The construction industry (and buildings) is one of the largest energy consuming and CO 2 emitting sectors in the world. To counter this, more lightweight structures are being used and energy saving applications are being developed. Phase change materials (PCM) are materials that can be considered to tackle these new challenges. It has been proven that PCMs can be passively used to improve the thermal mass of lightweight structures, which improves thermal comfort and reduces peak cooling and heating loads and therefore provides energy savings. To use these materials in an active way, they should be used together with ventilation, cooling or heating equipment, and collectors to accumulate or return the energy stored in the rooms through these systems. PCMs in buildings are predominantly experimentally applied and tested in hot climates, but they have not been extensively studied for either cooling or heating in a warm-summer humid continental climate. Within the framework of this research, an experimental compound consisting of five test buildings, used in previous studies to assess the performance of different building materials and heating, ventilation and air conditioning (HVAC) systems in the Latvian climate, was used. Two types of PCM were used to carry out the three different experiments in situ, as well as modelling and validation of obtained data: the main goal was to increase the thermal mass of lightweight buildings. The preliminary results indicated that, in the case of overheating, additional mechanical ventilation during the night should be used due to the high temperatures at night, which are a little below the solidification temperatures of PCM during overheating periods. The highest efficiency of PCM was obtained when it was used in conjunction with capillary ceiling cooling, providing a lower indoor temperature of 3–4 °C during the day, but additional investigations are necessary to calculate the economic gains. In general, experiments have shown that PCMs can be used in buildings to increase their energy effectiveness in the Latvian climate, but complex control systems are required to operate such systems with the highest efficiency.