Looking strictly at the economic costs and benefits of three different roof types—black, white and "green" (or vegetated)—Lawrence Berkeley National Laboratory (Berkeley Lab) researchers have found in a new study that white roofs are the most cost-effective over a 50-year time span. While the high installation cost of green roofs sets them back in economic terms, their environmental and amenity benefits may at least partially mitigate their financial burden.

A new report titled "Economic Comparison of White, Green, and Black Flat Roofs in the United States" by Julian Sproul, Benjamin Mandel, and Arthur Rosenfeld of Berkeley Lab, and Man Pun Wan of Nanyang Technological University in Singapore, provides a direct economic comparison of these three roof types. The study will appear in the March 2014 volume of Energy and Buildings and has just been published online. "White roofs win based on the purely economic factors we included, and black roofs should be phased out," said study co-author Rosenfeld, a Berkeley Lab Distinguished Scientist Emeritus and former Commissioner of the California Energy Commission.

The study analyzes 22 commercial flat roof projects in the United States in which two or more roof types were considered. The researchers conducted a 50-year life cycle cost analysis, assuming a 20-year service life for white and black roofs and a 40-year service life for green roofs.

A green roof, often called vegetated roofs or rooftop gardens, has become an increasingly popular choice for aesthetic and environmental reasons. Rosenfeld acknowledges that their economic analysis does not capture all of the benefits of a green roof. For example rooftop gardens provide stormwater management, an appreciable benefit in cities with sewage overflow issues, while helping to cool the roof's surface as well as the air. Green roofs may also give building occupants the opportunity to enjoy green space where they live or work.

 "We leave open the possibility that other factors may make green roofs more attractive or more beneficial options in certain scenarios," said Mandel, a graduate student researcher at Berkeley Lab. "The relative costs and benefits do vary by circumstance."

However, unlike white roofs, green roofs do not offset climate change. White roofs are more reflective than green roofs, reflecting roughly three times more sunlight back into the atmosphere and therefore absorbing less sunlight at earth's surface. By absorbing less sunlight than either green or black roofs, white roofs offset a portion of the warming effect from greenhouse gas emissions.

"Both white and green roofs do a good job at cooling the building and cooling the air in the city, but white roofs are three times more effective at countering climate change than green roofs," said Rosenfeld.

Continue reading: http://newscenter.lbl.gov/news-releases/2014/01/21/white-green-or-black-roofs-berkeley-lab-report-compares-economic-payoffs/