IRRIGATED GREEN ROOFS IN A MEDITERRANEAN CLIMATE—EVIDENCE FOR INDOOR COOLING EFFICIENCY

Energy has been one of the most important factors dictating the growth of the world economy. Green roofs are known to alter the energetic dynamics of buildings and were found to be effective in terms of reducing temperature inside buildings and reducing energetic requirements in many studies performed mainly in temperate climate zones. The main cooling mechanism by which green roofs contribute to this phenomenon is evapotranspiration, which is defined as the sum of all processes by which water moves from the land surface to the atmosphere via evaporation and transpiration. In the current experiment, we tested directly the effect of an irrigated green roof on the indoor temperatures in a Mediterranean climate in Israel. Green roof plots were established over five out of ten office rooms located on the top floor of one of the wings in a Haifa municipality building. Three plots were planted with Aptenia cordifolia, a perennial succulent, and the other two with Thymus serpyllum, a sprawling perennial. Five additional offices were left plotless, being control roofs. Temperatures were continuously monitored in all rooms during the summers of 2021, 2022 and 2023 and relative plant cover was measured. During summer 2023 two of the green roof plots were cleared and left with no plants with the objective to evaluate the importance of transpiration by the plants on the cooling effect. In 2021 and 2022 we found a significant cooling effect of the green roof plots. The 2023 results showed a cooling effect of the vegetated plots that was higher than that of the irrigated empty plots. We therefore conclude that in the Mediterranean and other semi-dry climates, green roofs have a significant effect to the cooling of buildings.