Evaluation of the energy performance and irrigation requirements of extensive green roofs in a water-scarce Mediterranean climate

Green roofs are generally seen as a desirable building element, providing numerous benefits where water availability does not restrict their implementation. However, most Mediterranean locations have long, dry summers, requiring irrigation to sustain vegetation throughout extended dry periods. The cooling effect and water use of several types of plants suitable for extensive green roof systems were assessed using small test cells, which were insulated and equipped with internal thermal mass to provide a thermal response comparable to that of real buildings. The water requirements of the plant species tested ranged from 2.6 to 9.0 L/m2 per day. Aptenia cordifolia was the most efficient in its use of water, providing the highest cooling benefit per unit water required for irrigation. However, the cooling efficiency of all roof variants studied was very low, and the reduction in the sensible heat load of the model building attributed to the green roof system was less than 5% of the latent heat content of the water lost to evapotranspiration. In this context, it is hard to justify green roofs in such environments on the basis of their contribution to building energy conservation, although other benefits may nevertheless make green roofs attractive.