SRI Value

Benefits of Cool Roof

A cool roof can benefit a building and its occupants by:

• Reducing energy bills by decreasing air conditioning needs

• Helping older inefficient (or undersized) air conditioners provide enough cooling for today’s hotter summers

• Qualifying for money-saving rebates from utilities or government programs

• Making it possible to downsize new or replacement air conditioning equipment, saving money and potentially increasing cooling efficiency

• Improving indoor comfort and safety for spaces that are not air conditioned

• Decreasing roof temperature, which may extend roof service life

• Incur a winter heating penalty. That is, absorbing less sunlight at the roof reduces heat conduction into the building, increasing the need for mechanical heating in winter

• Beyond the building itself, cool roofs can also benefit the environment, especially when many buildings in a community have them

• Lower local outside air temperatures, thereby lessening the urban heat island effect

• Slow the formation of smog from air pollutants, which are temperature-dependent, by cooling the outside air

• Reduce peak electricity demand, which can help prevent power outages

• Decrease power plant emissions by reducing the demand for energy to cool buildings

• Help offset global warming by reflecting more sunlight to outer space

Benefits of Cool Pavement

Conventional paving materials can reach peak summertime temperatures of 50 to 65 degrees Celsius (120 to 150 degrees Fahrenheit), heating the air above them.11 There are many kinds of paving options that are lighter in color and create more reflective paved surfaces. Additionally, many kinds of permeable pave­ments, including reinforced grass pavements, can also cool a pavement surface through the evaporation of moisture stored in the pave­ment. If pavements are too bright, they can cause undesirable glare, but there are many shades of gray that are reflective that do not cause too much glare.

There are a number of additional benefits to light colored pavements beyond cooling.

Improved durability Testing and research are underway to evaluate the durability and longevity of cool pavement materials in a variety of usage conditions. Asphaltic pave­ments that stay at lower temperatures may be less likely to rut.

Cool roofs and pavements can help cool down buildings and cities.

Studies of a city’s “urban fabric” indicate that about 60 percent of urban surfaces are covered by roofs or pavements. About 20 to 25 percent are roofs and 30 to 45 percent are pavements.3 Because these surfaces are dark and typically absorb over 80 percent of sunlight that con­tacts them and convert that solar energy into heat, our built environment exacerbates the warming effects of climate change. Replacing and upgrading roofs and pavements with more reflective materials could reverse this warming, turning urban surfaces into assets instead of burdens. Vegetated roofs, permeable pave­ments, and shade trees are other cooling strategies that are complementary with cool roofs. Cool roofs paired with appropriate levels of roof insulation help keep buildings more thermally comfortable. Cool, reflective roofs and pavements should be a priority strategy because they are cost-effective, typically pay back within one year, and help cities both mitigate and adapt to climate change while making them more desirable and comfortable places to live.

Key Cool Roofs Terminology

Solar Reflectance (SR or albedo)

The fraction of sunlight (0 to 1, or 0 percent to 100 percent) that is reflected from a surface. SR typically ranges from about 0.04 (or 4 percent) for charcoal to 0.9 (or 90 percent) for fresh snow. High solar reflectance is the most important property of a cool surface.

Solar Absorptance (SA)

The fraction of sunlight (0 to 1, or 0 percent to 100 percent) that is absorbed by a surface. Surfaces with high solar absorptance tend to get hot in the sun. If the surface is opaque, solar absorptance equals 1 minus solar reflectance.

Thermal Emittance (TE)

The efficiency (0 to 1) with which a surface emits thermal radiation. High thermal emittance helps a surface cool by radiating heat to its surround­ings. Nearly all nonmetallic surfaces have high thermal emittance, usually between 0.80 and 0.95. Uncoated metal has low thermal emittance, which means it will stay warm. An uncoated metal surface that reflects as much sunlight as a white surface will stay warmer in the sun because it emits less thermal radiation. TE is the second most important property of a cool surface.

Solar Reflective Index (SRI)

A coolness indicator that compares the surface temperature of a roof on a sunny summer after­noon to those of a clean black roof (SRI = 0) and a clean white roof (SRI = 100). SRI is computed from solar reflectance and thermal emittance, and can be less than 0 for an exceptionally hot surface (e.g., a solar collector) or greater than 100 for an exceptionally cool material (e.g., a very bright white roof). 

Thermal Resistance (R-value)

A measure of a material or system’s ability to prevent heat from flowing through it. The thermal resistance of a roof can be improved by adding insulation, a radiant barrier, or both.