Quantifying the Effects of Vegetation and Irrigation on the Thermal Performance of Extensive Green Roofs: A Factor-Based Experimental Study

Urban areas increasingly face summer overheating, highlighting the need for passive cooling strategies. Extensive green roofs offer cooling potential, but the individual roles of vegetation and irrigation remain insufficiently quantified. This study addresses this gap through a controlled field experiment using a 2 × 2 factorial design combining vegetated and non-vegetated surfaces with irrigated and non-irrigated conditions. Surface and waterproofing membrane temperatures were monitored during dry conditions and a three-day irrigation period and compared with a meteorologically similar reference day. A factor-based decomposition approach was applied to quantify the contributions of vegetation, irrigation, and their interaction. Results show that vegetation alone provides limited cooling under dry conditions, while irrigation acts as the dominant cooling factor by increasing substrate moisture and thermal capacity. The combined application achieved the most effective performance, reducing the 90th-percentile waterproofing membrane temperature (TM,90) by 8.51 °C relative to the non-vegetated, non-irrigated reference configuration. The proposed framework supports performance-based design of green roofs under summer heat stress.