Sustainable Overhead

The Green Institute’s new rooftop is a vision of progress

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A Meadow in the Sky: Extensive Green Roof (Image copyright The Kestrel Design Group, Inc.)

by Evan Reminick

August 2004—On the roof of the Phillips Eco-Enterprise Center, home of The Green Institute, most of the surface that is not covered with solar panels is planted with greenery. It’s an extraordinary sight not only for visitors to the building but also for commuters by the thousands who are whisked by daily on the elevated tracks of the Hiawatha LRT.

The rooftop features not only the largest photovoltaic array in the five-state area of Minnesota, Iowa, the Dakotas and Wisconsin, but also the Twin Cities’ first green rooftop built specifically to increase a building’s energy efficiency and manage stormwater runoff.

Michael Krause, director of The Green Institute, said it was planned to be a high-visibility demonstration project that could generate useful data for designers, planners and academics.

LRT riders get a close look at the green roof atop the Phillips Eco-Enterprise Center

Where the green part of the rooftop ends, the 5-state area's largest photovoltaic array begins

Growing interest

“We are living proof that these progressive technologies can be built around here,” said Krause. “We represent the next generation of environmental thinking that it's not only the right thing to do, it’s the smart thing economically. We’ve taken the barren, unproductive space of an industrial rooftop and put it to work to generate electricity and improve the drainage of our entire property.”

Krause said that over the past several months he has given tours to numerous local developers interested in green roofs and solar energy. Most, he said, represent smaller-scale, high-density urban projects.

Acting as an advocate or consultant, The Green Institute has also influenced decisions to install green rooftops on several prominent buildings in the area.

These include Abbott Northwestern Hospital (installing a green roof atop an old helipad), the new downtown central library now under construction (committed to building a green rooftop), and Minneapolis City Hall (studying a green rooftop for its inner courtyard).

Types of green roofs
An ”intensive” green roof is created through the use of planters, beds or pots. Intensive roofs are mainly decorative, as with a typical “roof garden.” Intensive green roofs may use plants with foliage from one to 15 feet tall and may require several feet of soil depth, according to Emory Knoll Farms, a grower of green roof plants in Street, Maryland.

An “extensive” green roof is a working system planned to manage stormwater, reduce heating and cooling costs, and extend the roof’s lifespan. Extensive green roofs usually use plants with foliage from 2 to 6 inches and from 2 to 4 inches of soil, according to Emory Knoll Farms. Plantings for extensive roofs are made directly atop the roof surface, with intermediary layers of engineered soil and extra waterproofing.

The Phillips Eco-Enterprise Center's green rooftop features both "extensive" and "intensive" plantings.
Michael Krause, executive director of TheGreen Institute (right) with board member Jeff Karr at a recent open house celebrating the green rooftop.

The Green Institute’s green rooftop distinctively features both extensive and intensive plantings. It was designed and funded in part by The Kestrel Design Group, Inc., of Edina.

Construction cost/benefit
Krause says that as a rule of thumb, green rooftops cost twice as much as typical flat roofs to install (ranging from $14 to $25 per square foot for an “extensive” roof) but last twice as long (35-50 years for a green roof vs. 15-20 for conventional membranes).

Corey Zoll, who directs The Green Institute’s GreenSpace Partners program, said the 4,000 square-foot green roof of the Phillips Eco-Enterprise Center cost about $75,000 to install. He said the cost of installing both the green roof and the solar panels was lowered significantly by an abundance of volunteer labor, a benefit few developers can count on.

Zoll noted that living roofs require more maintenance, mostly weeding, during the first years after installation, but that the cost comparison evens out as time goes on.

Yet for green roofs—like other new building technologies vying for market share—cost comparisons can be the weakest selling point. Though their use is common in Europe, extensive green rooftops are just taking hold the United States.

As a result, demonstration projects like the Chicago City Hall and the Phillips Eco-Enterprise Center here in Minneapolis remain too few in number and too new in lifespan to generate the kind of meticulous benchmarks most large developers require when costing construction projects.

A case in point is the Midtown Exchange, the ongoing redevelopment of the former Sears complex in south Minneapolis. After being approached by The Green Institute, the project’s developer, Ryan Companies US, Inc., conducted two separate feasibility studies of green roofing systems, said Mike Ernst, the project manager.

Ernst said vendors’ estimates for the installation costs of green roofs were incomplete. Because of the size of the Midtown Exchange project and the attendant financial risks, Ryan Companies decided not to take the chance on green roofs. Ernst said that Ryan Companies remains interested in working green roofs into future projects that prove more suitable.

Energy savings
Zoll said the 10,000 square foot green rooftop on Chicago’s city hall has yielded a $10,000 annual savings in air conditioning costs alone.

Widespread application of green rooftops, he added, would greatly reduce the “urban heat island” effect. Urban heat islands occur when concentrated development produces significantly higher (up to 10 degrees Fahrenheit in the summertime) temperatures than occur in surrounding countryside, leading to increased energy consumption, health risks and building maintenance.

Visitors to the rooftop deck take in the view.
Green rooftops can help the region, especially the downtown areas, reduce its trend toward becoming an "urban heat island".

Heat islands are subject to regulation by the U.S. Environmental Protection Agency. Krause said the Twin Cities are trending slowly but predictably toward non-attainment of the applicable Clean Air Act standards. He said government and businesses have estimated the cost of meeting additional federal regulations at as much as $250 million per year should the region reach non-attainment.

Stormwater management
Under the building codes of most cities, each property must be able to manage runoff from stormwater so that it does not spill onto adjoining properties. Krause, who sits on the Minneapolis Planning Commission, said most property developments must contain 15-20 percent of “pervious”, or water-permeable, surface to offset the runoff created by impervious surfaces like driveways, parking lots and normal rooftops.

Green roofs are so effective at containing runoff from buildings that the Minneapolis planning staff has begun to offer a one-to-one surface area credit against impermeable surfaces on the ground. Thus, the installation of green roofs could grow to have great applicability in commercial construction, particularly big-box retail development, said Krause.

Working in concert with swales, ponds and other natural drainage measures in high-density development, green rooftops could lower the high cost of installing extensive sewer grids, Krause added. Again, he pointed to an example close to home: the green roof on the Phillips Eco-Enterprise Center is helping The Green Institute to realize its goal of managing 100 percent of its stormwater runoff on-site.

Zoll said green roofs will soon demonstrably lower utility bills in Minneapolis. The city will soon begin to bill property owners for stormwater utility fees according to the amount of impervious surface on a property. Zoll said the city has an opportunity to use this restructured fee to create incentives for property owners to install green rooftops, rain gardens, parking lot green islands, permeable paving and other on-site stormwater management features.