Urban heat islands are a significant environmental problem faced by cities worldwide. Higher urban temperatures can impact communities in a number of ways—increasing the concentration of air pollution, impairing water quality, increasing the demand for electricity, and causing heat-related morbidity and mortality.
Urban overheating impacts low-income populations more since these populations often live in lower quality buildings in hotter neighbourhoods. The energy consumption in these buildings is much higher than the average energy use and the people who live in them are often unable to afford the cost of running air conditioning. As extreme weather events escalate it’s increasingly important to identify strategies to mitigate urban overheating. One potential mitigation strategy is the installation of cool roofs.
Cool roofs are one of the most efficient methods to mitigate urban overheating. A cool roof is designed to reflect more sunlight than a standard roof. This lowers the temperature of the building, reducing the need for air conditioning and improving the comfort and safety of the occupied space. Unfortunately, the existing research and data on cool roofs is fragmented, which makes it difficult to gain a holistic understanding of the success of cool roofs as a method to reduce urban overheating.
Researchers from the School of Built Environment at University of New South Wales (UNSW), along with their colleagues at the University of Adelaide and the University of Calcutta, have conducted a comprehensive comparative analysis of the impact of urban-scale implementation of cool roofs on the local urban climate. Samira Garshasbi and her colleagues, Jie Feng, Dr Riccardo Paolini, Dr Jean Jonathan Duverge, Dr Carlos Bartesaghi-Koc, Samaneh Arasteh, Assistant Professor Ansar Khan, under the coordination of Professor Mat Santamouris, performed building performance simulations to quantify the energy and indoor air temperature benefits of the application of cool roofs in 17 different types of buildings.
The researchers of the High Performance Architecture Lab of UNSW, used the Weather and Research Forecasting (WRF) model to create accurate mesoscale climate modelling. The climate simulation analysed the existing conditions of urban microclimates and evaluated the impact of the large-scale implementation of cool roofs. The simulation was then coupled with a DesignBuilder model—modelling software which provides in-depth analysis for buildings—to accurately calculate how much cool roofs changed energy demand or indoor thermal conditions when applied to one building, multiple buildings or across an urban area.
The research team analysed the application of cool roofs in 17 different types of buildings in major Australian cities during January and February. Cool roofs have the greatest cooling potential in residential buildings, reducing heat and energy consumption by potentially up to 63.6%, with a slight but still significant impact on office and commercial buildings. The implementation of cool roofs has the highest impact on low-rise buildings with low levels on insulation but can still be noticeable in high-rise buildings with high levels of insulation.
The researchers found that during the summer months cool roofs are estimated to significantly reduce indoor air temperature and the number of overheating hours. The researchers calculated that cool roofs could reduce the maximum indoor air temperature by up to 12 C. This would result in a significant improvement in the temperatures in low-income households, improving the health and mortality of the people living there.
Urban heat islands are a problem that cities increasingly need to address. Cools roofs are a cost-effective strategy to mitigate urban overheating but there is limited documented information on the energy impact of cool roofs in big cities around the world. Research like this fills an important gap in the available information on cool roofs. Quantifying the impact of cool roofs allows researchers and policy makers to gain a deeper understanding of the benefits of cool roofs, providing evidence that large-scale application is a cost-effective solution to make our cities more heat resilient.