A recent study out of the University of Notre Dame has examined whether green or reflective roofs could help over-heated cities cool down.

Conducted by the University’s Environmental Change Initiative, the study researched the effects of plant-covered green roofs and the effects cool roofs surfaced with reflective materials might have on cities that are known to create urban heat islands, or UHIs.

These UHIs absorb more solar energy during the day than the rural areas around them, and they emit heat back into the atmosphere at night. Although it may not sound like a serious problem, these UHIs increase all-around demand for cooling (and therefore, have an effect on cooling costs for the area). They can also make the entire population uncomfortable and can even raise death rates.

Although this particular study focused on reducing UHI effects in the Chicago area, cool and green roof tactics have been implemented in major cities throughout the U.S. (including New York, Los Angeles, and Portland) and throughout Europe. Even developing cities throughout Asia are trying out the idea.

Both green and reflective roofs have their positive points. Reflective roofs do not require care and generally cost less than green roofs. But green roofs can improve urban air quality and add valuable greenery to cities. Exploring the impact that these different types of roof structures have on urban temperatures could make a big difference for the comfort of city residents.

The strategy does make a lot of sense, as changing out the material used for roofing has been shown to have a significant impact on how cool an interior structure can be. Switching to a metal roof in lieu of asphalt or tiles can reduce the energy costs associated with cooling your home by up to 20%, and can reduce peak cooling demand by up to 15% in many cases. But the outcome of plant-covered and reflective roofs on these urban areas as a whole is not so black-and-white.

The best news is that when these types of roofs were used in Chicago’s downtown area during peak daylight hours, roof surface temperatures were lowered by seven to eight degrees celcius. Comparatively, when these types of roofs were used across the Chicago metropolitan area, roof surface temperatures decreased only by two to three degrees celcius. In other words, these tactics are significantly more effective in city centers.

Although the new roofing approach was able to reduce the UHI effect, the results weren’t consistent throughout the entire city. In Chicago, they actually had a surprising adverse effect in that they ended up reducing the regional air circulation in the form of Lake Michigan’s breezes. This highlights the need to take regional differences into account when planning for cool and green roofs in order to fully reap the benefits.

Since UHIs continue to be a growing problem throughout the world, these measures will need to be studied and improved upon. UHIs pose a risk to our health, our economy, and our energy resources. Only with methods like these — along with the addition of other natural elements like parks, trees, and greenery — will we be able to see results for our planet and our people.