Thermal characterization of different materials for extensive green roofs

In recent years, extensive green roofs have been consolidated as a passive system of energy savings in buildings. This fact provides a more environmentally friendly point of view in the building field. To better understand how the extensive green roof works, it is important to note that there are two main mechanisms which regulate its thermal behavior. On one hand, there is the “shadow effect” produced by the plants. On the other hand, there is the “insulation effect” provided by the layers beneath the plants, specially the substrate and drainage layers. The coverage of plants cannot ensure uniformity, this fact causes that the “shadow effect” cannot be considered as a constant parameter. However, the abiotic materials that compose the internal layers (substrate and drainage) provides a constant “insulation effect” depending only on its physical properties (density, thickness, thermal conductivity and specific heat capacity). Despite previous studies have investigated the thermal response of extensive green roofs, there are gaps that have to be solved. The complexity of disaggregated materials for extensive green roofs (substrates with different composition, porous stone materials and recycled materials) implies a lack of real data about its thermal properties. Due to this fact, the literature about thermal response of these materials is an approximation that can differ significantly from the reality. The main objective of this study is to determine experimentally the physical properties of different disaggregated materials for the internal layers of extensive green roofs. To carry out the experiments, an apparatus created and assembled by GREA Group of the University of Lleida has been used. The apparatus allows to calculate the thermal transmittance in steady-state (U-value), the heat storage capacity, and the dynamic thermal response under daily thermal oscillation. The materials under study are three different compositions of substrates for extensive green roofs. The key point of this study, compared to the previous studies with the same apparatus, is to analyze the disaggregated materials instead of compact materials like gypsum, concrete, rammed earth, single layer mortar, and plaster board samples.