The load on a building's HVAC system can be most brutal in urban areas because of an effect known as the "urban heat island." Caused by energy-absorbing roofs and pavement-and the attendant scarcity of trees and bushes-the phenomenon boosts peak-hour cooling demand by as much as 2 percent for every 1 degree F rise in temperature, according to the U.S. Environmental Protection Agency. In fact, urban temperatures across the United States have risen by an average of 2 degrees to 4 degrees F over the last 40 years.
One way to offset the challenge of the urban climate is with prudent landscaping designs, says Katrin Scholz-Barth, director of sustainable design with HOK Planning Group, Washington, D.C. "Trees and plants provide natural cooling in urban areas by shading surrounding surfaces and through 'evapo-transpiration,' which cools the air through the plant's natural evaporation process," she says, suggesting two areas for attention:
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Parking lots. "Most parking lots are built up to shed water," says Scholz-Barth, "but by planting shrubs and trees in the narrow strip where cars face each other, and channeling rainwater runoff into that 'bio-retention' area, the stormwater is filtered while the landscaping produces a cooling effect."
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Building envelope. Another design innovation is what Scholz-Barth calls "vertical green walls": Trellis-like structures attached to building façades that support climbing plants. "Vertical green walls can be used to help block solar gain in new buildings where building orientation can't be controlled due to lot limitations, or in buildings already in place," she says.
"All this takes is a narrow strip of ground alongside the building, maybe 4 or 5 feet," says Scholz-Barth. "Any number of climbing vines can be planted." Deciduous plants can be used to block summer sun and allow some solar gain in the winter when the leaves have fallen off, she notes.
