The mission for the energy-efficient renovation of the C1C2 building of the Vésinet site of the Institu de Radioprotection et de Sûreté Nucléaire (IRSN) aims to improve the thermal comfort for its occupants during the summer and winter, as well as to reduce the carbon footprint of the building. Energy-efficient studies have been conducted on each façade in order to propose relevant and cost-effective thermal, architectural and environmental solutions following the High Quality Environmental procedure HQE® : The occupants’ comfort, the economic and energy-efficient performance, the respect towards the environment and the responsible management of the building.