01 Dec 2009

Up on the Green Roof

Shad’s new living roof shows how even the most inhospitable environments can be transformed into models of sustainability.
by Roger Thompson


When it came time to replace the leaky roof on Shad Hall — runners joked of dodging buckets placed on the indoor track to catch ceiling drips — the HBS Operations crew responsible for the project opted for a novel approach. Shad’s runners now have a green roof over their heads, a first for HBS.

What’s a green roof? Think garden in the sky. In the case of Shad, the garden covers 5,200 square feet (about a quarter of the roof’s total flat surface), requiring 64 cubic yards of “dirt” spread to a depth of three and a half inches. Planted at eight-inch intervals are 9,000 tiny perennials, including eight varieties of drought-tolerant sedum, a low-growing, flowering plant with water-storing leaves.

Rather than dirt, Shad’s plants grow in expanded shale, a gravel-like growing medium that won’t blow away or compact over time. While it retains water, it also drains well to protect plants from drowning even in heavy rainfall. Beneath the layer of shale lies a moisture-retention fabric to hold water for thirsty plants when needed, making irrigation unnecessary. A leak-detection system can pinpoint holes in the waterproof membrane within a foot, minimizing the need to disturb established plantings during repairs. While it’s hard to imagine today, the thumb-sized seedlings planted in September will spread to completely cover the green roof surface within two years.

A living roof can cost more than twice as much as a conventional roof, but given its positive environmental impact, energy efficiencies, and extended life, it offers benefits over time that outweigh the additional costs. A green roof is actually two roof systems in one. On the bottom is an impermeable membrane similar to that of a conventional roof. Atop that is insulation, a water retention and drainage system, several inches of growing medium, and a variety of hardy plants (see diagram above). The extra layers create a thermal blanket that keeps a building cooler in the summer and warmer in the winter.

In addition to the green roof, another 15,000 square feet atop Shad not suitable for planting were covered with sheets of white PVC (polyvinyl chloride). The light surface provides energy savings because it reflects the sun’s heat and reduces cooling costs.

With completion of the living roof project, Shad has become the School’s showpiece for sustainability, says Andy O’Brien, HBS chief of Operations. “We have photovoltaic panels on a portion of the roof, and we run a cogeneration plant in the basement that uses natural gas to fire a generator to help power the building, with the waste heat going to create domestic hot water,” he explains. This trio of projects, supplemented by other energy- and water-saving measures, anchors the School’s application for Shad to receive LEED (Leadership in Energy and Environmental Design) certification. HBS now has five LEED-certified buildings — Aldrich, Gallatin, Hamilton, McCollum, and Wyss, — and two additional buildings striving for LEED status — the Class of 1959 Chapel and McCulloch. The School’s goal is to achieve LEED Gold Certification for all new construction and major renovations, says O’Brien.

The Shad project underscores HBS’s commitment to doing its part to help the University meet its ambitious goal of reducing greenhouse gas emissions by 30 percent from 2006 levels by 2016, explains Doug Scatterday, director of facilities. The School has already made significant strides toward that goal. Since 2003, HBS has completed more than 45 energy conservation measures, offsetting greenhouse gas emissions by the equivalent of 2,495 metric tons of carbon dioxide, reducing operating costs $920,000, and maximizing utility rebates totaling upwards of $830,000.

Shad’s green roof may be the School’s latest sustainability innovation, but living roofs aren’t a new idea. They’ve been used in Europe for hundreds of years and were common in 19th-century sod homes across the American prairie. The recent surge of interest in green roofs stems from a modern-day concern for mitigating the environmental impacts of conventional roof construction.

In blazing summer heat, the daytime temperatures on a conventional asphalt roof can reach 160 degrees or higher compared to the 80s and 90s for a green roof. A Columbia University study found that green roofs can reduce average daily energy demand of a building by about 15 percent. Because a green roof protects the underlying waterproof membrane from temperature extremes and the sun’s ultraviolet radiation, it can last up to twice as long as a conventional roof.

Green roofs also help reduce the urban heat island effect, a condition created when dark, impermeable surfaces like asphalt absorb heat and radiate it back into the air, forming an urban heat dome.

When it rains, a green roof acts much like a lawn to absorb, retain, and filter runoff. The Shad green roof will retain an estimated 75 percent of annual rainfall, says Charlie Sinkler, whose Somerville, Massachusetts, firm, Apex Green Roofs, installed all the components of the system. Collectively, green roofs in urban settings can help reduce storm-water runoff that causes sewer systems to overflow.

In Germany, Switzerland, Austria, and Japan, laws require green roofs on buildings with roofs of low pitch. In the United States, a number of cities offer builders and companies tax breaks, rebates, and higher-density building allowances for installing green roofs. Chicago, a national leader in green roof installation, now has more than 400 constructed or planned green roofs in the downtown area. The Boston metro area has fewer than two dozen, estimates Sinkler of Apex Green Roofs. Harvard has five of those.

“HBS is an innovation leader in sustainability,” says Heather Henriksen, director of Harvard’s Office for Sustainability. “Andy O’Brien has an incredibly dedicated team that is always looking for continuous improvement.”


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