Energy savings bloom on rooftop landscape — study
Posted on September 20th, 2007By Michael Burnham
Greenwire: From atop the American Society of Landscape Architects’ headquarters in downtown Washington, D.C., a grid of low-rise office buildings fans out in every direction.
It’s a great place to see one of urban America’s most glaring energy problems: building bald spots.
Adjacent rooftops of century-old brick structures are layered with black tar. Even sleek glass-and-steel buildings erected in the past 20 years sport hulking heating and cooling units and thick paint on their chrome domes. On steamy summer days sunlight bounces off those barren surfaces in wavy sheets, cranking up the mercury.
Forty-six percent of the District of Columbia’s 61.4 square miles is considered impervious to rainwater. In any given year, this urban jungle of parking lots, rooftops and streets deflects about 2.5 billion gallons of untreated runoff into the Anacostia and Potomac rivers.
During the study period, ASLA’s roof prevented about 27,500 gallons of rain from rushing into the sewers. What’s more, the roof did not appear to add nitrogen to the runoff that did drain into the watershed. Nitrogen and other nutrients create major water quality problems in waterways.
Price, permits
Despite the project’s purported environmental benefits, cost and permitting issues are major barriers facing developers, industry experts say.
ASLA paid about $110 a square foot for it’s new roof, not counting the cost of adding a stairwell to the top.
But the association’s roof is a cool oasis in the urban heat island. The 3,300-square-foot roof is planted with hearty grasses and flowers that soak up rain and insulate the building from winter snow and summer sun. And if Washington’s new mayor has his way, rooftops like ASLA’s will sprout across the city.
The association released findings yesterday of an energy and environmental study of its living rooftop at 636 Eye St. NW. Between May 2006, when the rooftop was planted, and July 2007, the roof retained almost 75 percent of the 29 inches of rain that fell upon it.
And on hot days, the “green roof” was recorded as much as 32 degrees Fahrenheit cooler than neighboring rooftops, according to the study conducted by Howard University civil engineering professor Charles Glass and the University Park, Md., environmental consultant ETEC LLC.
Just like dressing for D.C.’s notoriously fickle weather, layering is the building’s secret to staying comfortable in any season.
Plants, pollution control
Between its structural cross-beams and its surface sedum — plants with thick, fleshy, water-storing leaves — the roof features layers of waterproof membrane, rigid insulation and soil.
About 2 to 6 inches of dirt and pebbles cover the roof’s flat plane where shallow-rooted sedum live. Thicker soil covers berm-like decking that hides the building’s heating and cooling units. There are even a few trees atop the raised elevator shaft.
The mix of vegetation on the berms includes drought-tolerant plants such as butterfly weed and purple lovegrass. After the plants get fully established in another year or so, they should require little or no artificial watering, said Nancy Somerville, ASLA’s chief executive officer.
At least that’s the hope. The roof’s grasses held up well, the study showed, but perennials such as the black-eyed Susans wilted in the summer sun.
“With the perennial and the grass community, we’ve done very little watering, which is why you’re seeing some die-back of the perennials right now,” Somerville said from atop the 12-year-old building. “We’re actually having some philosophical discussions now about what we want this area to be when it grows up.
“Do we want to continue doing a little supplemental watering, or do we want to cut off all of the watering in this area and let the plants that can handle it be the ones to fill in?”
It’s not an inconsequential question.
Evapotranspiration — the natural cycle of water through the air, soil and plants — is a key reason the ASLA roof is able to stay cooler than surrounding rooftops, Somerville explained. Indeed, U.S. EPA says that a well-insulated roof with plants and soil can reduce a building’s temperature by up to 90 degrees F on hot days (Greenwire, June 14).
The environmental benefits of ASLA’s green roof, however, go beyond reducing heating and cooling costs, the study noted.
Green Roofs for Healthy Cities, a Toronto-based industry group, puts the typical cost of a green roof in the range of $10-$24 per square foot.
That’s still about twice the cost of a conventional roof.
“Cost is a factor in promoting green roofs, but more important is the lack of knowledge and lack of policies to support green roof infrastructure,” said Jennifer Sprout, Green Roofs for Healthy Cities’ local market development director. “People want to make a difference and support the idea of green roofs, but they need greater incentives from government to build them.”
For all of its tar-covered rooftops, Washington is widely regarded as a green roof leader. As of last year, the city ranked second in the nation for the most green rooftop space — about 300,000 square feet, according to Sprout’s group.
City officials hope to cover more of Washington’s building bald spots by offering developers technical assistance and referrals to sustainable design experts.
Last December, Washington became the nation’s first big city to require all new construction and renovations of buildings 50,000 square feet and larger to earn Leadership in Energy & Environmental Design certification from the U.S. Green Building Council, starting in 2012 (Greenwire, Feb. 20).
Under the LEED rating system, developers can earn points for installing green roofs.
Anita Hairston, chief of staff in the city’s Office of Planning, said “greening” the city is a major priority for Mayor Adrian Fenty (D), who recently announced the formation of a “Green Collar Jobs Advisory Council” to explore the nascent demand for jobs and businesses related to stormwater management, river restoration, energy efficiency and sustainable design. Whether the broad economic effort — and a little more rooftop gardening — help boost real estate prices and sharpen Washington’s competitive edge remains to be seen.
“We know that globally competitive cities are sustainable cities,” Hairston said.