Green and cool roofs provide relief for hot cities, but should be sited carefully
Posted by admin on 30th June 2016

More than half of the world’s population lives in cities, and the United Nations projects that this share will rise to 70 percent by 2050. During the daytime, these expanding urban areas absorb more solar energy than the surrounding countryside. At night they radiate the heat back to the atmosphere. Higher temperatures in cities compared to the areas around them create what are known as urban heat islands (UHIs).

Urban heat islands threaten urban sustainability and public health in cities. The heat island effect raises death rates, increases energy demand for cooling, and makes life uncomfortable even for healthy residents. It affects various communities differently: poor and elderly residents and small children are most vulnerable, followed by people who live in highly industrialized areas.

In a recent study conducted by Notre Dame University’s Environmental Change Initiative, we examined whether green roofs, which are covered with plants, and cool roofs, which are surfaced with reflective materials, could reduce UHI effects in Chicago. These roofing strategies are widely used across Europe and have recently been adopted in multiple U.S. cities, including New York, Los Angeles, Chicago and Portland, Oregon. Developing cities in Asia, such as Indore in India are also implementing cool roofs to mitigate UHIs.

Post-doctoral researcher Ashish Sharma on a green roof at the Morris Inn, University of Notre Dame, USA.
Author provided

We found that these roofing strategies did reduce the urban heat island effect, but their impacts were not uniform within the city. They also reduced breezes from Lake Michigan and altered regional air circulation in ways that could offset some of their cooling and air quality benefits. While the roofs still had a net cooling impact, urban planners will need to understand impacts on energy consumption and air quality to maximize benefits from installing green and cool roofs.