February 18th, 2014 by

By John-Miguel Dalbey

Green Roof in Ontario

Green Roof in Ontario – Photo Credit sookie/Flickr

The urban heat island effect, in which darkly colored construction materials such as asphalt and tar shingles absorb heat and make their urban surroundings warmer, has been well documented for years. However, a recent study published in Proceedings of the National Academy of Sciences shows that refitting buildings with white-painted roofs or green roofs (those with planted trees) can negate this effect. This stands to reason, as white or green colors absorb less heat energy than darker colored materials. Furthermore, green roofing could even go so far as to sequester carbon and reduce warming. This effect is furthered by plants’ process of evapotranspiration, in which plants absorb water in the soil through their roots, and release it through their leaves, adding moisture to the surrounding air. In a city, this increased moisture reduces ambient heat, as water vapor can absorb large amounts of heat energy. Green roofing strategies will also combat warming indirectly, as their direct effects on city temperatures will reduce air conditioning and heating usage. However, the study notes that white roofing only increased winter cooling, as white paint does not insulate as well as plant material and soil, leading to an increase in heating usage.

This study points out that the beneficial effects of green or white roofing may vary between different areas. According to E&E News,Cool roofs in Northern California had a small impact on rainfall compared to Arizona, while other parts of the country faced stronger effects.” This is most likely due to variances in regional weather patterns and ambient heat. But, in total, both green and white roofing are viable, easily implemented means of combating rising city temperatures. Green roofs in particular seem to be a more beneficial solution, as they provide insulation while absorbing carbon and are more aesthetically pleasing.

To learn more about the benefits of green infrastructure like green roofs, check out our book, “Urban Forests Case Studies: Challenges, Potential and Success in a Dozen Cities.”