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There are two things that keep a roof cool. High reflectivity and high emissivity. Cool roof coatings reflect 70% to 90% of the sun’s energy when newly installed. Most of the energy is radiated away. Conventional roofing materials have reflectivity of 5% to 25%, meaning they absorb 75% to 95% of the sun’s energy. A typical “white” capsheet roof only reflects 25% of the sun’s energy. The absorbed energy is transferred to the building. Emissivity is the ability to radiate away absorbed heat. Most cool roof coatings emit 90% or more. Even though aluminum coated roofs are highly reflective they are not considered a cool roof because they only emit 50% to 60% of the absorbed energy. In southern California conventional roofs can get up to 180º on hot sunny summer days. A cool roof coating keeps the roof at just 10º-15º above the air temperature. This means a cool roof can be 80º to 90º cooler than a conventional roof |
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Even at relatively cool temperatures there is a big difference in how much heat is absorbed. The chart on the left, done by Lawrence Berkeley National Laboratory, demonstrates how different roofing materials are affected on a sunny day when the air temperature was only 55º. Cooler roof surfaces means less heat is transferred into the building. If you air condition your building you will save money on cooling energy. How much you will save depends on many factors including roof insulation, cooling system equipment and other variables. In the right situation, a reflective roof can bring cooling savings of up to 50%1, with a reduction in peak cooling demand of 10% to 15%. |
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