Evaluation of Different Roof Materials for the Mitigation of Urban Warming in a Subtropical Monsoon Climate

Different roof materials are deployed for mitigating the urban heat, which significantly affects our life. However, the answer to the question of which roof material has the best cooling performance in urban areas remains uncertain. We need to compare different roof materials together under a certain climate condition to find geographically appropriate mitigation strategies. This study performs field experiments using four different roof materials (gray, white, and grass surfaces, and rooftop solar panel) to evaluate their effectiveness on temperature reductions in a subtropical monsoon climate region 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. On the other hand, 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 the adoption of grass roofs requires consideration of the availability of water resources even in humid regions. The solar panel reduced the daily surface temperature by 0.59 °C but exerted strong warming effect (7.36 °C) during midday and cooling effect (4.03 °C) during midnight because of its low albedo, low emissivity, and low heat capacity. Our results suggest that the grass roof with a native plant species is still not comparable with the white roof for mitigating urban heat in a subtropical monsoon climate under the present climatic conditions and especially for drier climates predicted for the future, and rooftop solar panels may heat the urban atmosphere by their warm surface during daytime.