Both urban expansion and climate change are expected to raise the temperature of cities and the built environment. 

A new report, Urban adaptation can roll back warming of emerging megapolitan regions, published in Proceedings of the Journal of the National Academy of Sciences, suggests that cool roofs and green roofs can counteract both processes. The report was authored by Matei Georgescu of Arizona State University and Philip E. Morefield, Britta G. Bierwagen, and Christopher P. Weaver of the US Environmental Protection Agency.

The continued growth of metropolitan areas, which the report calls “megapolitan expansion,” is expected to raise temperatures by one to two degrees Celsius by the year 2100 across much of of the US, and not just in individual cities. This rise in temperatures is separate from that caused by greenhouse gasses. The report states that design and urban planning tactics can mitigate rising temperatures.

The authors used climate simulations to evaluate “green, cool roof, and hybrid approaches to ameliorate the warming.

“Our results quantify how judicious choices in urban planning and design cannot only counteract the climatological impacts of the urban expansion itself but also, can, in fact, even offset a significant percentage of future greenhouse warming over large scales,” the authors said.

Australia may see even greater change. Temperatures in Australia will rise by two degrees Celsius or more according to the Intergovernmental Panel on Climate Change. The State of the Climate 2012 report says average temperatures will rise by one to five degrees Celsius by 2070. The report was prepared by CSIRO and the Bureau of Meteorology.

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Other recent research agrees with the cool roofs study. White roofs, the Lawrence Berkeley National Laboratory study found, are three times more effective than green roofs, and five times more effective than black roofs, in directly offsetting global warming by reflecting sunlight back into the atmosphere. Over a 50-year timeframe, white roofs are also the cheapest option, the study found.

In colder climates, critics argue, a cool roof comes with a “winter heating penalty,” meaning that it will reflect heat that a black roof would transfer into the structure. One of the study’s co-authors, Ben Mandel, acknowledged as much here, saying “it is important to consider ways to conserve the most energy over the whole calendar, not just during summer.”

The US EPA acknowledged that cool roofs can lead to greater heating needs in winter, but noted that on the whole, “cool roofs result in net energy savings, especially in areas where electricity prices are high.”

The EPA also cites a California study that found that “cool roofs provide an average yearly net savings of almost 50 cents per square foot,” or $5.38 per square metre. That figure includes increased winter heating costs, the higher cost of cool roofing materials, as well as savings from downsized cooling equipment and reduced labor and material costs thanks to cool roofs’ longer life.

Options for cool roofs

The Cool Roof Rating Council, a non-profit membership organization, rates the radiative properties of roofing materials. Solar reflectance is the fraction of solar energy reflected by the roof, while thermal emittance is the relative ability of the roof to radiate absorbed heat. Both properties are scored from 0 to 1, with higher scores “cooler.”

Several different product types can be considered cool roofs.

Cool roof coatings are like thick paint with reflecting pigment, and are applied to low-sloped roofs in good condition. They contain additives to maintain flexibility, resist algae growth and shed dirt. Cool roof coatings are compatible with asphalt cap-sheet roofs, metal roofs, and various single-ply materials. Cool roof coatings will exhibit solar reflectance of 65 per cent or greater when new, and thermal emittance of 80 to 90 per cent or more.

Single-ply membranes are also applied to low-slope roofs, with either adhesive, mechanical fasteners, or ballast. Seams get tape, glue, or heat welding. Membranes are made of EPDM, TPO, PVC, or CSPE.

Built-up roofs consist of a base layer, fabric reinforcement and a top protective layer that often contains gravel. By using light-colored reflecting gravel, the roof can be made cool.

Modified bitumen sheet membranes are made up of multiple layers of plastic or rubber membranes combined with fabric reinforcement, and topped with mineral granules or left smooth.

Spray polyurethane foam roofs combine two chemicals that adhere to the roof surface. Since the foam is vulnerable to moisture, mechanical, and UV damage, a surface coating that can also serve as a cool roof material is required.

Steep-slope cool roofs have many options. Asphalt shingles offer solar reflectance of 25 to 65 per cent with very low cost. Manufacturers of metal roofs make tiles and panels that may have reflecting pigments and offer solar reflectance of 20 to 90 per cent. Traditional clay, slate, or concrete tiles offer solar reflectance of 10 to 30 per cent, while cool roof tiles use reflecting pigments to provide solar reflectance of 25 to 70 per cent. 

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