Recycled agricultural plastic waste as green roof drainage layer within the perspective of ecological transition for the built environment

Although green roofs are considered among the most sustainable building envelopes, an expanding tendency is the use of recycled or reused materials and low-cost products to improve their environmental and economic sustainability. This article evaluated the use of polyethylene granules, derived from the recycling of agricultural plastic waste coming from greenhouse covering, as drainage layer in green roofs. To this end, the physical properties (i.e. particle size, specific weight, minimum and maximum density and permeability) of five different recycled plastic granules were determined. Moreover, the thermal conductivity of one of these recycled products was assessed through a customized testing methodology varying the water content. Thermal conductivity measurements were conducted by using two pieces of equipment to cross-referencing the measurements. The same surveys were carried out on other granular drainage materials (i.e. perlite, expanded clay) to validate the proposed methodology as well as to compare the thermal performance of the examined materials. The results of the thermo-physical tests showed that the recycled plastic granules have features within the range requested for green roof drainage layer, very similar to those of perlite and, therefore, they may be used as an alternative to natural drainage materials. In addition, recycled polyethylene had mechanical and environmental performances better than commercial products, indeed it has the lowest weight increase in saturated conditions, about 80 kg/m3, so reducing the extra load applied on existing building rooftops, as well it produces very low Global Warming Potential (GWP) impact, that is 2–4 kgCO2eq/FU. These outcomes could have wide dissemination considering that Europe is one of the major consumers of agricultural plastic products and their reuse/recycling comply with the ecological transition objectives.