Thermal and energy performance of two distinct green roofs: Temporal pattern and underlying factors in a subtropical climate

This study evaluated and compared the longterm thermal and energy performance of two distinct large-scale green roofs in a humid subtropical city of China. One is an extensive sedum roof (EGR), and the other is an intensive one (IGR) with much deeper soil layer, higher plant diversity and more complex biomass structure. Roof surface temperature (T-s), air temperature at heights of 10 and 150 cm (T-10, T-150), roof heat flux, and cooling/heating load were analyzed for the green roofs and a control bare roof on hourly, daily, and seasonal basis over an entire year. The two green roofs displayed a similar and consistent performance pattern across the year, characterized by cooling of the roof surface during the day and warming of it at night, and, conversely, warming of the ambient air during the day and cooling of it at night. IGR was more effective than EGR in decreasing daytime and nocturnal T-s, cooling the ambient air, and cutting the summer cooling load, but it added more heating load to the building in winter. Findings from the research suggest that green roofs do not always function in favorable ways for urban heat island (UHI) mitigation and energy conservation but may actually have adverse impacts under certain weather conditions. Despite having a much more complex structure, IGR did not seem to outperform EGR commensurately in thermal benefits. The results can shed light on green-roof design and management for optimization of thermal and energy performance in subtropical areas. (C) 2019 Elsevier B.V. All rights reserved.