Durability of High-Albedo Roof Coatings

Several high-albedo coatings were identified and, for each coating, several roofs that had been surfaced with the coating were selected. The albedo of each roof was measured 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.