Cool roof technology has been discussed a lot in the literature in the past decade and, by definition, a cool roof must strongly reflect sunlight and efficiently emit heat to the surroundings – i.e., solar reflectance and thermal emittance determine the coolness. All is well and good initially, but as roofing is exposed to sunlight and weathering the reflectance properties and heat emittance properties will change. Therefore, usually a minimum of 3-years outdoor exposure testing is required in various locations. So I was pleasantly surprised to read a recent report from the COOL ROOF RATING COUNCIL (CRRC) about a new method that allows labs to simulate three-year weathering of roofing materials in three days. This was actually reported on by Berkeley a year ago but I wasn’t aware of it. So if there are others not aware of this, please read on!
According to their press release, a collaboration led by scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) has established a method to simulate soiling and weathering processes in the lab, reproducing in only a few days the solar reflectance of roofing products naturally aged for three years. This protocol has been approved by ASTM International, a widely referenced standards body, as a standard practice for the industry, and published as ASTM D7897-15.
“What we’ve created is essentially a cool roof time machine,” said Ronnen Levinson of Berkeley Lab’s Heat Island Group. “By reducing product rating time to three days from three years, our new ASTM standard practice will speed the introduction of high-performance cool roofs not only in the United States, but around the world.”
Because the ASTM standard has been endorsed by the Cool Roof Rating Council (CRRC), it is in turn accepted as part of California’s building code, specifically California’s Title 24 Building Energy Efficiency Standards. Research is underway to adapt the method for use in China, India and Europe.
The laboratory practice involves putting a piece of the roof material in a commercial weathering apparatus, which exposes the material to cycles of heat, moisture and ultraviolet light, for one day. This “conditions” the material before soiling. Then a soiling apparatus developed at Berkeley Lab sprays a calibrated aqueous soiling mixture of dust, soot, particulate organic matter, and salts for about 10 seconds. After it dries, it goes back in the weathering apparatus for one more day, to simulate the cleaning effects of dew and rain.
This method was applied to 25 different roof products, including single-ply membranes, coatings, tiles and asphalt shingles, and reproduced—in less than three days—the CRRC’s three-year aged values of solar reflectance.
The researchers devised different soiling mixtures to mimic site-specific features of three environments: a hot and humid climate (Miami, Florida); a hot and dry climate (Phoenix, Arizona); and a polluted atmosphere in a temperate climate (Cleveland, Ohio). A fourth soiling mixture was devised to replicate the aged solar reflectance averaged over all three sites, which is the aged value reported by the CRRC on rated product labels.
A team of Berkeley Lab scientists and research associates, including Mohamad Sleiman, Hugo Destaillats, Sharon Chen, Thomas Kirchstetter, Haley Gilbert, Paul Berdahl and Levinson, worked with Hashem Akbari at Concordia University, the CRRC, and more than 40 industrial partners to develop the protocol.
It seems hard to believe that the data can be attained in only three days and yet the correlation data speaks for itself. An amazing time machine indeed!
More information can be obtained from Lawrence Berkeley National Laboratory and especially from the CRRC. They are working with different countries and test labs on successful implementation of the ASTM protocol.