BERKELEY, Calif. — Looking specifically at the economic costs and benefits of three different roof types — white, black, and green (or vegetated) roofs — Lawrence Berkeley National Laboratory (LBNL) 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 economically, their environmental and amenity benefits may partially ease 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 LBNL, and Man Pun Wan of Nanyang Technological University in Singapore, provides a direct economic comparison of these three roof types. “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 acknowledged 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 LBNL. “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.
The 50-year life-cycle 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.
The researchers acknowledge that their data are somewhat sparse but contend that their analysis is valuable in that it is 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,” Mandel said.
Publication date: 4/28/2014