Perhaps the most closely watched element of ASLA's vegetative roof is its performance in terms of energy savings.
"We did do a pre- and post-electric bill (review), and we did have a 10 percent savings during the winter, due to the insulating properties of the roof because the (outside) temperatures aren't able to bear down on the roof itself," Swann says. "We also looked at it during the summer months, and we had about a 3 percent savings.
"What we found is that we were overcooling the building during the summer months. The solution was adjusting the temperatures in the building, as well as telling people that they have to dress for the office, not outside, and that comfortable temperatures will be maintained in the building. We even put thermometers out so that people can see exactly what the temperature really is."
ASLA also found the temperature on the green roof on the hottest summer days can be as much as 43.5 degrees cooler than conventional roofs on neighboring buildings.
The association also installed monitoring equipment to track storm-water runoff, water quality, and air temperature to compare with data from the conventional roof on the building next door, Swann says. During a 10–month monitoring period, ASLA found the roof prevented 27,500 gallons of storm-water — nearly 78 percent of all precipitation hitting the roof — from flowing into the district's sewer and storm-water system. Water-quality testing also showed that the water runoff from the roof contains fewer pollutants, including nitrogen, than typical runoff.
Despite initial concerns from district officials, the green roof's performance and track record has led to at least one important change related to controlling water runoff from roofs.
"The D.C. government actually has been back over here — the permitting division," Swann says. "They're now charging an impervious-surface fee in the city. They came back over here to take a look at our green roof and our monitoring information to see what this green roof is doing for our building (in order) to help them write the regs for this impervious-surface fee. Originally, they weren't too familiar with green roofs, and now they're actually insisting on them."
Vegetative Roof Matures into Showcase for Sustainability
Installing Green Roofs can be Complex Process
Vegetative Roof Evolves Throughout its Life
Facility Managers See Energy Savings with Vegetative Roofs
