Chicago: Green roofs cut energy bills

Chicago, like many other American cities, is battling the urban heat island effect, which is common in cities where many building surfaces are dark-coloured. A heat island is an urban area, significantly hotter than its surroundings. The urban heat island effect causes overheating of cities during the summer, leading to pollution and increased energy consumption from cooling and air conditioning. The condition occurs due to the use of concrete and asphalt in the city. Green roofs are one way of beating the problem.

In Chicago, the municipality realized the effectiveness of green roofs, when a team of scientists flew over Chicago in a police helicopter in 2007. Thermal cameras pointed at a county building with a regular black rooftop (connected to City Hall) measured more than 40 degrees Celsius, whilst City Hall itself with its green roof only measured 21 degrees Celsius.

Since then, a green roofs fad has taken off in Chicago. In 2008, more than 450 green roofs were either in place or under construction – schools, fire stations, museums, etc. Mayor Richard M. Daley has promoted green roofs vigorously and developed a series of policy programmes to get the private sector and civilians to acquire green roofs. One initiative is a tax offer increment financing green building projects.

Green strategy requirements for building projects depend on the type of building in question. However, when it comes to looking at big box buildings from an energy savings point of view, green roofs are the way to go, according to Sadhu Johnston, Chief environmental officer at the Chicago Mayor's office. Not only do green roofs cut down energy bills on cooling but they also cut down energy bills on heating, due to the plants' ability to reflect heat and provide insulation, thereby conserving both heating and cooling energy.  

Positive effects of plants and gardens on roofs

The plants making up a green roof naturally help with building heating and cooling. Plants reflect heat, provide shade and help cool the surrounding air through transpiration.

Rooftop gardens absorb rainfall and reduce runoff that would otherwise collect pollutants and flood sewers. Also, a roof top garden filters and moderates the temperature of any water that does end up in the sewer.

Furthermore, plants filter the air. They improve air quality by using excess carbon dioxide and producing oxygen. On a neighbourhood or regional level, temperatures are lowered and air pollution is reduced when the overall area of dark surfaces is reduced and the area of reflective and shaded surfaces is increased.

The layers of a rooftop garden or green roof protect the constructed roof from damage and can extend both its warranty and its useful life.

The green roof may also add usable leisure space to a property that is attractive not just to people, but to wildlife such as birds and butterflies.