Cool roofs

In warm climates, cools roofs can pay for themselves in energy savings
By Brian Sloboda
Cool roofs

Most homeowners dread the thought of roof replacement or repair. But by installing a “cool” roof you can save money — and energy — for little to no additional cost and effort.

Cool roofs reflect the sun using materials that have a special coating. During summer, they stay 50 to 60 degrees Fahrenheit cooler than traditional construction. Because these roofs maintain a lower temperature, less energy is needed to cool the space beneath them.

According to the U.S. Department of Energy (DOE), cool roofs trim cooling loads by up to 15 percent. This not only cuts electric bills, but also extends roof life, reduces wear on cooling systems, and leads to more comfortable indoor temperatures — especially in houses with limited insulation or no air-conditioning at all.

Before purchasing a cool roof, consider adding insulation to your attic or crawl space because it remains affordable and provides year-round energy savings. For ceilings and roofs, R-30 to R-60 is usually sufficient, depending on climate. DOE offers a calculator that helps determine the insulation you need based on your ZIP code at ornl.gov/~roofs/Zip/ZipHome.html.

In addition, consider installing attic vents — continuous peak, soffit or turbine — especially if you’re replacing your roof. This shrinks heat transfer to living spaces. For more information on insulation and attic vent selection, visit EnergySavers.gov.

If you decide to go with a cool roof, research the type of roofing you want and how much protection you need for your area. The coolness of a roof is determined by two properties: solar reflectance and thermal emittance. Solar reflectance simply equates to the amount of solar radiation reflected, while thermal emittance spells out how efficiently the roof cools itself by re-radiating that heat.

The combination of these two properties, called the solar reflectance index (SRI), is typically shown as a rating from 0-1. Higher ratings mean increased reflectivity and emissivity. Cool roofs boast an SRI of up to 0.85, while a conventional roof may only rate 0.05.

Cool roofs work best in sunny, warm climates where daily temperatures average above 80 degrees Fahrenheit for at least three months of the year. In northern, colder regions the opportunity for energy savings may not be as large because there are fewer cooling degree-days. But there’s no disadvantage in choosing a cool roof in those places because your attic should already be well-insulated.

The main cost of installing a cool roof involves the type of material you choose. DOE estimates you’ll spend an average of 75 cents per square foot extra for a cool roof, but you’ll experience quick payback for the investment thanks to energy savings and a longer roof life.

Here are common cool roof options for residences:

  • Tiles. Roof tiles made of clay, slate or concrete have low reflectivity and high emittance and are naturally cool roofs. Cool-colored coatings or glazes can be applied to the tiles to boost reflectivity and waterproofing. You can apply a cool coating on-site or purchase pre-coated tiles, which don’t cost much more than regular tiles and are offered in traditional colors, such as brown, green and terra cotta.
  • Shingles. Cool asphalt shingles are made with specially coated granules. Unlike tiles, however, cool-colored coatings are not normally recommended for shingles. Wood shakes are naturally cool roofs if they are kept bare and not stained with darker colors.
  • Metal. Unpainted metal is naturally reflective but has very poor thermal emittance. It’s a good candidate for cool coatings, either applied in the field or at the factory.

About the Author

Brian Sloboda is a program manager specializing in energy efficiency for the Cooperative Research Network, a service of the National Rural Electric Cooperative Association.

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