The energy-efficient design of sustainable green roofs in Mediterranean climate: An experimental study

Green roofs were proposed to broaden nature-based solutions within the perspective of the ecological transition for the built environment. In this paper, an innovative and sustainable green roof was designed to optimize the energy performance in Mediterranean area during summer season by means of local-available and recycled materials, thus reducing the environmental impacts. This new green roof technology consisted of recycled polyethylene granules from the regeneration of disused agricultural plastic films as drainage layer and a substrate made of local-sourced materials with high percentage of organic matter and it was compared to two traditional green roofs and to the existing roof. The energy-efficient design was based on the thermo-physical parameters of the green roof (surface temperatures, heat flows and volumetric water content) and of the building (surface temperature of the ceiling) and on the thermal dynamic parameters (decrement factor, time lag and cooling potential). In addition, these thermo-physical parameters were correlated to the Mediterranean climate during summer season. The result demonstrated that although the proposed green roof reached higher surface temperatures than commercial solutions, it maintained an almost constant volumetric water content, it reduced daily fluctuations between minimum and maximum temperatures, it resulted in lower thermal flow through the cross-section, it reduced the surface temperatures inside the building by about 2 °C compared to the traditional roof, thus decreasing the energy consumption for building cooling and greenhouse gas emissions, and it had better dynamic thermal performance than commercial green roofs. Once the energy-efficient design of the proposed green roof technology was proved, the sustainability and the life cycle performance of the recycled polyethylene was discussed, also considering the costs.