The cool roofs strategy, which involves converting dark surfaces, such as roofs and street pavements, to light colored and reflective surfaces is being implemented in cities across the United States to help reduce the heat island effect. Roofs can be converted simply by applying a special reflective coating or by selecting materials that are lighter colored and have a higher solar reflectance. Pavements can also have a special reflective coating applied or permeable grass pavements can be added, which also help with stormwater filtration. The conversion to cool surfaces can help reduce pollution, energy costs, and reduce heat since 60 percent of city surfaces are covered by roofs and pavements surfaces.
Source: Global Cool Cities Alliance. (2012). A Practical Guide to Cool Roofs and Cool Pavements. Available at: https://www.coolrooftoolkit.org/wp-content/pdfs/CoolRoofToolkit_Full.pdf
Solar reflectance, also known as albedo, is measured from 0-1 to determine the efficiency to keep a surface cool (see Figure above). Lighter colored roofs and pavements have a higher albedo, meaning, with a higher reflection, the sunlight and heat bounces off of a surface more quickly. The power of reflection helps keep neighborhoods and buildings cool. Dark surfaces, with low albedo, on the other hand do the opposite. The dark surface reflects sunlight slower and absorbs more heat that is then slowly released throughout the day, making a neighborhood warmer. The Figures below compare a black and white roof and their solar radiation into the environment and absorption within a building. The 3 percent difference of heat entering the building from a lighter colored roof can be substantial for households in heat islands.
Source: Global Cool Cities Alliance. (2012). A Practical Guide to Cool Roofs and Cool Pavements. Available at: https://www.coolrooftoolkit.org/wp-content/pdfs/CoolRoofToolkit_Full.pdf
