Dark roofs are heated by the summer sun and, thus, raise the summertime cooling demand of buildings. For highly absorptive roofs, the difference between the surface and ambient air temperatures may be as high as 50°C (90°F), while for highly reflective roofs with similar insulative properties, the difference is only about 10°C. For this reason, "cool" roofs are effective in reducing cooling energy use. Typically, cool roofs incur no additional cost if changes are incorporated into routine reroofing schedules. Several experiments on individual buildings in California and Florida show that painting roofs white reduces the air-conditioning load between 10% and 50% (corresponding to savings ranging from $10 to $100 per year per 100 m²), depending on the thickness of insulation under the roof. The savings, of course, are strong functions of the thermal integrity of a building and climatic conditions.

Darker roofs quickly warm the air over urban areas, leading to the creation of summer urban "heat islands." On a clear summer afternoon, the air temperature in a typical North American city can be about 1°C - 5°C (2° F - 9°F) hotter than the surrounding rural area. The additional air-conditioning use caused by this urban air temperature increase is responsible for 5% - 10% of urban peak electric demand at a direct annual cost of about a billion dollars. On the community scale, increasing the albedo (solar reflectivity) of roofs can limit or reverse an urban heat island effectively and inexpensively.

The effect of temperature on smog is also very significant. Measured data and computer simulations studying the impact of the temperature in Los Angeles smog show that a significant reduction in ozone concentration is achieved by lowering the ambient temperature. The simulations predict a reduction in population-weighted smog (ozone) of 10% - 20%, resulting from a 2°C cooling in ambient temperature. This reduction, in some scenarios, is comparable to ozone reductions obtained by replacing all gasoline on-road motor vehicles with electric cars. This paper focuses on field data documenting the impact of cool roofs in reducing cooling energy use in several residential and commercial buildings in northern California and in Florida. Simulated savings for several U.S. metropolitan areas are also presented. Finally, policy and implementation issues such as ratings and ASHRAE standards are briefly discussed.

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