Influence of the thermal insulation position in standard and cool roofs on their contribution to urban heating during heatwaves

The urban heat island, coupled with increasingly intense, frequent and long heat waves, leads to a significant risk of excess mortality and increased energy requirements in urban areas during the summer season. Various cooling techniques are therefore being studied to moderate the intensity of this phenomenon during the hot season. The aim of this study is to quantify the contribution of roofs to urban climate during heatwaves, depending on the position of the insulation in the structure and its radiative properties. This study is carried out in the laboratory, using an experimental set-up to submit roof samples to heatwave conditions. The experimental set up includes a temperature and humidity-controlled climate chamber with a solar simulator. Two roofing configurations are studied: roofs with external thermal insulation (ETI) and roofs with internal thermal insulation. Each configuration is studied for two types of roof: standard (low albedo), and reflective (high albedo). It appears that ETI roofs heat the outside air during the day, but not at night. While ITI roofs heat the ambient air less during the day, they release some of the stored daytime heat during the night. Whatever the structure, increasing solar reflectivity significantly reduces the contribution of roofs to urban heating, although the impact is stronger for internally-insulated roofs, since it reduces the amount of heat released. Finally, for the externally insulated structure, the increase in solar reflectivity reduces daytime convective exchange by 41% and heat entering the building by 75%. For the internally insulated structure, daytime convective heat exchange is reduced by 43%, and the heat stored in the concrete layer is reduced by over 50%.