Green-roof thermal effects in the context of climate change and sustainable urban design

Abstract

With the growing urbanized population, cities have become a major contributor to global energy consumption and greenhouse gas (GHG) emission. The urbanization processes also cause local climate change through excessive anthropogenic heat emission and modification of the land biophysical properties. The resultant urban heat island (UHI) effects and aggravating human heat stress have become key environmental issues in city management. Cities can be designed to be climate-conscious and energy-efficient not only to contribute to urban sustainability, but also address global climate-change issues at the local level. Green roof presents a feasible strategy for climate-conscious urban design (CCUD). With the notable thermal effects in microclimate amelioration and energy conservation, it has great potential to help cities tackle local and global climate issues. Understanding the status and underlying mechanism of green-roof thermal effects can inform optimal design and management, and provide scientific basis to promote green-roof application. This study formulates a multidisciplinary framework to assess green-roof thermal effects at building, neighborhood and district scales, based on a case study in Hong Kong. Firstly, the building-scale field measurement found that the 484 m^2 experimental extensive green roof can significantly ameliorate rooftop microclimate and cut building energy consumption. The peak surface and air temperature can be reduced by 11℃ and 4.5℃ on sunny summer days, and 7.2℃ and 2.3℃ on cloudy days, leading to an electrical energy saving of 2.80×〖10〗^4 kWh for a summer of air conditioning. The thermal performance was sensitive to background environmental factors. Solar radiation and relative humidity governs the seasonal and diurnal variation of air and surface temperatures reduction. Substrate moisture can significantly regulate the subsurface temperature but has limited effect on evapotranspiration (ET). Based on correlation and scenario analyses, this study has formulated an irrigation scheme which could balance between plant growth, thermal performance and water efficiency to achieve sustainable management of tropical extensive green roofs. Secondly, the neighborhood-scale modeling revealed that greening all roofs in residential communities can extend the cooling effects from the rooftop to the entire neighborhood. Urban design factors such as building height, distance, site coverage and orientation can affect the diurnal, horizontal and vertical pattern of the “cool-islands” created by green roofs. Green roof can also enhance the rooftop thermal comfort by alleviating the intensity and duration of heat stress. The findings suggest that compact cities can green the roof and podium space to provide thermally comfortable and recreational venues for urban residents. Thirdly, the district-scale cost-benefit analysis found that large-scale construction of green-roof infrastructure in Hong Kong can be well justified by its thermal benefits. The extensive green roof has an annual monetary benefit of HK$0.9–1.7 billion, and the intensive, HK$1.4–2.6 billion, in terms of energy saving, CO2 and air pollutant reduction. The life-cycle benefit-cost ratio (BCA) is 3.7–7 for extensive green roof, and 1.4–2.7 for the intensive, indicating the high cost-effectiveness of both types, with the extensive being more economically attractive than the intensive.