An experimental study on the thermal and energy performance of self-sustaining green roofs under severe drought conditions in summer

Abstract

Self-sustaining green roofs can considerably reduce the building cooling demand in summer. In recent decades, extreme weather events including heatwaves and drought have increased, while there is a lack of knowledge of the dynamic thermal performance of green roofs during a severe drought. The present study is based on in-situ measurements of a room with a self-sustaining green roof and a reference room with a bare roof. The temporal evolution of the thermal performance of the green roof during a severe drought is investigated. Long-term observations for several years showed that Sedum lineare on the green roof survived in summers, except for a period during the severe drought. Although extreme drought weather led to extensive mortality, plants revived in the next year. The status of the green roof during the drought period could be divided into three phases: (I) normal, (II) substrates dried, and (III) plants withered and substrate dried. The respective contributions of live plants and substrates are quantified based on the results from these phases. In phase I, the average heat transfer into the indoor space through the green roof was 81% less than that of the reference bare roof. In phase II, the plants and the substrate dried out gradually, and the average heat transfer through the green roof was 69% less. In phase III, the dry substrate without live plants acts as a normal thermal insulation layer, and the average heat transfer through the green roof was still 40% less than that of the reference bare roof. In those three phases, the equivalent overall heat transfer coefficient of the green roof was 0.26, 0.49, and 0.88 W/(m2K), respectively. It suggests that the use of drought-tolerant plant species and substrate with a low thermal conductivity could improve the thermal performance of green roofs during a severe drought.