A simple effective idea. Cut air conditioning costs in hot climates by 20% by painting your roof brilliant white. Roofing lasts for about 20-25 years so that the change could be phased it gradually at low cost – only about 15% more than a dark roof. According to the NYT:
‘The physics behind cool roofs is simple. Solar energy delivers both light and heat, and the heat from sunlight is readily absorbed by dark colors. (An asphalt roof in New York can rise to 180 degrees on a hot summer day.) Lighter colors, however, reflect back a sizable fraction of the radiation, helping to keep a building — and, more broadly, the city and Earth — cooler. They also re-emit some of the heat they absorb….
Before the advent of central air-conditioning in the mid-20th-century, white- and cream-colored houses with reflective tin roofs were the norm in South Florida, for example. Then central air-conditioning arrived, along with dark roofs whose basic ingredients were often asphalt, tar and bitumen, or asphalt-based shingles. These materials absorb as much as 90 percent of the sun’s heat energy — often useful in New England, but less so in Texas. By contrast, a white roof can absorb as little as 10 percent or 15 percent’.