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Cool roofs use light-reflecting roofing materials or coatings and are a simple and accessible means for individuals and communities to directly reduce carbon emissions, energy costs, and urban heat island effects.

Source: Owens Corning cool roof

Project Drawdown , a global effort to drawdown carbon emissions, estimates that cool roofs, along with green roofs (where roofs are covered with soil and vegetation for insulation) could reduce up to 1.1 gigatons of CO2 emitted globally by 2050.

In the City of Los Angeles, 18,000 residential cool roofs were installed as of 2018. They cover over 25 million square feet and save over 3 million kilowatt-hours annually.

How do cool roofs work?

Cool roof materials are similar to other roofing materials but are often lighter in color and reflect, rather than absorb, solar energy as compared to darker, traditional roofs. Less energy is needed to cool buildings with reflective roofs and less thermal heat is released into the surrounding air. Using less energy for cooling helps mitigate greenhouse gas emissions.

Cool roof materials vary and are rated on their solar reflectance and thermal emittance by the Cool Roof Rating Council (CRRC). The solar reflectance index (SRI), incorporates both measures. The higher the SRI rating, the cooler the roof will be in the sun, with ratings ranging from 0 to 100.

With a traditional dark roof on a day with a temperature of 99 degrees, just 5% of solar energy is reflected back into space, with 95% heating the building and surrounding air, according to Drawdown projections.

Source: Cool Roof Rating Council.

How much cooler are cool roofs?

A cool roof in contrast reflects up to 80%, leaving the building and surrounding air cooler.

Southern California Edison reports that cool roofs can reduce roof temperature by as much as 60 degrees during the summer. Similarly, the U.S. Department of Energy reports that under hot summer conditions a cool roof could stay more than 50 degrees cooler than a traditional roof.

Are there state regulations or local requirements regarding cool roofs?

California Title 24, Part 6 establishes building energy efficiency standards, which can be met by meeting an overall building energy efficiency performance standard, or by specifically requiring a cool roof under a prescriptive approach, for new construction, additions, and roof replacements, with some exceptions. But it gets complicated.

Cool roof requirements vary by building type (residential, commercial, high rise vs. low) roof type (low or steep slope) and by climate zone.

California has 16 climate zones which allow tailoring to local conditions. There are no state cool roof requirements for low-rise residential buildings in climate zones 1 thru 9 and 16. Changes to residential cool roof requirements are planned for the 2022 Energy Code revision of Title 24.

Local governments can establish cool roofs requirements, as has been done in Los Angeles since 2015. The Los Angeles city building code requires residential roofing materials to meet a minimum SRI level for new construction and reroofing in residential and commercial buildings.

Pasadena’s Building and Safety Division requires cool roof materials be used for reroofing of nonresidential, high-rise residential, and hotels/motels, roof replacements for more than 50% of the roof area or over 2,000 square feet of roof. There is no Pasadena cool roof requirement for reroofing of residential structures three stories or less.

Are cool roof materials available, and do they look similar to “hot” roofs?

Cool roofing materials are available now. Shingles look like conventional roofing material and are available in tile, shake, shingle, and coating forms. They come in a wide range of colors , often lighter shades than traditional roofing material.

Are cool roofs more expensive than traditional roofs?

Cool roof costs are comparable to that of a standard roof of the same type, according to Cool California . However, darker asphalt shingles may cost slightly more than light-colored cool roof materials. But because cool roofs last longer than traditional roofs according to the CRRC, extended use can reduce the lifetime cost of cool roof materials.

Additional costs may be offset by financial incentives for adopting cool roofs.

The Los Angeles Department of Water and Power offers a rebate of between 20 and 30 cents per square foot of roofing meeting specific reflectivity values. Pasadena Water and Power offers a rebate of 20 to 25 cents per square foot for cool roofs. SCE offers a rebate of 20 cents per square foot of qualifying cool roof.