Investigating the cooling effect of a green roof in Melbourne

Abstract

‘Green Our Rooftop’ aims to transform the rooftop of Treasury Place, a state government building in the inner city of Melbourne, into an intensive green roof under the notion of ‘Garden of Victorian Landscapes’. The concept behind the innovative green roof design is to break down perceived barriers to green roof retrofitting, limit the global temperature rise and help cool the city by advocating modifications in urban infrastructure (e.g. greening projects). This study quantified the cooling effect of the complex green rooftop of Treasury Place (which is characterised by a diverse range of plant types and topographies) by using ENVI-met to assess the air temperature and outdoor thermal comfort at rooftop and pedestrian levels. After verification through field measurements, the study also investigated how the adjustments in the green roof's design settings (e.g. leaf area index [LAI], plant height, soil moisture and additional green coverage) can further improve the green roof's thermal performance. The findings from our study indicate that the implemented green roof configuration effectively lowered the air temperatures at the roof level by 1.5 °C, simultaneously enhancing thermal comfort by 2.38 °C during hot summer days. This optimum performance was achieved when soil moisture levels were set at 0.6, plant height at 0.6, and LAI at 2.5. Our statistical analysis indicates that all these scenarios exhibited equivalent cooling benefits. Thus, a holistic approach optimizing LAI, plant height, soil moisture, and tree coverage combined is essential to maximise cooling impact when integrating green roofs into future developments in inner city areas.