Cool roofs can help keep buildings cool, thus lowering the building’s energy use, while also mitigating the urban heat island effect by reflecting sunlight away from buildings and cities. But as cool roofs age and get soiled, how much of their reflectance do they lose?
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. Now 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 U.S., but around the world.”
Roof reflectance is rated when new, and after three years of outdoor exposure. 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 sec. 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.
The process was described in Solar Energy Materials and Solar Cells last year.
Source: Lawrence Berkeley National Laboratory
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