Long-term performance of high-albedo roof coatings

Cooling energy savings of 10 to 70% have been achieved by applying high-albedo coatings to residential buildings in California and Florida. Since dirt accumulation can alter the performance of high-albedo roofs as an energy efficiency measure, we examined some high-albedo coatings at various stages of exposure to determine the magnitude of this effect. We conclude that most of the albedo degradation of coatings occurred within the first year of application, and even within the first two months of exposure. On one roof, 70% of the drop in albedo for the entire first year occurred within the first two months. After the first year, the degradation slowed, with data indicating small losses in albedo after the second year. We use measured data to estimate the effects of weathering of white roofs on seasonal cooling energy savings and estimate a 20% reduction from first year energy savings for all subsequent years (2-10). Although washing the roofs with soap is effective at restoring original albedo, calculations show that it is not cost-effective to hire someone to clean a high-albedo roof only to achieve energy savings. Instead, it would be useful to develop and identify dirt-resistant high-albedo coatings.