Green roof cooling effect as climate-adaptation tool for tropical cities

Abstract

Global warming has accentuated the urban heat island (UHI) effect in cities. Urban green infrastructures can provide ecosystem services and attendant climate-change adaptation. Compact cities have urban green space (UGS) deficit due to deficient ground-level sites. Numerous building envelopes, including rooftops, facades and walls, furnish ample yet largely untapped greening opportunities. Their efficacy in mitigating climate-change impacts deserve to be investigated by empirical experiments. In compact humid-tropical Hong Kong, two green-roof and a control bare-roof plots were installed on a high-rise building. Precision temperature sensors were installed in a holistic vertical profile extending from outdoor air to roof surface, green-roof material layers, and indoor ceiling and air. The apartments under the plots were kept unoccupied to monitor air-conditioning energy consumption. The comprehensive-systematic data allowed in-depth analysis of thermal performance of vegetation (Sedum and Perennial Peanut) and weather (sunny, cloudy and rainy) in summer. Intense solar radiation at Control plot triggered significant material heating, which in turn warmed near-ground air to intensify UHI effect and indoor space to lift energy consumption. Sedum plot with incomplete plant cover, low transpiration rate and limited substrate moisture experienced feeble evapotranspiration cooling. The warmed roof passed heat to near-ground air and subsurface layers to impose an indoor cooling load. Peanut plot with high transpiration rate can significantly cool foliage surface and near-ground air to ameliorate UHI. Its high moisture-holding capacity, however, can generate a heat-sink to push heat downwards and increase indoor cooling load. Practical hints on green roof design and management were derived from the findings for application in tropical region to contribute to climate-resilient cities.