The Department of Energy (DOE) defines cool roofs as "roofs that are designed to maintain a lower roof temperature than traditional roofs while the sun is shining." Cool roofs often are touted as having two major benefits when compared with traditional roofs: they contribute less heat load to building interiors, making buildings less expensive to cool, and they help reduce urban heat islands.
But cool roof membranes become less reflective (and less cool) as they age, mostly as a result of climatic exposure. Sometimes, to restore a cool roof membrane's reflectivity, the membrane is cleaned. However, before you prepare to clean a cool roof membrane, there are a few things to consider.
What are cool roof membranes?
There are many forms of cool roof membranes, but the most common is a roof membrane with a high solar reflectance index (SRI). SRI is calculated from solar-reflectance and thermal-emittance values according to ASTM E1980, "Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces." The higher a roof system's SRI value, the cooler the roof system will be in the sun. The highest SRI value is 100 (white), and the lowest SRI value is 0 (black). As in-service roof surfaces become soiled, reflectivity is reduced and SRI values decrease.