The paper presents the results of a computer simulation of a building under the assumption of the presence of a phase change material in wall elements. The influence on the cooling energy requirements and, also, the risk of overheating of the low-energy building (without HVAC systems) have been analyzed. The obtained results were compared with the ones of a low-energy building without a phase change material in walls. The comparison calculations were performed for a model of a service building located in Silesia, which is built according to an energy-efficient standard. It has a light-frame construction with light covering (external panels, heat-insulation, interior plasterboard). In the study, the presence of a 1 cm thick PCM board placed under the inner surface of plasterboard is assumed. Tests were conducted for organic materials that undergo phase transition in different temperatures, such as: 23°C, 25°C and 27°C. Based on the results of operative temperature measurements, it is possible to determine the influence of PCM on the risk of building’s overheating.