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Study: White Roof Beats Green, Black

Thursday, February 6, 2014

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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.

Roofing types
Images: Berkeley Lab

The study analyzed 22 commercial flat roof projects in the U.S. in which two or more roof types were used.

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.

Significant Study

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.

Figure

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

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.

   

Tagged categories: Climate Control; Cool roof coatings; Energy efficiency; Green roofs; Lawrence Berkeley National Laboratory; Research; Roofing contractors; Roofs

Comment from Steve Black, (2/6/2014, 7:52 AM)

What climate areas was this economic study based on? Cooling and Mixed only? What about in heating dominated climates? Who sponsored this study, TPO and PVC manufacturers? Did it consider the moisture build up under cool roof membranes that used to "cook out" from under darker roofs. I am all for doing what makes sense in a given environment to have the greatest possible postive impact and do not feel that white is always right. While white roofs do reduce heat island effects, so do green roofs. Green roofs have an added benefit of delaying and reducing storm water discharge peak flows. One could also argue that green roofs do offset climate change by consuming atmospheric carbon as the plantings grow, something that no white roof to date can do. Light colored roofs definitely have their place, however they are not the end all, be all that this article would indicate.


Comment from tim barrett, (2/6/2014, 10:39 AM)

An admittedly incomplete study that is flawed beyond the point where it provides meaningful information, courtesy of pinhead university scholars that have lost touch with real-world conditions.


Comment from Benjamin Mandel, (2/6/2014, 11:48 AM)

Regarding Mr. Black's comments: I am one of the study's authors and can answer some of your questions. This study received data from projects in which a white or green roof was considered instead of a black roof in cities as far north as Portland, OR and Chicago. As such, there were no heating-only buildings in our sample. I agree that the conclusions change when you consider such Northern climates where there is no cooling energy savings and only a heating energy penalty. Our paper (which is regrettably behind a paywall online), is careful to point out that white or green roofs are preferable to black roofs in warm climates. We did not account for the moisture effect you describe, because we have not seen that white roofs are systematically more prone to moisture accumulation. You are correct to point out several important environmental benefits of green roofs, which our study also mentions. Some of these benefits are difficult to monetize, however, and remain beyond the scope of the economic analysis for the time being. We do not conclude that either white or green is "best," simply that white is most cost-effective, and either is preferable compared to black in cities subject to urban heat island concerns. Our study was not sponsored by industry in any way.


Comment from Tom Schwerdt, (2/6/2014, 2:17 PM)

I'm happy with my white roof in Austin, installed about 4 years ago. Attic is cooler (though still VERY hot) in the summer. Completely dry.


Comment from Steve Black, (2/7/2014, 8:13 AM)

Mr. Mandel, thank you for clarifying and answering my questions. It is the title of the article that makes the absolute statement and while I understand headlines sell articles it is important that the content be balanced and accurate. I am glad to read that the study was not funded by the white roof manufacturers. With all of the various groups funding research it is important to understand where the money is coming from and what motives are behind the study.


Comment from KARL BRINKMANN, (2/7/2014, 8:42 AM)

How do green roofs handle harsh winters, we are topping 60" of snow this winter in Chicago, with two months to go? How are businesses supposed to clear off snow and drifts of snow on a green roof without tearing up the greenery?


Comment from M. Halliwell, (2/7/2014, 10:23 AM)

Actually, they can handle the cold and snow quite well, depending on the species of vegetation used. I'm currently working on a project that, once completed, will have a green roof. I'm considerably further north than Chicago or Portland here in Edmonton, Alberta. I'm not able to comment on the architect's plans regarding snow clearing and drifts, unfortunately, but I do know that the project is not unique here so there must be some measures used to deal with snow and ice.


Comment from Tom Schwerdt, (2/10/2014, 10:33 AM)

If it were my green roof, I would just clear the pathways and leave the greenery under the snow.


Comment from EPDM Roofing Association, (2/20/2014, 3:58 PM)

For over a decade, The EPDM Roofing Association (ERA) has been the leading researching body and voice for manufacturers of both white and black EPDM single-ply roofing systems. Based on our concern that this study, released by Lawrence Berkeley National Laboratory, is flawed, ERA assembled a team of experts to provide analysis of the study and detail its extreme shortcomings. Their analysis found that the LBL Researchers failed to follow certain scientific processes, and that the report frequently relied on anecdotal data, potentially biased or incomplete interpretations of data, and lacked quantitative sources of data. The complete analysis can be found at http://www.epdmroofs.org. Furthermore, ERA rejects the recommendation contained in the study that black roofing be banned in warmer climates. Due to the complexity of roof and building science, prescriptive requirements that limit choices available to the architectural and building owner community are not in the best interest of good roof system design. Our overriding concern is that building owners and their design professionals are provided with science-based, field-tested information to help them make the choice of a roofing system that will meet their needs. Additionally, ERA feels that it is imperative to defer any decision related to roof design to architects and/or roof consultants, who have the proper training and understanding of all of the components found within a roofing system.


Comment from Andrew Piedl, (2/21/2014, 3:36 PM)

My impression is that there is very little research done to study these types of issues. There are so many variables involved, it is difficult to accept general statements. Clearly, in more recent years, the idea of 'cost' is shifting as individuals are beginning to expand their concept of 'bottom line' to include environmental impact in a broader sense (yes, that burger was just 99 cents, but we had to clear a small rain forest to make it so cheap!). How does one go about deciding what to include in cost and then assign values? I think that we are just beginning to understand the issues and that we have a long way to go. I thank LBNL for their work and hope that there is plenty of real privately and publicly funded building science research performed in the coming years.


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