Dark materials absorb more heat from the sun -- as anyone is well aware if they've worn a black T-shirt on a sunny day.
Black surfaces in the sun can become up to 70 degrees Fahrenheit hotter than the most reflective lighter and white surfaces and if one of those dark surfaces happens to be the roof of your home, some of the heat is transferred inside -- and onto your energy bill.
So called "cool roofs" at peak condition are designed to reflect the sun's heat energy, and stay 50 to 60 degrees cooler than darker roofs and reduce cooling energy demand by 20 to 70 percent -- for a energy savings of more than $125 a year.
Energy Star-labeled roofing products can reduce temperatures by up to 100 degrees, according to the U.S. Environmental Protection Agency.
Reflecting the sun's damaging heat rays also prolongs the life of the roof and reduces maintenance expenses, according to Lawrence Berkeley National Laboratory's Heat Island Group.
In a study funded by the U.S. EPA, the group studied the energy-saving potentials of light-colored roofs on 10 commercial and residential building prototypes in 11 U.S. metro areas and estimated a savings potential of about $175 million per year for switching to light-colored roofs on that many buildings in 11 metros.
The Heat Island Group also monitored real buildings in Sacramento with lightly colored, more reflective roofs and found that the buildings used up to 40 percent less energy for cooling than buildings with darker roofs.
What's more, the Florida Solar Energy Center performed a similar study with similar results on residential properties.
"It's surprising that something as simple as choosing a different color can make such a difference," said Danny Parker, an energy center spokesman. The center conducted the study for Florida Power and Light Company (FPL) with cooperation from the Habitat for Humanity of Lee County.
In the Florida study, six roofing materials were evaluated: dark gray shingles, white shingles, white flat tile, white S-shaped tile, terra cotta S-shaped Spanish tile and white metal. A seventh home had a new concept: dark gray shingles and a sealed attic with the insulation on the roof decking.
The roofs were constructed on seven identical and adjacent Habitat for Humanity homes in Fort Myers and the study found that energy savings were most strongly influenced by the solar reflectance of roof materials.
Dark gray roofs reflected a mere eight percent of the heat associated with sunlight, while white shingle and terra cotta tile roofs reflected 25 and 34 percent, respectively.
White metal and cement tile roofs revealed the most dramatic results, reflecting 66 to 77 percent of the sun's energy.
"The results of the study clearly demonstrate that white, galvanized metal roofing material saves the most energy as a result of its high reflectance and superior ability to cool quickly at night," said Craig Muccio, evaluation coordinator for FPL's Conservation Program.
The white, galvanized metal roof should save a customer who lives in an average-size 1,770 square foot home approximately $128 or 23 percent annually in cooling costs, compared with a dark gray shingle roof on the same home.
For the same size home, white, S-shaped cement tile produced the second-best savings of $110 or 20 percent of annual cooling costs followed by white, flat cement calculated at $96 per year for a 17 percent savings compared to the dark gray shingles.
White shingled roofs trimmed $24 or about four percent off the annual cooling bill, while terra cotta S-shaped cement tiles netted a modest $15, or three percent compared to dark gray shingles.
The unvented attic with dark shingles and foam insulation applied directly to the roof decking resulted in an average energy savings of about 9 percent or $50 annually in cooling costs. While this is notable, it is not nearly as significant as the energy savings associated with lower-priced reflective roofing materials.
The unvented attic concept also led to peak roof decking temperatures that were 20 degrees higher than a ventilated attic. As a result, the researchers concluded the unvented attic concept should only be used with light colored roofing or tile roofs to avoid endangering the life of the roofing system.
"Choosing a roof for a home, whether it's a new home or a replacement roof, is a major decision for most homeowners. The purpose of this study is to provide homeowners with additional information to help them make the best, most informed decision. Every little bit we do to help our customers stay informed should ultimately conserve energy as well as save them money," Muccio said.