Article presents the key findings from extensive research on the effect of increasing a building's thermal mass on the effectiveness of preventing overheating during prolonged heat waves in temperate climates. Experimental investigations were conducted in two full-size detached single-family laboratory buildings with different walls technologies: traditional (B1) and skeletal (B2) located in the Science and Technology Park of the University of Zielona Góra. The research was carried out during the summer months for the period 2015-2020. Because the effect of thermal capacity is particularly important during very high outdoor temperatures, 10 heat waves were selected throughout the research period and for which separate analyses were conducted. The thermal behavior of entire buildings and their individual rooms was analyzed. The research were conducted with the doors to rooms open (2015-2018) and closed (2020), because it was assumed that this factor may have a significant influence on the formation of temperatures. In 2019, the thermal floor insulation in the building with higher thermal mass (B1) was removed to account for the additional effect of the thermal capacity of the underneath ground. Additional goal of the study was also to compare the performance of traditional heavy-weight materials and PCMs. Research in the experimental rooms in building of Centre of Sustainable Building and Energy were made in 2020. The effects of lowering the average and maximum temperatures during oppressive heat waves reached 1,7-3,6°C. Incorporation of the ground heat capacity beneath uninsulated slab-on-grade resulted in a significant maximum temperature reduction of 7-8°C. The resulting additional heating energy demand of 5.5 kWh/m2 of building floor area in 2019/2020 may be compensated by using horizontal insulation on the outside of the building and in the strip next to the external walls. The use of increased thermal mass of traditional materials as a source of cooling may be sufficient to prevent excessively high temperatures and may not require any additional passive methods, which often depend on the habits and skills of the occupants. Additional experiments with PCMs suggest great caution in their use in non air-conditioned residential buildings. The material was found to be less effective than traditional types of material due to the great difficulty in selecting the optimum extent of the transition phase at the building design stage.