Cool Roofs

February 12, 2014
 Green roofs and cool roofs have unintended consequences

These days, global warming is a pretty hot topic. A lot of attention is being paid to how industrialization, especially in urban areas, contributes to warmer temperatures. It’s been reported that the temperature in an urban area can be increased by several degrees. This is known as the urban heat island effect. To counter this, engineers have come up with a number of heat reducing technologies. Two of these include roofing materials.

Two heat reducing roofs

One of the factors believed to contribute to the urban heat island effect is dark roofing materials. By far the most common roofing material is asphalt shingles. These darker roofs absorb heat. It’s theorized that in urban areas where there are many dark colored roofs packed close together, the heat absorbed by those roofs increases the temperature in that region. This has led to two alternative roofing types.

The first is cool roofs, also known as white roofs. A cool roof can be made from many different materials. What makes it a cool roof is the white reflective coating placed over it. The white coating, rather than trapping heat, reflects it back.

Green roofs, use a different method for reducing the urban heat island effect. The vegetation on a green roof absorbs the energy from the sun. The layer of vegetation prevents the heat from becoming trapped and raising the temperature.

Unexpected consequences

While many proponents of cool and green roofs are optimistic about these alternative roofing options, a new study casts some doubt on their effectiveness, at least in some areas. The study was carried out by Arizona State University's School of Geographical Sciences and Urban Planning. Their conclusion was that when it comes to these alternative roofing options, geography is everything.

One consideration often overlooked is the climate of the area in which the roofs are installed. For instance, cool roofs are effective at reducing cooling costs in the summer. Because they reflect heat, homeowners don’t need to spend as much money keeping their homes cool. However, in northern climates where winters are much colder, the cool roofs can do more harm than good. By reflecting heat during winter, cool roofs can cost homeowners more on heating costs in winter than they save on cooling costs in summer.

Another interesting finding is that cool roofs can have an effect on precipitation. According to simulations for Florida for example, urban areas with cool roofs could reduce precipitation by up to 50% leading to water shortages for the region.

According to researchers, the data suggests that a blanket approach for all areas will not work. Different regions will have to adopt the heat reduction strategies that will work best for those areas.

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