A: A cool roof is a white or light-colored roof that significantly increases the amount of solar radiation that's reflected back into space, rather than being converted to heat energy and contributing to global warming. Cool roofs have the additional benefit that they result in much less solar heat gain in the house itself - meaning, during summer, that you don't need to run the air conditioner as much - or sleep in discomfort if you don't have an air conditioner.
Or, as the scientists say: "Increasing the solar reflectance of urban surfaces reduces their solar heat gain, lowers their temperatures, and avoids transferring heat back into the atmosphere."
There are multiple technologies for cool roofs, ranging from painting an existing roof white or another light or reflective color, to using light materials when a building is reroofed during its natural lifetime.
Cool roofs are an ancient design feature that has been around for thousands of years, because they're more comfortable. Buildings in hot climates throughout the world have been whitewashed or built of white materials to take advantage of the effect.
Q: How do cool roofs help with global warming?
A: Anthropogenic ("caused-by-humans") global warming has two aspects. One is the so-called "greenhouse effect," in which the atmosphere traps heat instead of letting it flow out into outer space. Human production of carbon dioxide (CO2) and other greenhouse gases has significantly increased the greenhouse effect on Earth.
However, there is also the question of the heat that is trapped by the greenhouse gases - where is it coming from? Much of this heat is caused naturally, by the sun shining on the surface of the earth, on vegetation, and the oceans, where visible light is converted to heat energy. This has been going on for millenia, of course. But humans have changed the amount of heat generated in this way, by putting lots of dark material - such as roofs, parking lots, roads, and other structures - on the surface of the earth. In fact, some analyses suggest that the contribution of this warming each year is the equivalent to the emissions from world’s approximately 600 million cars road for 18 years!
Note: Another big chunk of the heating is all the fossil fuel we burn. That generates heat, much of which is used in turn to generate electricity or to move our cars, but much of which ends up as waste heat in the environment. Scientists estimate that 15-20% of the increase in the Earth's temperature since the dawn of the Industrial Age is due to our generation of heat, rather than the greenhouse effect from power generation.
Q: OK, 600 million cars is a lot, but how does that relate to me?
A: According to (which summarizes a ), of Lawrence Berkeley National Lab and the California Energy Commission, 1,000 square feet of white or very light roof (about half of a normal-sized house in Menlo Park) offsets about 10 tons of CO2 emissions over its lifetime. In comparison, 10 tons of CO2 is approximately the yearly emissions from a typical house in the U.S, and the average car emits about 4 tons of CO2 in a year. So, painting your whole roof white or very light would offset one year of both your house's and your car's greenhouse gas emissions. And keep you cooler in the summer and lower your air conditioning bill.
Q: How do cool roofs compare to other steps I can take, like buying a Prius or putting solar PV on my roof?
A: According to , driving a Prius results in about 15 tons less CO2 over ten years versus a typical car, saving about 1.5 tons per year. So a cool roof has about 2/3 the effect of buying a Prius in terms of lifetime greenhouse gas reduction.
Actions I Can Take
Q: What can I do?
When you put on a new roof, use a light or white colored roofing material. There are several lists of good roofing options, in all kinds of materials. One is the , at the Lawrence Berkeley Lab website.
If it's not yet time to put on a new roof, in many cases you can paint your existing roof with a cool color. Asphalt tile, composite, and gravel roofs can all be painted.
Q: How much does it cost?
If you are already planning to re-roof, there is little to no additional cost for making it a cool roof. Simply work with your roofing contractor to choose an appropriate color in your desired roofing material.