Thermal and physiological performance of roof and façade greening under summer-time heatwave conditions in Hanoi, Vietnam

Roof and façade greening is widely considered as a measure for urban heat mitigation. However, plant physiology is often overlooked in studies on its thermal performance, which is critical to ensure practical designs and cost-effective maintenance. This study bridges this gap by showcasing the establishment of roof and façade greening on an apartment building and quantitatively assessing its thermal and physiological performance under subtropical heatwave conditions in Hanoi, Vietnam. Using automated field measurements, we validated high-resolution numerical microclimate simulations, achieving index of agreement values exceeding 0.85 in most cases. Results suggest substantial variations in surface temperature reductions, with the strongest cooling effects on the rooftop (mean of 10.9 ℃), followed by west (3.6 ℃) and south façade (2.7 ℃) during sunshine hours. We also used leaf temperature, stomata resistance and transpiration flux as physiological proxies to further understand plant cooling behavior. Depending on the location, parts of the rooftop exhibited highest leaf temperatures (37.9 ℃) and stomata resistance, while façade vegetation remained hotter throughout the afternoon hours. By integrating plant vitality metrics with thermal performance analysis, we highlight the benefits of native species selection and strategic vegetation placement to mitigate urban heat islands and optimize irrigation in dense urban contexts.