CHICAGO, IL — A local landscaper can be an air conditioning contractor’s best friend when commercial customers want energy savings and have environmental concerns.

That might be one of the messages coming from a decidedly offbeat project in downtown Chicago.

On the roof of the 14-story City Hall, structural engineers, architects, and landscapers have put in 12,750 cu ft of a special soil, 20,000 plants, and two full-size trees. It won’t be seen from street level, and chances are it won’t be open to the public.

It’s purpose? Primarily air conditioning energy savings. Studies have shown that rooftop temperatures often run up to 40°F higher than at ground level during the summer in northern Illinois cities. Environmentalists say this is due to the rooftops’ hard surfaces and the common use of black tar, which absorbs heat.

City officials are hoping that the rooftop garden will knock $4,000 annually off the building’s air conditioning costs and extend the life of the mechanical equipment.

An added spin is the use of the foliage to reduce ozone pollution, officials said.

Rooftop Garden Recipe

The project, which got underway in mid-May, starts with a layer of gravel, then a sheet of water-permeable membrane. On top of that is placed a special soil mixture of compost, mulch, and sponge-like ingredients.

The soil mixture is designed to have less weight than typical soil and be able to retain more water. The two full-size trees (a cockspur hawthorn and a prairie crabapple) will be placed over two of the building’s structural columns.

The use of vegetation for energy and environmental reasons is based on rooftop garden ideas Chicago Mayor Richard Daley noticed on his travels in Europe. Some other Chicago buildings are also targeted for such work.

Comparable non-rooftop-landscaped buildings will be monitored to see how much of an impact such efforts have.

That research aspect, in part, justifies the $1.4 million price tag for the project. City officials also noted the money is coming from part of a settlement the city won in a dispute with the electrical company that serves the city.

The project’s impact on cooling costs will not be fully measured until next summer.

Publication date: 09/04/2000