Cool Roofs
What are cool roofs?
A cool roof is made of a material or has a coating that can lower the roof surface temperature, decreasing the amount of heat transferred into a residential or commercial building. In general, traditional roofs absorb sunlight during the day, heating the building and the surrounding air. Roofs that are lighter in color or are reflective stay cooler than roofs that absorb sunlight.
There are two ways to help cool a roof. First, a cool roof can reflect away sunlight, so it stays cooler. Such a roof is said to have a high solar reflectance. Second, a cool roof should also release or emit heat (infrared radiation) so it stays cool. Such a roof is said to have high thermal emittance. An ideal cool roof is a roof with both high solar reflectance (SR) and high thermal emittance (TE). While lighter color roofs tend to have the best SR and TE, new coating and material technologies now exist for other colors that have high SR and TE.
Exterior building walls are exposed to only about half the amount of sun as roofs, but they can also absorb heat. The same basic principles apply to cool walls as to cool roofs.
What are the benefits of cool roofs?
- Cool roofs have several benefits for both for building owners and the environment.
- Keeping buildings cooler on hot days to improve indoor comfort and safety
- Reducing building air conditioning costs and reducing the strain on the electrical grid during peak energy demands
- Reducing the pollution and greenhouse gas (GHG) emissions associated with building energy use
- Decreasing roof temperature which can extend the life of the roof materials (slows degradation)
- Contributing to lower temperatures in the surrounding air which helps reduce the urban heat island effect in cities. EPA has detailed information on how cool roofs can help communities reduce heat island impacts and risks at https://www.epa.gov/heatislands.
What are the benefits of cool roofs?
According to Lawrence Berkeley National Lab Heat Island Group on a typical summer afternoon a clean white roof that reflects 80% of sunlight will stay about 50°F cooler than a grey roof that reflects only 20% of sunlight. In general, cool roofs work best (save more energy) in hot sunny climates, like the Southern U.S., on buildings with low levels of roof insulation. Energy savings for buildings with cool roofs in Northern climates are predicted to grow as the climate warms.
Selecting the Right Roof
Cool roofs can be installed on residential and commercial buildings. There are 2 basic types of roofs: sloped and low-sloped (or flat). A low-sloped roof is generally a roof with a pitch that is less than or equal to 2:12 (9.5 degrees from horizontal). Some common cool roof materials include membranes, shingles, coatings, stone/rock, metal, and tile with high solar reflectance. Other cool roof strategies include ballasted roofing and green roofs. Learn more about roof types and product types from the Cool Roof Rating Council (CRRC). https://www.energystar.gov/products/cool_roofs_emissivity
