Lightweight extensive green roof for building renovation: Summer performance analysis and application in a living laboratory

Extensive green roofs are considered an effective energy conservation measure for increasing buildings’ energy efficiency and reducing the heat wave effect in dense build environments. In this context the present work has a two-fold objective: the first is to test and analyse a commercial extensive lightweight green roof sample through an experimental monitoring campaign carried out in a hot climate during the summer time; the second is to provide a practical case study application showing the architectural integration of the extensive green roof technology for existing buildings. The experimental monitoring campaign has been set for analyzing the temperature levels of an extensive green roof compared with a traditional horizontal roof finished with cement tile. The temperature levels have been analysed through a set of sensors positioned at different levels to characterise the green roof response to the climatic forces during summer. The results show that the air temperature in proximity to the green surface (15 cm above the greenery) is warmer than the undisturbed ambient air temperature during the day and lower during the night by 2–2.5 °C. The soil substrate and the vegetative layer contribute to increase ambient air humidity levels. As expected, the evapotranspiration of the green layer increases during a typical sunny day resulting in more water content in the air above the vegetative level of about 4–8 %. The surface temperature of the ground below the vegetation layer and the temperature of the ground layer (8 cm deep) shows beneficial attenuation and time shift properties with respectively 12–15 °C and 3–4 h. Compared to the traditional cement tiles the green roof shows lower intralayer temperature with differences ranging from 6 to 8 °C. Moreover, the renovation case study represents a practical example of green roof technology integration in a real environment. The study has high replicability, and it is meant to be an interesting example for researchers and professionals to boost the green roof technology application for higher-quality built environments. Keywords: Extensive green roof; Hygrothermal performance; Monitoring; Multi-layer dry construction; Mitigation Strategies; Building renovation