Mitigating the warming in urban areas: Experimental study of different roof materials in a subtropical monsoon climate

Different roof materials are deployed for mitigating the urban heat, which significantly affects
our life. However, the performance of specific roof materials could be influenced by the
background climate. To evaluate the effectiveness of roof materials on temperature reductions in
a subtropical monsoon climate region, this study performs field experiments using four different
roof materials (gray and white surfaces, solar panel, and grass surface) from December 2017 to
July 2018. The results show that the white surface reduced the average daily surface temperature
by 3.37 °C. This cooling effect increased with the increase in surface albedo and incoming solar
radiation. However, the average cooling effect of the grass surface was much lower (0.43 °C).
This is attributable to the low soil moisture, which was influenced by the monsoon, thereby
indicating that irrigation is required to improve the thermal performance of grass roofs even in
humid regions. The solar panel reduced the daily surface temperature by 0.59 °C but exerted
strong warming (7.36 °C) during midday and cooling effects (4.03 °C) during midnight because
of its low albedo, low emissivity, and low heat capacity. Our results suggest that, for the roof
treatments explored here, white roofs are more effective for mitigating urban heat in a
subtropical monsoon climate under the present climatic conditions and especially for drier
climates predicted for the future, while grass roofs are not a sustainable method as they require
irrigation to achieve a cooling effect and solar panels may heat the urban atmosphere.