A method for quickly simulating the weathering of roofing materials over time could accelerate the deployment of cool roofs, according to the US’s Lawrence Berkeley National Laboratory.
Cool roofs can help buildings stay cool by reflecting sunlight away, and also help to mitigate the urban heat island effect. However, solar reflectance can be affected by weathering and soiling.
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Roof reflectance in the US is rated by the Cool Roof Rating Council when new and after three years of outdoor exposure, meaning there is a lag time between creating new high-performance cool roofing material and getting its three-year exposure reflectance rating.
Now a team led by scientists at Berkeley Lab has developed a method to simulate the weathering and soiling roofs undergo to reproduce in less than three days the solar reflectance and thermal emittance of roofing products naturally aged for three years.
The process has now been adopted by standards body ASTM International as standard practice for the industry.
“What we’ve created is essentially a cool roof time machine,” Ronnen Levinson of Berkeley Lab’s Heat Island Group said. “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.”
The new standard has been endorsed by the CRRC, which in turn means it is now part of California’s building code, specifically Title 24 Building Energy Efficiency Standards. Research is now underway to adapt the method for use in China, India and Europe.
The method involves putting a piece of the roof material in a commercial weathering apparatus, exposing it to cycles of heat, moisture and ultraviolet light for one day. Then a soiling apparatus 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.
The 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.