any of the competitive alternatives. If you are really trying to achieve sustainable design, what can be better than that?
Mike Petersen is the president of Petersen Aluminum Corp., Elk Grove Village, Ill. For more information, visit www.pac-clad.com.
