Assessing practical measures to reduce urban heat: Green and cool roofs

As cities continue to grow and develop under climate change, identifying and assessing practical approaches to mitigate high urban temperatures is critical to help provide thermally comfortable, attractive and sustainable urban environments. Green and cool roofs are commonly reported to provide urban heat mitigation potential; however, their performance is highly dependent upon their design, particularly green roofs that vary in substrate depth, vegetation species, and watering regime. This study compares the insulating properties, the radiation budget and surface energy balance of four experimental rooftops, including a green roof (extensive green roof planted with Sedum) and a cool roof (uninsulated rooftop coated with white elastomeric paint), over the summer of 2011-12 in Melbourne, Australia. For the roof treatments explored here, results suggest that cool roofs, combined with insulation, provide the greatest overall benefit in terms of urban heat mitigation and energy transfer into buildings. The high albedo of the cool roof substantially reduced net radiation, leaving less energy available at the surface for sensible heating during the day. Under warm and sunny conditions, when soil moisture was limited, evapotranspiration from the green roof was low, leading to high sensible heat fluxes during the day. Irrigation improved the performance of the green roof by increasing evapotranspiration. Daytime Bowen ratios decreased from above four during dry conditions, to less than one after irrigation, yet sensible heat fluxes were still higher than for the cool roof. These results demonstrate that rooftops must be designed accordingly to target specific performance objectives, such as heat mitigation.