There are cool roofs and not so cool roofs, and it has nothing to do with fashion. A cool roof reflects the non-visible part of the sun into space, reducing the atmospheric temperature, and reducing the temperature of whatever is beneath the roof. The same phenomenon is true on a global scale, if you consider the Arctic circle to be the roof of the world. As global average temperature increases, the Arctic becomes warmer, the snow melts, and seawater takes its place. Snow is a great reflector of invisible light; seawater isn’t.
In the cities of the United States, it has been common to roof buildings with tar and gravel. But the result of that practice is that the summer temperatures in many large cities is up to 5°F higher during the day and 20°F higher at night than in the surrounding suburban and rural areas, where there is more green space. Lately, in cities like Chicago and San Francisco, builders are using white roofs or colored roofs that reflect light in the invisible range.
A typical white roof is made using a highly reflective elastomeric covering. But there are colored roofing shingles that look a lot like the traditional composite shingles that you find everywhere on houses. Only the reflective shingles reduce roof temperatures by 50°F to 60°F in the summer, reducing cooling load and air conditioning bills. Scientists at Lawrence Berkeley National Laboratory helped produce the reflective coatings that allow shingles to be something other than pure white.
The Florida Solar Energy Center in Cocoa, Florida, does research on cool roofs at its Flexible Roof Facility, where they measure the effects on attic temperature and heat flux using various roofing materials. Experiments show that a white metal roof decreases heat flux by 44% compared to traditional black shingles. This translates into 15% less cooling energy use in the house below and a 15% drop in cooling bills. Not a huge amount, but imagine if even one home in ten had a cool roof; the total effect on U.S. energy use in the summer would be significant.
In the Central Valley of California, simulations done by the California Energy Commission predict annual savings of more than 400 kWh for every 1,000 square feet of highly reflective roof surface. Because of the energy and money savings, cool roofs have been a prescriptive requirement in California’s Title 24 building codes since 2005.