Green Roofing Requires Careful Planning

According to the UN, in the next 30 years nearly 70% of the population will live in cities, compared to 20% only 60 years ago. Bigger cities and more people mean more problems with urban heat. Big cities absorb more heat than the surrounding areas thanks to the all the steel, concrete and auto emissions, to name just a few factors. At night, the heat is radiated back from the cities, keeping the temperatures high. Heat islands interfere with weather and wind patterns and make living uncomfortable, and even dangerous for residents.

(Credit: Chicago City Hall Green Roof by TonyTheTiger under Creative Commons license.)

Green roofing can be part of the solution to the urban heat problem. Cool roofs include those covered with reflective surfaces, or more literally covered in gardens and plants, and they can cut back on the heat. However, a study in Chicago has shown some interesting statistics and some surprising results that researchers didn’t expect.

For example, green roofing did drop temperatures in metropolitan Chicago 3-4 degrees. However, roof surface temperatures in downtown Chicago dropped even more — by 7 – 8 degrees. What researchers didn’t expect was that the cool roofing also altered wind patterns in the city. Normally, the winds over Lake Michigan sweep in and cool the city, especially during the summer months. However, the cool roofs weakened the winds, thereby increasing temperatures in the neighborhoods that normally were cooled by the winds.

Even more interesting is that the winds and heat radiation affect the atmosphere not just above the roofs but up to several miles into the sky. That prevents air flow and air mixing, which causes the air to stagnate at ground level and thereby increases the pollution problem.

Installing green roofs could provide great recreation areas and local garden space, as well as habitat for birds and insect, while cooling the city. In addition, plants can help improve the air quality by removing carbon dioxide and other pollutants. Will that offset the stagnant air problem? Well, researchers are still trying to figure that out. One way planners believe they can offset the problem is by designing streets to channel breezes from outlying areas.

In short, unless cities act to reduce urban heat islands, the rising temperatures can and will threaten human health and strain energy resources. The trick is to find a way to make green and reflective roofing work, while not making the problems worse.