Green roofs are cool!
The first life-cycle assessment of green roofs confirms that they save energy as well as improve the larger environment.
The conventional wisdom that green roofs save energy and reduce pollution is correct, according to the first life-cycle assessment (LCA) of these rooftop gardens. In the analysis posted today on ES&T’s Research ASAP website (DOI: 10.1021/es0517522), researchers from the University of Toronto found that replacing the standard flat roof of an 8-story apartment building in Madrid with a green roof would reduce the need for air conditioning by 6% and overall energy use by 1.2%. The study also details how green roofs benefit the larger environment by reducing runoff and pollution.
Green roofs are popular throughout Europe, especially in Germany; their use is also burgeoning in U.S. cities such as Chicago. Two different approaches exist to adding greenery to rooftops. Low-growing, drought-resistant plants, such as sedum—usually introduced on an existing building—form a lightweight extensive green roof. Intensive roofs feature taller, more-conventional plantings—flowers, vegetables, even trees—and have the advantage of doubling as green spaces for residents or the public. However, because they are heavier, they are usually incorporated into the design of new buildings.
The researchers performed the LCA because they felt that potential environmental benefits or costs were being ignored when green roofs were compared with other types of roofs, says coauthor Brad Bass. An LCA helps experts understand the impacts of a complex process or system by classifying and delineating the various parameters that play a role and detailing the environmental benefits and impacts. The Madrid building was chosen because lead author and architect Susana Saiz provided both blueprints and complete materials specifications, essential for an effective LCA, Bass says.
The study compared the LCA for an extensive green roof with LCAs for reflective white and conventional roofs. “LCA revealed a larger number of impacts [than expected], which the green roof could reduce,” says Bass. He adds that most of the potential benefits accrue to the larger environment. These include 5% decreases in eutrophication (the addition of excess nutrients, such as nitrate, to water) and abiotic depletion (the loss of minerals and other natural resources to the environment) and a roughly 2.5% drop in potential human and aquatic toxicity. When the analysis was broadened to model the reuse of water from a collection system within the green roof to maintain the vegetation, the benefits increased. For example, decreases in eutrophication and abiotic depletion rose to 7%.
A group headed by architect and LCA expert Robert Ries at the University of Pittsburgh recently completed its own LCA of a green roof atop a grocery store and obtained similar results, including a 1% decrease in overall energy use and greater decreases in the need for summertime cooling.
Although LCAs do not address cost issues directly, they can be useful research tools, says Steven Peck, the founder of the industry group Green Roofs for Healthy Cities. For example, LCAs can aid in optimizing green roofs as a technology to reduce pollutants. In effect, this is “the greening of the green roof,” he adds.
Working with faculty at Ryerson University, the City of Toronto recently completed a cost–benefit analysis of green roofs. By examining various factors—such as storm-water runoff and its impacts on beach closures—the city determined that the benefits of these roofs exceeded their costs, Bass says. As a result, green-roof advocates successfully lobbied the City of Toronto for a bylaw that mandates that every city-owned building will get a green roof unless physical reasons preclude it.
“When you look at the big-picture impact of green roofs, when you find out that they can reduce the urban heat island by 1–2 °C, and you further find out that a 1 °C reduction in the temperature of the city in Toronto reduces peak [electrical] demand by 5%—we’re starting to talk about hundreds of millions of dollars,” Peck points out.
Meanwhile, the University of Toronto has declared that every new building on its campuses will be designed to be “green-roof-ready” according to Bass. “If the roof can support it and the budget is there, there are more benefits with an intensive roof. You get social benefits, and you may get even a stronger energy benefit too,” he adds. Peck agrees. Emphasizing the need to integrate green roofs into design, he says, “The roofs of our cities are the last great opportunity we have to generate a positive impact from that space.” —BARBARA BOOTH
