This paper proposes a novel passive cooling method utilizing the heat capacity of the ground under an uninsulated floor slab in the summer to cool the inside air. The calculation results and measurements of the indoor air temperature inside two full-scale laboratory rooms located in western Poland were validated with simultaneous statistical analysis. The similar, terraced rooms differed from each other only in the presence or absence of thermal insulation over the whole floor slab area. Experimental research fully confirmed the simulation results. Omitting the ground floor thermal insulation decreased the indoor air temperature in the room during a 2 week heat wave by almost 4 K; thus, the cooling potential of the uninsulated floor is approximately 0.2 MJ/m(2)day. Moreover, the cooling effect remained stable independent of the heat wave duration. The differences in the average floor surface temperature did not exceed 0.5 K; therefore, the "cold floor" effect was negligible. The energy demand for the two rooms was significantly different but can be easily offset by alternative energy sources, e.g., solar energy. (C) 2017 Elsevier Ltd. All rights reserved.