Impact of roof watering on urban cooling during heat waves

• The effects of watering four roof structures under heatwave conditions is studied. • Watering reduces daytime atmospheric convection for all studied structures. • Watering reduces the contribution of internally-insulated roofs to UHI. • The optimum watering rate (max cooling for minimum water) depends on the structure. • Watered reflective roofs are cooler than watered standard roofs. Due to their morphology, lack of vegetation and the materials used, cities overheat compared to their natural surroundings. This phenomenon, coupled with heatwaves, can cause public health problems and increased energy consumption. In response, public authorities are deploying a range of cooling techniques, such as vegetation, reflective materials and urban watering. In response, various cooling techniques are deployed by public authorities. This study focuses on the cooling effects of roof watering conducted in a lab-scale experiment under heatwave conditions. The experimental set-up includes a temperature and humidity-controlled climate chamber with a solar simulator and a watering system to analyse the thermal and microclimatic behavior of different roof structures. Temperature, heat flow and the surface heat balance are measured or calculated to this aim. When the watering rate is sufficient, regardless of the roof sample studied, convective exchanges become negative during the day. As the surface temperature of the roof is lower than that of the outside air, the latter is cooled on contact with the roof. On the other hand, watering has a different impact depending on the position of the insulation in the roof structure. For external thermal insulation (ETI), watering cools the air both day and night. For internal thermal insulation (ITI), watering significantly reduces the heat stored during the day and therefore the amount of heat released at night. For these structures, watering cools the air during the day and limits their contribution to nighttime urban heat island (UHI). This study also shows that there is a different optimum watering rate for each structure, which maximizes the cooling effects of watering while minimizing water consumption. Finally, watering applied to cool roofs reduces the water consumption associated with this cooling technique, while being highly effective in moderating urban overheating during heatwaves.