A Berkeley Lab study compared two Fresno homes for a year; the reflective tile roof ran 13.8°C cooler on a summer day, cut cooling electricity by 26%, and peak demand by 37%

Imagine two brand-new houses standing side by side in the same neighborhood. Same size. Same layout. Same blazing California sun overhead. The only real difference between them is the roof: one is topped with dark asphalt shingles, the other with light-colored reflective tile. Now imagine keeping track of all the changes in temperature and all the electrical power that these houses consumed throughout the entire year. That was roughly the case in Fresno, where the findings showed how much your roof can affect comfort and power bills. The study from Lawrence Berkeley National Laboratory, ‘Measured temperature reductions and energy savings from a cool tile roof on a central California home,’ found that the home with the reflective tile roof used much less cooling power compared to the other house, although both houses were exposed to the same weather conditions.

A real experiment, not just a computer model

Simulations have been the basis for the majority of all research performed on roofs, but that is not the case for this study. Researchers chose two single-story houses constructed side by side in Fresno, California, in 2010. One of the houses had a typical dark asphalt shingle roof that was low in reflectivity, while the other had a high-reflectivity concrete tile roof with an air gap. Data was collected for almost a year, from May 2012 to April 2013, on the temperature of the roofs and attics, as well as how much air conditioning was used in each house. Researchers chose Fresno because it sits in California's Central Valley, where homes typically use air conditioning from May through October.


The heat difference was hard to ignore

On a representative sunny day in July, the researchers noticed a marked temperature difference: the reflective tile roof's surface reached a peak temperature 13.8°C lower than the peak temperature of the dark shingle roof that day. In addition, the attic space under it had a temperature that was 10.5°C cooler. This difference matters because attic temperature can affect indoor room temperature, forcing the air conditioner to work longer to cool the home. Over the summer season, the surface temperature of the tile roof was approximately 3.4°C lower than the shingle roof. It's worth noting that the tile roof wasn't just a different color; it also had a ventilated air gap beneath it and nearly twice the thermal mass of the shingle roof. The study's own authors acknowledge that their design didn't fully separate these three factors, though their analysis points to the ventilation gap, more than the color change or the added mass, as the biggest driver of the daytime cooling difference.

Lower bills, not just lower temperatures

Cooler roofs do not just feel nicer; they show up directly on the electricity meter. The report found that the house with the reflective roof had used 26 percent less electricity for cooling throughout a whole year compared to its dark-roofed neighbor. What is even more interesting is the fact that the peak hour demand, which means how much energy the house uses during the hot afternoon hours in summer, decreased by 37 percent. That also helps utilities, because lower peak demand eases pressure on the grid when it is busiest. Combined, the results showed that the home with a reflective roof saved an estimated 20 percent annually in conditioning costs, a dollar figure, not a simple average of the cooling and heating percentages. Electricity in the area cost roughly nine times as much per unit as the natural gas used for heating, so the 26 percent cut in cooling electricity carried far more weight in the total bill than the smaller heating savings did.

What about winter?

Cool roofs may at times be accused of deflecting useful winter sunlight, increasing heating costs. The study found the opposite: the cool roof did not increase energy use and instead reduced it by almost 4 percent. The researchers suggested the slight winter advantage probably came from the tile's ability to retain heat overnight, not its light color, because reflectivity only matters when the sun is shining.

Why this matters beyond one street in Fresno

Rooftops cover a major part of any city when seen from above. As the U.S. Environmental Protection Agency explains, reflective roofing is an example of the most direct ways that urban areas can counter what is called the "urban heat island" phenomenon, in which built-up and paved environments retain much higher temperatures compared to other open areas around them. Replacing traditional roofs with reflective ones can help lower neighborhood temperatures and ease peak demand on power plants.

Is a cool roof right for your home?

As the U.S. Department of Energy says, cool roofs are designed to reflect more sunlight and absorb less heat compared to regular roofs; hence, they ensure that a building stays cooler, just as people wearing white clothes stay cool on hot summer days. However, this particular study was conducted in a hot Californian climate during one year, and the authors noted that cool roofs make far less sense in cold climates with long heating seasons and short summers. However, for those who live in an area that experiences hot summers and mild winters, the Fresno study provides an opportunity to see first-hand what changing the color of one’s roof can actually do for your comfort and your energy bill. The bigger takeaway is refreshingly simple. A roof isn’t merely a cover. It can be a critical, and easily forgotten, means of regulating heat within the home, along with energy and expense, and in some cases, the solution really can be as easy as changing colors.