The building sector is the third-largest consumer of primary energy in Belgium. This is partly because of the high percentage of old buildings (buildings constructed before 1945) in its building stock. Existing international standards on thermal comfort focus primarily on new construction and commercial buildings but tend to overlook old buildings. This study involves a thermal comfort assessment of fully functional (in use) residential buildings constructed before 1945 in Liege (Belgium). The research methodology is based on continuous long term monitoring of the indoor environment (November 2011 to May 2012) and followed by comfort surveys for selected houses in the city. The analysis of the collected data shows that family composition, envelope performance and the occupants' interaction with the indoor environment greatly affected occupant preferences and functioning of the indoor thermal environment. This study reveals that the occupants' interaction with the indoor thermal environment to restore comfortable thermal conditions varies throughout the day, by adjusting the temperature in different rooms of the house at different times of the day. This study argues that modern comfort standards have failed to estimate the comfort level in these old buildings and, if applied, they would lead to under estimation of their thermal comfort. (C) 2016 Elsevier Ltd. All rights reserved.