Wash Your Roof

"High-albedo" roof coatings can keep buildings cool and thus reduce building air-conditioning energy use. High-albedo--that is, very reflective--coatings lower the absorption of solar energy, reduce surface temperatures, and decrease heat transfer into the building. Link to: Roof Heat Transfer To maximize cooling energy savings, these materials should 1) have high solar reflectivity, 2) have high infrared emissivity, and 3) maintain these properties for many years. Link to: Energy > Ways to Save Energy While an ordinary roof coating of good quality will last ten years, there are little data on the change in reflectance of high-albedo coatings. The Heat Island Group has quantified the effects of weathering and the accumulation of dirt on high-albedo roof coatings.

In one study, we identified high-albedo coatings and, for each coating, we several roofs that had been surfaced with the coating. We measured the albedo of each roof with a pyranometer. Some of the roofs were then washed with soap and water in an attempt to return the albedo to the original value, and measurements were repeated.

The average reduction in albedo after one year of soiling was 0.15, ranging from 0.04 to 0.23. The reduction in albedo for most of the roofs that had collected dirt for two, four, and six years was similar, indicating that albedo decreased the most during the first year and that later reduction in the albedo through the sixth year may be negligible. Some roof coating distributors recommend applying another coat after ten years because of a rapid decrease in albedo after the tenth year, when microbial growth appears.

While the albedos of most roofs were nearly restored to their initial values by washing, the labor costs of the washing and the limited energy returns made the cost of conserved energy too high to be worthwhile. Thus resistance to the accumulation of dirt is an important consideration in selecting a high-albedo coating. These results were published in Recent Research in the Building Energy Analysis Group: Link to: "Durability of High-Albedo Roof Coatings"

In another study, membrane and steel samples that had been exposed to weather for a number of years were tested before and after washing with a mild soap solution. The results on the top graph to the left show a significant gain in reflectance for a light gray roofing membrane.

The middle graph to the left shows the difference between the two curves in the graph above. It shows that whatever was washed off has a smoothly varying optical absorption, which gradually increases with decrease in wavelength. Similar results were obtained on galvanized steel.

The bottom graph is a textbook spectrum for black carbon. Comparison with the middle graph suggests strongly that the reduction in solar reflectance, due to soiling, is primarily due to black carbon accumulation.

To learn more about the instruments we use to characterize roofing samples, link to: Coof Roofs Instrumentation

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This web page last modified by Brian Pon on April 27, 2000.
Questions? E-mail: SCChang@LBL.gov