‘Green’ roofs differ in capacity to cool interiors


Washington : Increased awareness of water and energy conservation has necessitated interest in vegetated roofs; but not all of them have the same capacity of cooling interiors or garnering rainwater, according to a study.

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A University of Texas (UT) team studied the performance of different types of green roofs. “Just having a green roof may not mean anything in terms of preventing water from reaching street level, for instance,” said Mark Simmons, a lead investigator in the study.

UT’s Wildflower Centre staff designed the first commercial green roof in Austin. Simmons, colleagues and Brian Gardiner from Austech Roof Consultants simulated green roof conditions by studying the manufacturers’ roofs atop metal insulated boxes.

The study of 24 experimental roof-tops during autumn 2006 and spring 2007 at the centre suggested a green roof could reduce a building’s air-conditioning bills by 21 percent compared to traditional, tar-based black-top roofs.

During one 91-degree day of the study, for example, a black topped box without air conditioning reached 129 degrees inside. Meanwhile, the green roof replicas produced indoor temperatures of 97 to 100 degrees Fahrenheit.

“That’s a huge difference to have a 20-or-so degree temperature drop,” Simmons said, noting that green roofs’ mercury-lowering capabilities are also believed to double the lifespan of roofing material.

An even greater temperature difference was found on roof surfaces, where black-top roofs reached 154 degrees Fahrenheit on that 91-degree day. By comparison, the soil temperature of the green roofs was between 88 and 100 degrees Fahrenheit.

Part of the rooftop differences, Simmons noted, resulted from the native plants used on the green roofs. Each had 16 different types of plants native to Texas in a similar arrangement as part of this first-ever study of their use on green roofs.

The study didn’t directly measure their cooling impact. However, plants cool surfaces by providing shade, and by shedding water to cool down, like humans do by sweating.

States such as Texas that experience flash flooding may benefit even more from the ability of green roofs to capture water, lessening runoff onto streets and storm drains.

Yet this feature varied the most among the six manufacturers. The better green roofs retained all of the water during a half-an-inch rainfall, and just under half the water when two inches of rain fell.

These findings were published online in Urban Ecosystems.