In this paper an experimental research is presented on a new use of Phase Change Materials (PCMs) in concrete floors, in which thermal energy provided by the sun is stored in a mix of concrete and PCMs. When this thermal energy is being released – in moderate sea climates during the evening and early night – it is aimed to reduce the need for thermal energy of conventional heating in houses. The temperatures of four concrete floors in closed environments were monitored to reflect on the influence of PCMs and type of insulation in relation to ambient temperatures and solar irradiation. The application of PCMs in concrete floors resulted in a reduction of maximum floor temperatures up to 16 ± 2% and an increase of minimum temperatures up to 7 ± 3%. The results show the relevance of an integral design in which the thermal resistance of the building shell, the sensible heat capacity of the building and the latent heat capacity of the PCMs are considered simultaneously.