Cool Roofs

Cool roofs, or reflective roofs, have both a high solar reflectance and high thermal emittance, which means it absorbs less of the sun’s light and heat.
Photo credit: WalMart via Flickr

Therefore, cool roofs reduce temperature fluctuations inside buildings, which lessens the heating and cooling load and saves energy.

Conventional roofs are typically made with dark asphalt or tile shingles, which can absorb up to 83% of incoming sunlight and heat. Dark surfaces, like these traditional roofs, can be up to 70 degrees F (or 40 degrees C) hotter than a white or reflective surface on a bright day. This heat will move through the roof and into the building, which makes it more difficult for the heating, ventilation and air conditioning (HVAC) system to do its job.

Instead, cool roofs can reflect up to 88% of the sun’s light and heat, which reduces the building’s need for temperature control and can cut energy bills.

Cool roofs also reduce heat island effect in urban areas, or the absorption of the sun’s heat by dark or non-reflective nonroof and roof materials that artificially increases the temperature of the surrounding area.

What Is SRI?

As previously mentioned, cool roofs have a high solar reflectance and thermal emittance. Reflectance is the ability of a surface to reflect sunlight, while thermal emittance is the ability of a surface to radiate solar energy. The combination of these two qualities is called the Solar Reflectance Index, or SRI.

The SRI measures the ability of a surface to reject solar heat on a scale of 0 to 100. The higher the SRI, the better the surface is at reflecting and radiating solar light and heat. A standard black surface has an SRI of zero, while a standard white surface has an SRI of 100. Very warm surfaces can go below 0, while very reflective surfaces can exceed 100. Obviously, when looking for a cool roof, the higher the SRI, the better.

Free LEED GA Practice ExamCost of a Cool Roof

Costs generally vary with type of roofing material, but a simple color change from black to white may drastically reduce temperature fluctuations for little or no cost difference. To predict the extra cost you should spend on roofing, you can calculate the roof energy savings and compare with the roofing costs by using www.roofcalc.com. This website will tailor the energy savings calculation by using a building’s size, the climate, the roof type, and other applicable data.

Cool roofs are usually a lot cheaper than green roofs (also known as vegetative roofs). Cool roofs can range from 75 cents per square foot for reflective paints to over $3 per square foor for PVC single-play membranes, while green roofs can cost $15-35 per square foot. Therefore, the payback period for a cool roof is typically much shorter, and reflective roof coatings can pay for themselves in a couple of years. Plus, cool roofs are much more low-maintenance (the same as an ordinary roof), where green roofs can rack up in maintenance costs because of the plants, potential leaks, etc. However, green roofs do have benefits that cool roofs do not, such as reducing stormwater runoff and providing habitat.

The Heat Island Group, an organization that concentrates on the study and development of more reflective surfaces for roadways and buildings, has monitored buildings in Sacramento with lightly colored, more reflective roofs. The group found that these buildings used up to 40% less energy for cooling than buildings with darker roofs. The Florida Solar Energy Center performed a similar study, also showing up to 40% cooling energy savings.

In another example, two separate buildings in Washington, D.C. and Pheonix switched from black EPDM rubber to white EPDM rubber and saved $600 and $1800 on the building’s lifetime energy use.

What to Look for in a Green Roof

According to the United States Department of Energy, three properties an energy-saving roof should have are high solar reflectance, endurance of high reflectance over time, and high emittance (“How to Buy”, Dept. of Energy). It provides guidelines for recommended solar reflectance, depending on roof slope:

Energy Star has rated roofs for their energy-saving capabilities. Energy Star has listed ranked roofs according to initial reflectance values and reflectance values after three years. A complete list can be viewed here.

Currently, Energy Star only considers reflectance, not emittance, when ranking roof material quality. The Cool Roof Rating Council considers both reflectance and emittance when rating roofs, so it may be a more useful resource. The site allows the user to search CRRC-rated cool roofs based on desired roofing type (shingles, tiles, etc.), model, brand, manufacturer, color, slope, and desired solar reflectance and thermal emittance.

Heat Reflective Paint

If you do not want to re-roof but want to save energy, you can paint it with heat reflective paint. They are very popular in places with warm climates, like the Southwest, Australia and Dubai, because they help to reduce cooling needs.

Covering a roof with reflective coating will reduce the indoor air temperature by 2-3 degrees Farenheit, which can decrease cooling costs by 20%. It is about $200 for two 5-gallons, so it can have a short payback period for buildings with high cooling costs.

Its important to remember that heat reflective paint is not insulating, though there are products that have that property. They merely reflect heat, they do not keep heat in or out of the building. They do decrease the amount of work the insulation has to do to keep, because of reduced temperature fluctuations, though.

Also, remember when choosing a heat reflective paint that it doesn’t need to be a light color to be effective. Check out LBNL’s “Cool Colors” database to learn about which colors will
Not all dark colors are a poor choice for heat reflective paint. While it's generally true that light colors are more reflective, there are some dark colors that can be more reflective. You can learn more about "Cool Colors" at the LBNL pigment database.

Cool Roofs and LEED

Cools roofs can contribute to LEED because they reduce heat island effect. In LEED for New Construction 2009, for example, a cool roof could contribute to Sustainable Sites Credit 7.2, Heat Island Effect - Roof. This credit requires that cool roofs have a minimum SRI of 78 for low-sloped roofs or a minimum SRI of 29 for steep-sloped roofs.

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