A heat wave in 1995 caused over 700 deaths in Chicago, spurring the city to champion the so-called “Urban Heat Island Initiative Pilot Project”, aiming to help cities mitigate the heat generated by urbanisation.
Reducing the Chicago heat while also keeping cooling costs down has been a major challenge, but the City of Chicago has found an innovative solution in rooftop gardens – a green solution to lowering both temperatures and utility bills.
An example to follow
Starting with the Chicago City Hall, the city has decided to create a system of green roofs designed to bring down the temperature. Conceived as a demonstration project, the Chicago City Hall Green Roof has sparked the city’s green roof movement and become a flagship for a green roof revolution.
Since the first waterproofing and root barrier layers were installed on the city hall’s 2,000-m2 existing roof structure, the city has seen over one million square metres of green roofs materialise.
Reducing temperatures by 30°C
Data on the Chicago City Hall Green Roof initiative show that it has reduced summer surface temperatures by 30°C, decreased stormwater runoff by 60% and saved the city an estimated USD 75,000 in cooling costs.
The total cooling effect from evapotranspiration (the sum of surface evaporation and plant transpiration) produced by the City Hall Green Roof amounts to 730% of that needed to eliminate the cooling load of the roof.
The surplus cooling effect available for the surrounding microclimate can reduce cooling costs in buildings downwind of the city hall.
These results have made the Chicago City Hall Green Roof a driver for change, and in 2011 almost five million square metres of green rooftops were installed in North America.
The Chicago City Hall Green Roof was retrofitted with 20,000 plants of more than 150 varieties native to the Chicago region.