Taming the Asphalt Jungle

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August 9, 2004 // UPDATED 2:56 pm - April 25, 2007
By: Sarah McKenzie
Sarah McKenzie

Green roofs cool the 'urban heat island effect,' save energy and are prettier to look at. Here's why you might see more lawns in the sky Downtown.

Most people get an eyeful of concrete and asphalt when they look out their windows onto Downtown's cityscape.

Some local leaders are looking to change that by outfitting new and old buildings

with green roofs -- grass and plant surfaces that filter storm-water runoff and improve energy efficiency.

City Councilmember Lisa Goodman (7th Ward), who represents the Downtown business district, Loring Park and Elliot Park, is pushing for a green roof in City Hall's elevated courtyard.

Green roofs are also proposed for the New Central Library at 300 Nicollet Mall, and proposed condominium projects such as the eight-building, 1,095-unit Pillsbury "A" Mill condo development on the East Bank; the North Loop's Tower Lofts conversion at 700 Washington Ave. N.; and Parc Centrale, a proposed 20-story Loring Park tower.

Goodman's office is also crafting a citywide policy to create incentives for property owners who install green rooftops: lower storm water fees, for example.

"The vast majority of people who live and work Downtown are above the second floor. They look out at a sea of 10,000 tar roofs," she said. "Green space softens the hard edges of being around a lot of glass and concrete."

However, Goodman doesn't just want to add soothing greenery to the vegetation-starved urban core. Green roof proponents tout the environmental benefits of cooling the "urban heat island effect."

A Chicago native, Goodman cites Chicago City Hall's green rooftop as a functioning example. Installed in 2001 on the building's city-owned side, the 20,000-square-foot roof is a demonstration project to compare green rooftops to traditional roofs.

Air above Chicago City Hall's green roof has been as much as 50 degrees cooler than neighboring roofs, said David Yocca, senior partner with Elmhurst, Ill.-based Conservation Design Forum, the project's landscape architecture firm.

The air-cooling is a drop in the bucket -- that could be replicated many times. In terms of cold cash, Chicago saves about $10,000 annually on cooling the building.

Other economic pluses and minuses:

- Plus: Municipal water-treatment costs go down because green roofs trap up to 60 percent of the storm-water runoff, allowing it to evaporate naturally.

- Minus: Green roof costs about twice as much to build -- $400,000 versus $200,000 in Chicago's case.

- Plus: A green roof also lasts more than twice as long (based on longer European experience), wiping out the higher installation costs over time.

- Minus: A green roof costs about $2 per square foot to maintain, mostly in staff time for weeding and watering, says Corrie Zoll of the Minneapolis-based Green Institute. Maintenance costs tend to taper off a few years after installation, Zoll said.

Chicago's Yocca said the City Hall demonstration project has spurred several other proposals for Chicago green roofs. They are going above museums and new housing developments, he said.

"It just makes for a richer environment," Yocca said, adding people are starting to look at what has been traditionally the "unused fifth side of a building" in a new way.

Local efforts

Craig Wilson, a University of Minnesota landscape architecture and regional planning graduate student, heads up Minneapolis City Hall's green roof project for Goodman's office.

Said Wilson, "I basically noticed last year that there's a lot of water collecting on the inner courtyard, which is a rooftop of City Hall. There's actually some moss growing, and I thought it would be a good place for a green roof."

Jose Cervantes, director of the Municipal Building Commission, which oversees City Hall maintenance, said the commission expected a report in October about installing a green roof atop the 7,500-square-foot space.

Current plans call for a $674,000 conventional replacement roof in 2006, he said.

Meanwhile, officials shepherding the New Central Library's construction are expected to decide this month if they want a green roof ready for the library's spring 2006 unveiling.

The roof is pricey: $700,000, including $400,000 for a special membrane, said Anne Ulseth, a New Central Library Project Office spokeswoman.

The New Central Library's current budget does include money for the green roof, Ulseth said; donations and city dollars are targeted toward the proposal.

Getting greener

The Green Institute's Phillips Eco-Enterprise Center recently added the Twin Cities' first rooftop garden designed specifically for its environmental benefits.

Downtown's existing green roofs -- such as Brit's Pub's manicured lawn bowling green at 1110 Nicollet Mall, and the rooftop garden at Crowne Plaza Northstar Hotel, 618 2nd Ave. S. -- are considered "intensive" roofs, meaning they're mostly about beauty and need significant maintenance.

Environmentally designed green roofs are "extensive," with largely self-sustaining, energy-efficient plant communities. Typically, they're not publicly accessible.

The Green Institute's roof is a hybrid: an "extensive" layer of water-retaining plants that tolerate extreme temperatures, along with spaces for people to walk.

The green roofs last longer because they are less prone to deterioration from temperature extremes. In summer, water evaporating from the vegetation has a cooling effect, and in the winter, the green layer serves as insulation.

According to Roofscapes, Inc., a Philadelphia-based green roof outfitter, the roofs can retain up to 60 percent of the total annual storm water runoff.

With all that water over their heads, property owners are naturally concerned about a green roof leaking.

However, in his research, Zoll has found green roofs leak less than traditional roofs because there are more layers over the roof membrane.

A typical roof membrane lasts 15 to 20 years compared to 35 to 50 years for a green roof, based on studies in Europe where green roofs are more common. In Germany, for instance, roughly one out of seven roofs is green.

Zoll said green roofs are most susceptible to leaks during installation, he said, but the technology is improving and problems are becoming more rare.

The Green Institute's rooftop starts with a liquid rubber membrane covered in 4 inches of polystyrene for insulation, Zoll explained. A drainage layer resembling an egg carton sits over the polystyrene, covered by light-weight gravel and finally, a layer of succulents, Minnesota bluff prairie grasses and flowers.

Besides improving energy efficiency, green roofs also attract nesting birds.

Green lots

Closer to the ground, there's also a push to make Downtown's parking lots more environmentally friendly.

In a special environmental demonstration project, the Downtown Minneapolis Neighborhood Association (DMNA) plans to spend a $40,000 grant to "green" a Star Tribune parking lot.

The 325-space lot, bounded by 3rd and 4th streets and 5th and Portland avenues, is one of five reserved for newspaper staffers and for event parking.

The proposal calls for a rain garden in the lot's corner to filter storm-water runoff and cool the heat island effect, said Michael McLaughlin, a Downtown Council staff member working on the proposal.

The project team has looked at several "green" options for the nearly shadeless pavement: a new tree canopy around the lot's perimeter; new draining systems; and porous pavement to replace the asphalt, according to a Lark Weller report for the city's Environmental Management office.

The lot slopes toward the 3rd Street & 5th Avenue intersection, and only has one drain to catch all the runoff. The lot now has wood chips that clog street storm drains during heavy rains.

Goals include improving air quality, reducing air temperature above the lot's surface and filtering storm-water runoff that drains into the Mississippi River.

Storm-water runoff can cause flooding and often contains pollutants that threaten water quality and harm aquatic life in streams, rivers and lakes. Excessive urban runoff also depletes groundwater, since the water can't seep back into the ground.

The "green lot" report cites a model Vadnais Heights parking lot that illustrates rain garden benefits. A rain garden is a manmade depression that collects storm water run-off in a "bio-retention" area that allows the water to filter slowly into the soil.

McLaughlin said the Star Tribune wasn't asked to pitch in for the project since its lot will be the model for others Downtown.

DMNA applied for the Mississippi Watershed Management Organization grant to plant more trees in the urban core but decided to use the grant for the parking lot project, said DMNA Chair Tom Hoch.

Karen Swenson, executive director of Groundwork Minneapolis, spearheads the project and works with several city departments, the Downtown Council and the University of Minnesota's Metropolitan Design Center and the Humphrey Institute on the project.