The main objective of the study was to evaluate the effects of PCM and traditional masonry wall and RC roof construction of a residential building on the summer thermal performance of the building. Three rooms on the top floor of the experimental building had identical flooring. The internal roof and wall surfaces of the 2 rooms were made as a light wooden frame construction. In one of them PCM on internal walls was covered by plasterboards from inside. In the third room a construction typical for residential buildings, masonry walls and a reinforced concrete roof was used. During the study, which was conducted from 15.06 to 15.09.2021, the course of air temperatures was recorded in three identical rooms with a large south-facing window with a ratio of 0.25 to the floor area. For the night period, mechanical ventilation with a capacity of 6 ACH was turned on in all rooms. Except for a few days when there was a sharp drop in outdoor temperature, the lowest maximum daytime temperature values were maintained in the room with massive walls and roofs, they were slightly higher in the room with PCM, and remained the highest in the room with light frame construction. During the heat wave build-up period, the greatest reduction in maximum daytime temperature of 3.5 K relative to the light room was obtained in the heavy room. In the PCM room, the difference was 2.2 K. While the applied PCM with a melting point of 24-26 C remained effective when the minimum night time indoor temperature dropped to at least 25-26 C, increasing the thermal mass of the walls and roof by using traditional heavy building materials was effective regardless of temperature. When lightweight construction was used, there were large diurnal temperature variations of up to 9 K. In the other two cases, the variations were much smaller.