Green Roofs Differ in Cooling and Water Handling

Oct. 1, 2008
The first study to compare the performance of different types of green roofs has been completed by the Lady Bird Johnson Wildflower Center at the University of Texas at Austin, the City of Austin, the Raleigh, NC-based Roof Consultants Institute Foundation, Austin Energy, and TBG Partners, also in Austin. The study suggests that buyers should not assume that all green roofs are created equal.

Interest in vegetated roofs has increased as water and energy conservation becomes more important to property owners. The study, completed on six different manufacturers' products, found that green roofs varied greatly in capabilities, such as how much they cooled down a building's interior and how much rainwater they captured during downpours.

"Just having a green roof may not mean anything in terms of preventing water from reaching the street level," says Dr. Mark Simmons, a center ecologist and the lead investigator on the study. "Green roofs have to be done right, and our hope is to help manufacturers understand how to improve their designs." Simmons and collaborators published their findings in Urban Ecosystems and will continue to collect real-time temperature and other data.

The study of 24 experimental rooftops during fall 2006 and spring 2007 at the Wildflower Center suggested that a green roof could reduce a building's air-conditioning bills by about 21 percent, compared to traditional, tar-based black-top roofs. During one 91-degree F. day of the study, a black-topped box without air-conditioning reached 129 degrees F. inside. Meanwhile, the green-roof replicas produced indoor temperatures of 97 to 100 degrees F.

The native plants used on green roofs also affect the indoor and rooftop temperature, as plants cool surfaces by providing shade and by shedding water to cool down. The presence of native plants also helps green roofs capture water. In comparison to sedums, a type of succulent traditionally used on most green roofs, native plants can take in more water and release more of it to the atmosphere.

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