Researchers have developed a new tool that allows residents of Los Angeles, Long Beach and other California cities to see whether their roofs reflect heat energy or contribute to the extra warming experienced by urban areas known as the "heat island effect."
All told, the team surveyed rooftops in five major California cities to get a sense of how much heat individual buildings reflect or absorb. Lighter colored roofs tend to reflect heat energy back into the atmosphere while darker roofs tend to absorb heat, warming the buildings they cover and others around them.
Click here to see how reflective your roof is.
The researchers found that roofs in Los Angeles are not nearly as cool as they could be, leading to higher temperatures across the city.
On average, roofs in L.A. absorb 83% of the solar energy coming down from the sun on any given day, said Ronnen Levinson with Lawrence Berkeley National Laboratory's Heat Island Group.
"Which is to say, they are not very reflective at all," he said.
Roofs that reflect large amounts of solar heat stay cooler and save energy.
"If you conduct less heat into the building your air conditioner doesn't have to work as hard to keep the interior of the building cool," said Levinson, who led the study.
These reflective roofs also help curb the "heat island effect" by sending solar energy back into space rather than conducting it to the area around them.
White roofs tend to be the most reflective, sending back 80% to 60% of all solar energy. Gray roofs typically reflect 20% and a dark roof with asphalt shingles reflects about 10%.
Cooler roofs, cooler cities
Los Angeles had the lowest reflectivity among the group, reflecting just 17% of solar energy. San Francisco didn't fare much better, reflecting only 18%. Bakersfield was the most reflective at 20%.
The researchers also looked at what kind of difference more cool roofs could make for Bakersfield in particular.
They found that if the rooftops on homes and businesses in that city were highly reflective, it would lower average afternoon temperatures by up to 0.2 °C in summer and winter.
Many flatter roofs can be retrofitted to reflect more light, Levinson said. Contractors can apply a reflective white coating relatively easily.
Sloped rooftops with clay or concrete tiles can also be coated, but asphalt shingles, which make up about 80% of residential roofs, are not easily changed to be more reflective.
Levinson suggests replacing such roofs when they wear out.
How the research was conducted
To create the map, Levinson and his team culled data collected in summer 2009 by the National Agriculture Imagery Program of the U.S. Department of Agriculture.
The survey tracked crop growth from planes and included information on how much near-infrared light is reflected back into the sky.
This gave them an aerial map showing the reflectivity of various urban areas.
The team combined that with city data on home shapes and sizes and surveyed around 200 samples of roofing material to come up with a sense of how reflective any given roof is.
"To our knowledge, this is the first study to estimate the albedo of rooftops at the scale of entire cities," said USC's George Ban-Weiss who also worked on the study.