Cooling effect of Sedum sediforme and annual plants on green roofs in a Mediterranean climate

Green roofs are often used to cool and insulate the buildings below them, but there is a lack of information about optimum design of green roofs to achieve this function in Mediterranean climates. In such climates, the cooling effect of green roofs can also be important on warm days during the winter. In this study, we compared temperatures of non-green roofs, green roofs planted with Sedum sediforme (Jacq.) Pau, and green roofs planted with the same density of Sedum together with annual plants. These plants were chosen because annual plant communities are an important part of semi-arid environments, which are analogous to green roofs. Temperatures were measured below green roof plots and in the substrate. We found that temperatures below green roofs were around 7 °C cooler than below non-green roofs, and that temperatures below Sedum roofs were, in general, 1.5 °C cooler than below annuals + Sedum roofs. The difference in temperatures between green roofs types was seen in summer in the substrate and in winter below the roof. Substrate moisture was higher in Sedum roofs in the summer. Vegetation cover was higher in annuals + Sedum roofs in winter and in Sedum roofs in summer, as Sedum grown alone grew more than Sedum grown with annuals. Overall, Sedum roofs appear to be more effective in providing cooling benefits in this climate, due to higher substrate moisture and vegetation cover in the summer. Thus, to achieve the cooling benefits of an extensive green roof in a Mediterranean climate, it may be prudent to use succulent species alone, without annuals that would compete with Sedum in winter, while not providing a cooling effect in summer.