Science NOW!

Green roofs could cut cooling bills

David Bradley

Rooftop temperatures in many parts of the world get high enough to fry an egg during the summer - up to a scorching 60° Celsius. Anyone inside the building can expect to get hot and sweaty if they don't have an effective way to dissipate the heat energy - an air-conditioning system, for instance.

The temperature beneath a grass roof remains several degrees lower than other roofing materials.

Now, a truly green solution is being investigated by researchers in the USA, who have found that a rooftop garden might help buildings keep their cool. Unlike a layer of heat-absorbing tarmac or other roofing material, a layer of grass on a roof will never get above a balmy 25° Celsius.

Cooling the city

Scientists at Weston Design Consultants in Chicago reckon that with enough green roofs they could lower the temperature of an entire city. According to Weston's study, a few degrees drop in roof temperature means a 10 per cent reduction in air-conditioning needs for the building below. If the building is just a single-story structure with a green rooftop, that figure increases to 20-30 per cent.

Saving energy

The scientists estimate that, if all the roofs in Chicago were carpeted with a lawn and planted with a few trees and shrubs, not only would they look nice from the air, but they would cut the city's energy needs by 720 megawatts a year. This would save its residents some $100 million (about £70m). The researchers point out that the new greenery would also mop up air pollution, bringing a breath of fresh air to the city.

Links

1. What biological processes are involved in helping a grass roof keep its cool?
2. During the winter, cities such as Chicago are exceedingly cold. What effects would a rooftop garden have on the energy requirements of the building below?