We are now a few decades into the era of cool metal roofing. “Era” is probably the wrong word, because cool metal roofing is here to stay. Many coatings producers and metal roofing manufacturers in North America now offer the benefit of cool roofing as a part of their standard product portfolio.
As “special” becomes “standard,” it is easy to assume that cool roofing is on autopilot and that nothing more needs to be done. Although in a practical sense this is mostly true, it is good to remember that research continues wherever energy savings is the goal, and cool roofing is included in this research.
Consider the following:
- Battened Roofing—Admittedly the R&D was done almost a decade ago, but battened, above-sheathing ventilation is still not a common building practice; however, it is one worth considering. This article published by Bill Miller, Joe Wilson, and Achilles Karagioszis, Oak Ridge National Laboratory and Metro Roof Products, or this presentation made by Marty Hastings, Valspar, are certainly worth re-visiting.
- Subambient Cooling—Picture a shiny crescent wrench sitting on your driveway on a hot, sunny summer day. You pick it up but immediately drop it, because it is so hot. You suddenly remember that—while the shiny wrench’s solar reflectance is quite high (it is a silvery, nickel-plated, reflective metal)—its emittance is quite low, because of the shiny surface. Most of the solar energy is reflected, but some is absorbed, and—once absorbed—the wrench cannot radiate the heat because of the low emissivity. So you conclude—rightfully—watch-out for shiny metal objects on a sunny, hot day. Now, picture an equally shiny film, with an equally high solar reflectance. But imagine your surprise to learn that this special film has a surface temperature lower than the ambient air temperature. Such films are being studied at a few laboratories around the world. The secret: These films are designed to emit what little solar radiation that they absorbed in range of the 8-14 microns. This wavelength range is the so-called “sky window,” and is largely free of incoming IR radiation. This “window,” therefore, allows emitted IR radiation to pass freely from Earth to outer space. The material is not yet ready for prime time, but if you wish to learn more about subambient cooling, visit this link for an example of the research being conducted.
- NIR Fluorescent Pigments—Speaking of hot, new, cool roofing research, check out this publication by the U.S. Department of Energy, authored by Paul Berdahl, Lawrence Berkley National Laboratory (LBNL), and Michael Zalich, PPG. You will find a very novel approach to cool pigmentation. Pigments typically both reflect some of the incoming solar radiation and absorb some of this energy. These new pigments do one additional interesting thing: they fluoresce in the near-infrared radiation (NIR) range. You may not realize it, but when you are in a room with black lights and your teeth, clothing (if treated with optical brighteners), or those bright fluorescent pigments appear to glow, well, they are, actually. These materials are absorbing the long-wavelength UV energy (which is not visible to the human eye) emitted by the black lights, and immediately fluorescing—emitting—light in the visible region that our eyes can detect. Similarly, the NIR-fluorescing pigments being studied both reflect and absorb some of the solar radiation and then re-emit some of the absorbed energy in the NIR range. This is equivalent to reflecting IR energy, and LBNL is reporting some astounding results. Coatings made with NIR fluorescing pigment have surface temperatures far lower than expected for a dark color when exposed on a hot, sunny day. This type of pigment will require quite a bit of study, but the concept—NIR fluorescence—is something to follow.
The gap between R&D and a commercially-viable product is large, but as you follow the links in this blog, you will see a real sense of practicality to the researchers’ work.
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