What colour is your roof?
An engineering study compares roof technologies to determine which is best for cold climates.
The colour and type of rooftop can make a sizable impact on its environmental performance, according to research from the Schulich School of Engineering. Joule Bergerson and her research team have found significant energy differences between three types of technologies in their paper, “Sustainability of rooftop technologies in cold climates: Comparative life cycle assessment of white roofs, green roofs, and photovoltaic panels,” published in the Journal of Industrial Ecology.
The study was the first of its kind to make a consistent comparison of rooftop technologies on a life cycle basis, looking at a wide variety of environmental impacts.
Comparing energy performance
The project assessed the environmental impacts from cradle to end of use — i.e. manufacturing, transportation, installation, and energy use phases — of rooftop technologies. The impact of roof type on building energy performance was assessed in Vancouver, Toronto and Calgary with a building energy simulation software (Energy Plus). A life cycle database (Simapro) was used to evaluate the impacts of the upstream (manufacturing, transportation and construction) phases.
The study compared the following types of rooftops:
- White roof — a rooftop that is painted or tiled white to reflect sun and heat
- Green roof — a rooftop that is covered with vegetation and a growing medium
- Photovoltaic roof — roof-mounted solar panels to generate electricity
“We found that roof-mounted photovoltaics are by far the best-performing option in all impact categories,” said Bergerson, assistant professor in the Chemical and Petroleum Engineering Department and the Centre for Environmental Engineering Research and Education.
“Considering energy and greenhouse gas (GHG) emissions, adding PV on the roof of a standard office building saves the equivalent GHG emissions as taking 15 cars off the road. Green roofs also result in GHG savings, although only the equivalent of taking 0.2 cars off the road. In cold climates such as Canada, buildings with white roofs use more energy for heating and add more GHG to the atmosphere than buildings with conventional roofs.”
Choosing rooftop technology
“The results are surprising, even in a city like Vancouver — with relatively low solar radiation and a relatively clean electricity generation system — PV was not only better on energy and GHG emissions savings, but also on other impact categories such as aquatic ecotoxicity where we expected green roofs to perform better,” explains Bergerson. “We were also surprised to see that white roofs have a net negative impact in most categories, including GHG emissions. Our results were consistent across the Canadian geography.”
Bergerson notes the potential advantages of green roofs, “Green roofs provide benefits we could not quantify, for example sound insulation, and there is a large variety of ways in which green roofs could be deployed.” According to Bergerson, “the contribution of this study lies in the fact that we have a potential resource (the rooftop) that can be used in many ways. I hope people can use this information to make informed choices considering regionally specific details and depending on the environmental outcomes they are hoping to achieve when constructing houses or buildings. How much people are willing to pay for the different technologies is an important next question.”
Bergerson worked with research associate Eduard Cubi and two former students, Nicholas F. Zibin and Sarah J. Thompson. The project was initiated by Zibin and Thompson in an undergraduate engineering class at Schulich.