The Wealth of Green Space

Do you know how hot your roof is?

On a 90 degree July day in downtown Minneapolis, the black asphalt on many downtown roofs can reach a steamy 170 degrees. That’s hot enough to burn your skin in a matter of seconds. Meanwhile, over on the Minneapolis Central Library’s green roof, it’s a relatively cool 92 degrees. Just two degrees above the air temperature.

That 78 degree difference is thanks to the library’s green roof, which uses plants, soil, synthetic membranes, and filter fabric to change how the roof functions. A green roof not only eases the work of an air conditioning system by cooling the air the system takes in, but can also manage rainwater and provide green space. Green roofs also last longer than an asphalt roof, the most common type of commercial roof.

The multiple benefits of green roofs, coupled with the move toward more environmentally friendly and sustainable building practices, are driving up demand for green roofs. And it’s a trend that many say is overdue.

Roof Mechanics

“Green roofs are becoming more viable because of the direct benefits that they can provide to buildings and communities,” says Corrie Zoll, independent contractor and facilitator of the Minnesota Green Roofs Council. “People are becoming more aware of their benefits. Of the suite of green building strategies, green roofs stand out as addressing multiple issues.”

Many of the benefits are achieved through evapotranspiration—the process by which plants pull water from the soil and secrete or transpire it through their leaves as water vapor. As the water evaporates, the surrounding air is cooled in the process. The greater the area that is covered by plants and the more water available to be transpired through them, the greater the cooling action.

Peter MacDonagh, director of design and science at The Kestrel Design Group, a Minneapolis-based design firm that specializes in sustainable design—green roofs in particular—estimates that cooling costs can be reduced by 20 to 30 percent in a building with a green roof. The City of Chicago recently predicted that it would save $4,000 each year in heating and cooling costs due to the 20,300-square-foot green roof on Chicago City Hall.