A Fresno home replaced dark asphalt shingles with reflective concrete roof tiles; a Lawrence Berkeley study reported 26% lower annual cooling energy use and 37% lower peak-hour demand

A house in Fresno, California, replaced its standard dark asphalt shingle roof with reflective concrete roof tiles and decreased its annual cooling energy use by 26 percent. Peak-hour cooling demand (the amount of air conditioning used at times of the day when electricity is most expensive) decreased by 37 percent. For homeowners facing higher summer bills, those are notable savings.

It's the result of a year-long field study by the Lawrence Berkeley National Laboratory, a US Department of Energy research center. The scientists there studied two nearly identical single-story houses built side by side in Fresno. One had a standard dark asphalt shingle roof. The other had a lighter, concrete tile roof. Sensors measured temperatures at the roof and in the attic, heat moving into the ceiling and ducts, and the electricity used to power the air conditioning in both homes, zeroing in on the difference the roofs made in each home's cooling energy use.

Roof material affects cooling costs more than people assume


Homeowners often think about adding more insulation or replacing old windows when their homes are heating up. Fewer people think about their roofs, even though they are constantly exposed to sunlight. The researchers measured how much sunlight different roofing materials reflected. The standard shingle roof had a solar reflectance of 0.07, while the reflective tile roof had a solar reflectance of 0.51. This means that the standard roof absorbed 0.93 of sunlight, while the reflective roof absorbed only 0.49. In other words, the tile roof reflected more than seven times as much sunlight as the shingle roof. The researchers then measured the attic air temperature under these two roofing types on a summer day. The data showed that the temperature under the tile roof was 18.9 degrees Fahrenheit lower at the hottest time of the day than under the shingle roof. This temperature difference will naturally affect how much and how long the air conditioner needs to run.

Annual and peak-hour savings by the numbers

These are not estimated results from a simulation, but rather data from an annual examination. The tile-roofed home used 26 percent less energy for cooling, for compressors and fans combined, in a whole year, than the house with the shingle roof. That means more than just one particularly hot afternoon. Peak-hour savings matter even more to consumers who want to lower their bills. These are the times when the California utilities charge the highest rates, usually around early afternoon to early evening on summer weekdays. The power needed to cool the house peaked at 37 percent lower for the tile roof: costs fell during the period when electricity is at its priciest. The same roof reduced heating energy used in winter by about 4 percent, since the thermal mass in the roofing kept attics warmer overnight. It is a small number, but it means that the lighter-colored roof has not sacrificed winter heating for summer cooling.

Reflectivity alone did not do all the work

One point often overlooked in discussions of cool roofs is that, in addition to being lighter in color, the tile roof in this study was constructed with an air gap at the leading edges of the tiles that allowed air to flow up and over the roof from eaves to ridge (above-sheathing ventilation). This contributed as much as the higher reflectance to the building's energy savings. This is a consideration, since a dark shingle lying flat on the roof with no airspace underneath would not be expected to perform nearly as well as the raised tile roof tested.

EPA and DOE guidance on cool roofs

According to the Environmental Protection Agency, cool roofs reduce temperatures on the roof surface, inside the building, and in the adjacent exterior air, and can cut maximum indoor temperatures in non-air-conditioned homes by about 2.2-5.9 degrees Fahrenheit. The U.S. Department of Energy's Energy Saver program also lists cool roofing as a direct means to reduce air conditioning use and increase indoor comfort, particularly in sun-drenched warm parts of the country. None of that means every home will see a 26 percent savings. Fresno's long, hot, dry summers make it close to a textbook climate for the result, and actual savings will depend on attic insulation, duct location, and local weather conditions. In different climates, the basic process is the same: a roof that reflects more sunlight passes less heat into the attic and ducts, reducing strain on the air conditioner and energy use for cooling.

Reflective tiles vs dark shingles: What the Fresno data shows

A roof is one of the few home improvements in which the homeowner makes a decision that continues to pay off for the life of the roof. The choice of roofing material determines how much heat the roof absorbs, how hard the air conditioner has to work to counteract the warming effect on the interior of the house, and, by extension, how much it will cost the homeowner in July and August. The next time homeowners consider a cool-roof replacement in a warm-climate state, they should ask the roofing contractor two questions: What is the solar reflectance of this roofing material, and will it be installed in a way that leaves space for air to escape underneath the roof? Given the results of this study, homeowners should consider these factors alongside the cost of the roofing material.