As other historical centres in Europe, the centre of Cahors-in the south west of France-is characterized by its historical heritage, but also by its high level of degradation, high vacancy rate of the dwellings and fuel poverty of its inhabitants. Consequently, the energy retrofit of the old housing stock is urgent and relevant. For this, the city council of Cahors had created several partnerships with Universities, craftsmen, and architects in order to elaborate sustainable technical solutions. The energy retrofit solutions have to preserve historical details, to be sustainable, to create a healthy indoor climate and also to preserve the building from pathologies. In-situ measurements have been running in two medieval apartments to study indoor comfort in the main room (room and wall temperatures, indoor relative humidity) and also some parameters in the outer wall such as humidity, temperature variations and heat transfer (U value). A large masonry brick and a timber frame wall have been monitored in summer and winter conditions, before retrofitting. Large thickness of wall, inhomogeneity of construction, aging of materials and uncontrolled indoor environment-in part due to natural ventilation-make difficult obtaining accurate and relevant results by computer simulations. The onsite campaign, offering more realistic boundaries, permits to offer a better understanding of thermal behaviour of historical buildings.