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Reflective roofs could reduce A/C costs

Researchers from the Lawrence Berkeley National Laboratory are investigating how white, reflective roofs can reduce a building’s air conditioning costs.

On a global level, the scientists are exploring the possibility that reflective roofs can cool the earth by reflecting solar radiation back into space. The scientists hope to publish the results in April.

“We are studying the radiative cooling benefit of employing reflective roofs in urban settings and what the radiative cooling benefit means to the extent that we would be able to cool the earth,” said Marc Fischer, co-investigator for the study and staff scientist at the lab.

Fischer said that using white surfaces instead of dark ones on roofs cool areas below. When buildings have white roofs in hot climates, it generally keeps the rooms below at a much cooler temperature, he said.

During sunny days, white roofs can reflect about 80 percent of the sun’s radiation back to space, while dark roofs usually absorb approximately 90 percent of that radiation, which then gets transferred to rooms below and increases air conditioning costs, according to Surabi Menon, lead scientist for the study and staff scientist at the lab.

“If you reflect away this solar radiation from a building, you don’t need to spend as much electricity or money with an air conditioning unit in warm climates,” Fischer said. “Having a reflective surface saves you energy and money.”

While the research studies a reflective roof’s impact on the individual level, it also focuses on measuring how much heat is reflected off the white roofs and back into space, said Francisco Salamanca, a co-investigator of the study and postdoctoral fellow at the lab, in an email.

When solar radiation is reflected off white roofs, some of it gets caught in the layer of pollutants that lingers in the earth’s atmosphere, which Fischer termed the “warming blanket.” Fischer said that this layer, which prevents solar radiation from exiting the atmosphere, is usually heavier in urban industrial environments, Fischer said.

Because a location’s air quality and climate can have such a significant effect on the efficacy of reflective roof, the researchers are conducting tests in India, which has both clean and dirty air sectors, according to Menon.

Although white roofs are advantageous in warm climates near the equator, reflecting solar radiation is not as beneficial in colder climates or locations that experience mostly cloudy conditions, Fischer said.

Fischer said that according to a past campus study, if reflective roofs were installed in most urban — where the presence of smog and poor air quality is typically higher — and low-latitude environments with hot climates, the installments could subtract the equivalent of three years of global fossil fuel

emissions from the atmosphere.
But although installing reflective roofs seems promising, Fischer said these roofs cannot replace better environmental practices.

“Coating roofs white once can subtract three years from the atmosphere, but it is no substitute for refusing fossil fuel CO2 emissions,” Fischer said. “Three years is nothing to a decade or a century. So it is just a small part of a solution to a much bigger problem.”

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