Study: White Roof Beats Green, Black
When it comes to roofing types, white wins.
White roofs are more cost-effective over a 50-year time span than green (vegetated) or black roofs, according to a new study by Lawrence Berkeley National Laboratory.
The lab's researchers have provided a direct economic comparison of the three roof types in “Economic Comparison of White, Green and Black Flat Roofs in the United States,” in the March 2014 volume of Energy and Buildings and online here.
White was the clear winner, the team found. While green roofs are costly, their environmental and amenity benefits may partially mitigate the financial burden.
Black roofs, however, “should be phased out,” according to study co-author Arthur Rosenfeld, a Berkeley Lab Distinguished Scientist Emeritus and former commissioner of the California Energy Commission.
The study analyzed 22 commercial flat roof projects in the United States in which two or more roof types were used, Berkeley Lab reported in a news release. The researchers conducted a 50-year lifecycle cost analysis—assuming a 20-year service life for white and black roofs and a 40-year service life for green roofs.
The chart shows that there is not a great economic difference in white vs. black roofs (middle bar). In both green vs. black and white vs. green; however, green proves more costly because of its large installation cost (gray) and maintenance (black).
The 50-year lifecycle cost analysis found that even the most inexpensive kind of green roof (with no public access and consisting of only sedum, or prairie grass) costs $7 per square foot more than black roofs over 50 years, while white roofs save $2 per square foot compared to black roofs.
In other words, white roofs cost $9 per square foot less than green roofs over 50 years, or $0.30 per square foot each year.
Limitations and Future Study
The researchers acknowledge that their data are somewhat sparse, but contend that the analysis is valuable in being the first to compare the economic costs and energy savings benefits of all three roof types.
“When we started the study, it wasn’t obvious that white roofs would still be more cost-effective over the long run, taking into account the longer service time of a green roof,” said Benjamin Mandel, a graduate student researcher at Berkeley Lab.
On the other hand, the study results highlight the need to include factors such as health and environment in a more comprehensive analysis.
“We’ve recognized the limitations of an analysis that’s only economic,” Mandel said. “We would want to include these other factors in any future study.”
More Green for Green
Green roofs, often called vegetated roofs or rooftop gardens, have become an increasingly popular choice for aesthetic and environmental reasons.
Rosenfeld admitted that the study’s economic analysis did 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, according to the scientist.
"We leave open the possibility that other factors may make green roofs more attractive or more beneficial options in certain scenarios," said Mandel.
"The relative costs and benefits do vary by circumstance."
Countering Climate Change
Unlike white roofs, however, green roofs do not offset climate change, the scientists reported.
White roofs are more reflective than green, reflecting roughly three times more sunlight back into the atmosphere and thus absorbing less sunlight at earth’s surface, the researchers said. By absorbing less sunlight than either green or black roofs, white roofs offset part of the warming effect from greenhouse gas emissions.
Arthur Rosenfeld, of Berkeley Lab, has been called "California's godfather of energy efficiency." He is a supporter of solar-reflective "cool" roofs as a way to reduce energy costs and address global warming.
“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.
Cool roofs, including white roofs, have been proliferating. They are used in about two-thirds of new roof or re-roof installations in the western U.S. In California, they have been part of prescriptive requirements of the Title 24 Building Energy Efficiency Standards since 2005 for all new nonresidential, flat roof buildings (including alterations and additions).
Black Roof Risks
Black roofs, meanwhile, pose a major health risk in cities that see high temperatures in the summer, the researchers reported.
“In Chicago’s July 1995 heat wave, a major risk factor in mortality was living on the top floor of a building with a black roof,” Rosenfeld said.
This study, then, also points out the importance of government policy in the field, he adds.
“White doesn’t win out over black by that much in economic terms, so government has a role to ban or phase out the use of black or dark roofs, at least in warm climates, because they pose a large negative health risk,” he said.
Julian Sproul of Berkeley Lab and Man Pun Wan of Nanyang Technological University in Singapore were also listed as co-authors of the new study.