## 1 Introduction  
Contemporary building structures are characterized with improving thermal insulation of partitions and solutions focusing on minimizing energy demand. The inability to counteract significant fluctuations in air temperature in rooms has a very negative impact on the microclimate and the perceptibility of people being inside, as well as on the effectiveness of using the available heat gains. The heat capacity of the building is primarily determined by the heat capacity of its partitions. The ability to store excess heat energy and release it when the room cools down, gives the possibility of better usage of solar radiation in winter and reduces the risk of overheating the interior in winter and summer. The lack of convergent in time between demand and possible supply of thermal energy contributes to the development of research on passive heat and cold stores. One of the effective methods of accumulation of thermal energy in buildings is the use of materials that change the state of aggregation. In this case, the heat of the phase transition is used to accumulate energy.) The authors [PERSON] et al. [1] remind and emphasise that a key component in buildings to reduce their energy consumption and improve their thermal performance is the envelope [2, 3, 4]. The latent heat energy storage through the use of phase change materials (PCMs) placed in the building envelope has become an interesting alternative to achieve these goals [3, 4, 5]. A successful implementation of PCMs requires that their melting temperature, thickness and location within the constructive system are selected according to the climate and use condition (air-conditioning or non-air-conditioning). One of the first and most important works on the thermal performance of specific PCM-constructive-systems and searching optimal values were melting temperature [6; 7; 8], thickness [9], location and melting temperature and thickness [10; 11]. Storage of energy in the form of latent heat has become the subject of interest in the last decade, which is clearly visible in the number of publications which increases every year. Most of the publications come from China and the United States [12], which means that the conclusions from the field research taking into account climatic conditions are so different from Poland, that they are useful only partially in our conditions. All simulation and research results contained in the analyzed publications indicate a high potential for the use of PCM in partitions of buildings. [PERSON] et al. [13] carried out the first tests with the use of prefabricated gypsum boards with the addition of PCM in the form of microcapsules. The test results show a large impact of external blinds and the method of ventilation of the room on the effectiveness of protection against overheating with the use of PCM. [PERSON] [14] used a material with two different melting points (23 \({}^{\circ}\)C and 26 \({}^{\circ}\)C) for the tests. The phase-change material was place in an aluminum foil located on the thermal insulation layer. In summer conditions, the reduction of the maximum air temperature in the tested room was 2.5 \({}^{\circ}\)C. Modification of the lightweight partition, by adding PCM-containing plates from the inside, resulted in a significant reduction in energy demand for cooling in summer and a reduction in the load on the heating system during the winter period. Extensive research and analysis of the use of various phase-changing materials, both in laboratory conditions and in real objects, were carried out by the team of [PERSON] [3; 15; 16]. The research proposes solutions for the use of PCM in finishing layers of internal partitions, as a filling inside the partition and as an external layer [17] or in window with one cavity filled with phase change material paraffin [18]. However, one the drawbacks of pure PCMs is their relatively poor thermal conductivities that results in an unacceptably slow heat transfer rate in some applications [19].