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American Society of Landscape Architects

 

November 2004 Issue

Chicago's Green Crown
It's a testament to Chicago's urban greening. But is it a model for other green roofs?

By Theodore Eisenman

Chicago's Green Crown Willard Clay

Chicago is undergoing a green makeover. Set atop City Hall, a 12th-floor meadow landscape symbolizes the city's new commitment to urban greening, and vegetated roofs in particular.

Under Mayor Richard M. Daley, Chicago has installed 400,000 trees, planted 63 miles of medians, and remediated over 1,000 acres of brownfield sites. The city requires all new municipal buildings to follow a green building standard patterned upon LEED (Leadership in Energy and Environmental Design), and Chicago has also instituted one of the most progressive green roof policies in the country: Over one million square feet of vegetated roofs are in various stages of development. Completed in 2001, City Hall's roof garden is a prominent element in this citywide greening.

Unlike many vegetated roofs that are driven largely by stormwater management concerns, this roof garden was designed as a pilot project with the U.S. Environmental Protection Agency Urban Heat Island Initiative. The urban heat island effect occurs when dark surfaces such as streets, parking lots, and rooftops absorb and retain heat, collectively increasing ambient air temperature. On hot summer days, for example, city air can be up to 10 degrees Fahrenheit hotter than the surrounding countryside. These elevated temperatures raise summertime cooling costs for buildings and exacerbate air pollution by increasing ground-level ozone (smog).

Chicago is the first U.S. city to undertake an urban heat island initiative of such an ambitious size and scope: The initiative includes testing and applying reflective roofing materials and light-colored paving materials and calculating the energy benefits to be derived from a citywide application of green roof systems. The City Hall roof garden is a crowning statement, and initial results have been encouraging.

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