•A model of unsteady heat exchange in light structure rooms is proposed.•Model takes into account radiator capacity and intermittent heating.•Reasonable agreement between calculation and measurement results is achieved.•Thermal comfort indices PMV and PPD are evaluated. Intermittent heating is one of the methods leading to savings in energy consumption. The intermittent heating system can work with reduced power or it can be completely cut off when the rooms are not occupied. At the beginning of the cut-off mode, the radiator remains warm for a specific period of time, due to its thermal capacity. This capacity is not negligible and should be considered for buildings with light or very light structures. This paper outlines a mathematical model of unsteady heat exchange in rooms with light wall structure with intermittent heating. The air heat balance of a given room takes into account the room air capacity, hot water radiator capacity, heat transfer through walls, ceiling, floor and windows as well as air infiltration. Reasonable accuracy between calculation and measurement results has been achieved. With known air and radiant temperatures, air humidity and velocity, thermal comfort indices predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD) were evaluated in order to verify how thermal comfort changes during radiator cut-off mode. The satisfactory convergence between measured and calculated internal air temperatures has been achieved.