Solar Reflectance Index and Cool Roofs

It’s no surprise that light-colored, reflective surfaces heat up less in the sun. That translates into less solar-generated cooling loads for buildings with white, reflective roofs. It can also mean lower ambient temperatures outside those buildings, because dark roofs and pavement heat the outdoor air, contributing to the “urban heat island” effect.

Late summer afternoon temperatures in cities are 2°F to 10°F (1.5°C–6°C) higher than in surrounding areas. Only about 1% of that increase is from heat generated directly by vehicles and equipment, according to Lawrence Berkeley National Laboratory (LBNL) research. The rest is from solar-heated surfaces. Those higher temperatures decrease comfort, increase air-conditioning costs, and exacerbate health problems, because ozone (a component of smog) is created when pollutants, such as nitrogen oxides and volatile organic compounds, heat up. A temperature drop of 3°F–4°F (2°C) can reduce smog by 10%–20%, according to LBNL.