Energy costs are rapidly rising around the world and it is important to find a way to reduce energy consumption. Energy consumption in buildings can account for up to 40% of total energy consumption in developed European countries due to an increase in living space per person, an increase in the need for air conditioning, an increase in the number of appliances used in households, and other factors. Due to the increasingly frequent high temperatures during the summer and the increase in electricity consumption, it is very important to find a way to decrease indoor temperatures in urban residential buildings without using more energy, given the severe energy crisis that has engulfed Europe and the high percentage of energy and electricity consumption in the residential sector. Installation of phase changeable materials is one method for improving the thermal envelope of a building by increasing the thermal capacity of the envelope and thus affecting the reduction of indoor temperature oscillations and, consequently, reduction of summer electricity consumption. In this study, the TRNSYS software package was used to simulate and compare the indoor air temperature in typical detached houses built in period 1946 and 1970 in the Belgrade area, for both, the case without and the case with installed phase change (PCM). It was shown that incorporating a PCM with a melting temperature of 25°C into the inter-floor construction of the house could significantly improve comfort conditions by reduce indoor temperature fluctuations during the warm period without consuming additional energy. When the temperature outside is around 30°C, the maximum air temperature in the house drops by about 2°C.