By Mike Petersen Cool Metal Roofi ng
A few months back, I was fl ipping through the Sunday funnies looking for some comic relief when I happened upon a Doonesbury strip addressing of all things, cool roofi ng—not cool metal roofi ng, per se, but the science behind cool roofi ng. In the episode, Michael Doonesbury and Bernie muse over the ultimate roofi ng system: a switchable roof system that would be maxi- mally refl ective during the summer but dark in the winter to absorb and retain as much heat as pos- sible. 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 roofi ng” 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 roofi ng such as solar refl ectance, albedo or emissiv- ity. Now, it’s one of the fi rst items discussed in the development of a roofi ng specifi cation. The metal roofi ng industry is uniquely positioned to play an important role as the nation shifts to more energy effi cient design and it’s essential that all the players in the metal roofi ng supply chain have an understanding of what constitutes a “cool metal roof.” Two elements essentially work together to cre-
ate a cool metal roof: solar refl ectivity and emissivity. Solar refl ectivity (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 refl ectivity rating of 0.05. The same system in Bone White would have a rating of 0.71. An increase of 0.01 in refl ectivity equates to a 1-degree cooler roof and less demand for cooling on a hot summer day. The coatings industry has worked with orga-
nizations such as the Lawrence Berkeley and Oak Ridge National Laboratories to develop new prod- ucts that would extend the color spectrum available to building designers while providing signifi cant improvement in refl ectivity ratings. Infrared refl ec- tive pigments are the result of this effort and there have been tremendous improvements in refl ectivity
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as the industry has switched to these new products. As an example, Elk Grove Village, Ill.-based Petersen Aluminum Corp.’s original fi nish, Classic Bronze, had a refl ectivity rating of 0.06. The reformulated color, almost identical to the eye, now has a refl ectivity rating of 0.27 and qualifi es 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 mea- sured 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 de- scribing 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 un-
coated galvanized steel) therefore has high refl ectiv- ity 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 refl ectivity and emissiv- ity to create a Solar Refl ectance Index or SRI: The higher the index number, the better the solar refl ec- tivity performance. The metal roofi ng industry enjoys a unique
position within the cool roofi ng 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, archi- tectural metal manufacturers have changed their standard product lines to include these infrared refl ec- tive pigments within their standard product lines. This sea change in standard inventories has been accom- plished at no additional cost to our customer base. One of the basic parameters in achieving an En-
ergy Star or CRRC-rated product is the requirement to provide a three-year, unwashed measurement of that product’s solar refl ectivity to prove that the
product’s refl ectivity 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 Refl ectivity (ISR) must be 0.65. The three-year rating must be 0.50 or greater. For steep slope roof- ing, 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 fl uropolymer coated metal panels (e.g., Kynar/Hylar fi nishes) are able to retain around 95 percent of their solar refl ectivity for more than 10 years and in some documented cases up to 20 years. This high level of long-term maintenance of refl ectiv- ity puts cool metal roofi ng well ahead of competing roofi ng products. In my opinion, the metal roofi ng industry should
push for longer-term measures of refl ectivity 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 effi ciency over the life of the building, why do we stop at three years and why do we allow a 40 percent decline in refl ectivity performance? How about requiring a fi ve- year, 10-year or even 20-year performance? Coated metal roofs would perform well under that criteria. To conclude, cool roofi ng has emerged as an
essential element in modern design. Its popular- ity 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 roofi ng that meets the requirements of cool roofi ng 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 roofi ng offers longer term refl ectivity/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 Alumi- num Corp., Elk Grove Village, Ill. For more informa- tion, visit
www.pac-clad.com.
The Bank of Georgia in Newnan, Ga., features straight and radiused 24-gauge Petersen Snap-On standing seam metal roof panels in PAC- CLAD Kynar 500 Medium Bronze.
www.metalconstructionnews.com
The Killeen, Texas, police headquarters features 22-gauge Silver Snap Clad panels and Silver and Zinc Flush panels and Flats.
The St. Joseph Mercy Health Center in Canton, Mich., includes 24-gauge, 18-inch Snap Clad panels; 24-gauge, 16-inch Tite-Loc panels; 12-inch Flush panels; and composite wall panels, all in Hemlock Green.
June 2012 METAL CONSTRUCTION NEWS 33
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