Berkeley Lab research associate Sharon Chen sprays roofing material with soiling mixture. Image: Roy Kaltschmidt/Berkeley Lab

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.

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  1. This will help speed up new research and development and bring new products to the forefront of the industry. Very good.

  2. I read this article with great suspicion.

    Yes, a Standard test time method has been created to test light coloured roofs, it appears from the photo using small roof samples, similar to the flawed and fraudulent Steady State Test Method Thermal testing a small piece of insulation at a mean temperature of 23?C for four hours, not really representative of real life conditions.

    What scientists here, OR are they directed to forget is in the real world, roofs are attached to buildings usually for their lifetime.
    How many people go onto their roofs, and clean their solar panels, let alone their roof regularly?

    The population aren’t made readily aware solar panel performance decreases, just as light coloured roofs efficiency does by the environment they are installed in, and actually clean these thoroughly, NOT just hi pressure water, but also the good old friction of a scrub with a broom.

    This is the only way to clean a metal roof and solar panels, tiles seems hi pressure water will do the job, but a broom scrubbing would be too expensive.

    What the scientists have overlooked is lichen of various types, AND the green substance that grows on the roof surface, such as I believe types of algae?

    These growths survive all types of weather, something the test method cannot replicate and accelerate three years growth in three days.

    “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.”


    How can light roofs be seriously considered as a contributor to building energy efficiency due to outside behaviour, of nature and occasionally human influence.

    I will be more than happy to have a reply to my blog.

    My experience in the real world, reflects my comments, as I regularly clean roofs and solar panels.