In this study, the thermal behaviour of mezzanine beam-slab floor sections of buildings under temperate climate conditions was experimentally investigated. Data were obtained from measurements on buildings under service conditions. Heat flow density and temperatures around the thermal bridges formed by the beam and floor elements were calculated. Thermal behaviours of mezzanine floor sections made of insulated cavity wall and beam was studied. The outcome measures were the general behaviour of components within the total measurement time of about one month; differences between wall and beam temperatures in all measurements; variation of section temperatures; variation of minimum, maximum and average temperatures measured in the environment and the surfaces; heat flow through the external surface of the beam; and damping ratios.