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Cool Roofs: Optimal Thermal Performance

David Summers, P.E., LEED®AP, Los Angeles Associate Principal

A cool roof is a term for a roof that has been treated with a coating to reduce the amount of absorbed solar radiation. In general, these coatings are white or light-colored, but new coatings are being developed in various colors that produce equal results. Cool roofs are best used in hot climates to maximize the energy savings in summer and minimize the energy penalty in winter.

In cold climates, dark roofs have a beneficial passive heating effect in the winter. Cool roof coatings increase the heating costs compared to a dark-colored roof. In areas where there are cold, cloudy winters and mild summers, the cooling benefits of a cool roof system are usually outweighed by the increased heating costs in the wintertime.

Over 90% of roofs in the United States and most roofs in the world are dark colored. These roofs can reach surface temperatures of 180°F or higher on a hot summer day. This places an increased load on the building's cooling equipment, decreases comfort levels of building occupants, and contributes to overall power grid demand during these critical peak hours.

In addition to these negative effects, high roof surface temperatures cause roofing materials to deteriorate more quickly and can increase maintenance costs.

There are many roofing options that can improve the thermal performance of a roof, although not all qualify to be called a "cool roof". Various definitions of a "cool roof" have been developed to set minimum performance standards.

The California Energy Code defines a "cool roof" as a roof that has a minimum thermal reflectance of 70% and a minimum thermal emittance of 75%. In the State of California, all new non-residential and high-rise residential buildings must have a cool roof that meets these requirements to comply with the energy code via the prescriptive compliance approach. The cool roof must be certified by the Cool Roof Rating Council, an independent third-party agency.

The U.S. Green Building Council has defined a "cool roof" as a roof that has a Solar Reflective Index (SRI) above 78 for low-sloped roofs and above 29 for steep-sloped roofs. The SRI is a combination of thermal emittance and reflectance factors. A building with a cool roof that meets these requirements will earn a LEED credit (SS credit 7.2) for reducing the heat island effect of the building. In addition to exceeding the SRI ratings, the credit can also be obtained either by having over 50% of the roof area as a vegetated roof or by having a combination of an acceptable SRI-rated roof and vegetated roof.

EnergyStar also certifies reflective roof products and maintains a list of manufacturers and products on their website. The EnergyStar requirements for low-slope roofs are an initial reflectance of 65% and a reflectance of 50% after three years under normal conditions. For steep-slope roofs, these values change to 25% and 15% to achieve the EnergyStar label.

For existing roofs, external coatings can be applied to achieve cool roof performance. Elastomeric coatings can be applied using an airless sprayer, paint brush, or roller. In addition to the energy benefits of these coatings, some shield the roof from UV rays, thereby protecting insulation and other roof components from UV damage. Some products also contain fungicides to prevent growth on the roof.

Energy Analysis

A case study was done for a warehouse building in Commerce, California near Los Angeles. Using the energy modeling program EnergyPro, the effect of adding a cool roof to this structure was studied. The cool roof selected was a Duro-Last roof that had an 88% initial reflectance and an 87% initial emittance as specified by the manufacturer.

Adding a cool roof with the properties listed had a net positive effect on the thermal performance of this case study. The following table shows a simple payback analysis.

Roof Type

Roof Area
(sq.ft.)

First Cost

Annual Energy Use

Annual Energy Savings

Simple Payback Period

kWh

Therms

Cost

Standard

187,000

$1.31 million

337,860

13,096

$103,129

-

-

Cool Roof

187,000

$1.68 million

243,529

19,238

$79,538

$23,591

15.9 years

The annual electricity usage is reduced due to the cool roof, but the amount of natural gas used for heating increases. Since energy use for cooling is much greater than that for heating in Southern California, the net effect of the cool roof is an overall energy use reduction. The energy cost was based on the Southern California Edison TOU-8 rate schedule for electricity and the Southern California Gas Company GN-10 schedule for natural gas.

For an existing roof, the first cost of a spray-on cool roof coatings would be about the same as the first cost difference shown above (approximately $2/sf), and the energy savings would be about the same as well. Therefore, the economic analysis of cool roofs is very similar for new construction and existing buildings.

Additional Resources

The following websites contain additional information on cool roofs.

Government websites

Industry websites

Manufacturer websites

Cool Roof with Temperature Variance Graph

Design Innovation: Cool roofs are best used in hot climates to maximize the energy savings in summer and minimize the energy penalty in winter

 

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