Cool Roofs Work Best When Needed Most

The people of Santorini, Greece, know something about building for hot climates the rest of the world should learn. Credit: pavant/Shutterstock

White roofs cool cities more when it matters most, during heat waves, a new study has found, making them more effective than previously recognized.

In the battle against a warming planet, painting roofs light colors so they reflect more sunlight sounds too trivial to matter. In fact, no less a source than former US Secretary of Energy, and Nobel Prize winning physicist, Dr Steven Chu has argued the extra energy reflected back into space can help cool the planet as much as eliminating 11 years' worth of greenhouse emissions from cars.

Although there is some benefit in directly increasing radiation into space, a reversal of what happens when ice is replaced by dark land or water, light roofs also help buildings not heat up, and this in turn reduces the need for energy-guzzling air conditioning. In winter there is a counter-effect where light colored roofs reflect some of the light that might otherwise be keeping establishments warm, but for most of the world this is minor compared to the two beneficial processes.

The advantages of a light colored roof can spread beyond the building topped in this way, counteracting the urban heat island effect, in which built-up areas are hotter than surrounding areas.

According to a paper in Environmental Science and Technology, Guangzhou, China, would experience a reduction of 0.8° C (1.4° F) on a typical summer day from a widespread coat of rooftop paint. More significantly, during six of the strongest recent heat waves, the temperature reduction would have been 1.2° C (2.2° F). The findings raise the possibility that related projects, such as urban tree planting, may prove similarly targeted.

"The hotter it is, the more cooling you get with cool roofs – and it is a significant difference, compared to the margin of error," author Dr Dev Millstein of the Lawrence Berkeley National Laboratory said in a statement. "We found that the stagnant conditions of a heat wave, where the air is just sitting over the city, was one of the main factors."

In ordinary summer conditions (left) cool roofs (bottom row) would cool Guangzhou's heat island, but the effect is larger during hear waves. Credit: Berkeley Lab.

Heat waves are associated with major spikes in deaths. These occur not only in hot cities, but locations that have normally cool climates and are consequently ill-prepared for high temperatures when they occur. Energy demand during heat waves can also induce huge financial costs. Electricity grids need to be built for maximum demand, requiring vastly expensive upgrades that may only be used a few days a year. Heat waves, Millstein noted, are “when the electric grid is the most stressed. Air conditioners are running at full speed and with no break, so a small change on the margin can have a bigger impact.”

Lightening roofs may appear such a self-evidently beneficial move that research like this becomes regarded as superfluous. However, cities worldwide continue to swelter with few requiring light-colored roofs on new buildings, let alone taking steps to paint existing ones.

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