## 1 Introduction  
During the last 20 years, phase change materials (PCM), mainly organic or inorganic, became a building fabric or a part of components in many different applications [1]. The most popular organic compounds used in buildings are paraffin. They are characterized by unique thermo-physical properties such as a very high latent heat of phase change with very low volume decrease [2, 3]. Additionally, they are non-toxic, non-corrosive, and chemo-physically stable [4]. The weakest feature of paraffin is flammability [5] which limits the possibilities of their application in building interiors [6]. Simultaneously, the inner location of PCM layer is the most effective solution for stabilizing the indoor temperature [7, 8]. Particularly, among the passive applications of PCM [9], lightweight construction buildings reveal the highest potential to improve thermal inertia [10]. This effect the storage of a large amount of energy with slight temperature fluctuations in the transition range [11]. Therefore, application of the PCM layer in an inner location was widely investigated [12], taking into account thermal comfort and the energy performance of buildings. Generally, PCM can be implemented into an inner part of a wall structure in different forms such as: impregnation of building materials, micro-encapsulation, or as a shape-stabilized thin plate [13]. Almost any construction material like concrete [14], gypsum [15] and ceramic [16] was considered as a matrix for paraffin or fatty acids. [PERSON] et al., 1993 [PERSON] et al. (1993) examined the performance of gypsum wallboard and concrete block which have been impregnated with PCM. [PERSON] et al. [PERSON] et al. (2014) investigated the improvement of thermal performance of residential building walls with a PCM thermal shield. [PERSON] (2015) studied the thermal performance of a gypsum board impregnated with phase change materials in a building scale. Additionally, improvement of heavyweight, passive solar collector wall was investigated experimentally [PERSON] et al. (1993); [PERSON] et al. (1993) and numerically [PERSON] et al. (1993); [PERSON] et al. (1993). Although a lot of research studies were dedicated to the investigation of PCM placed from indoor side of the wall the last decade brought a number of different external applications. One of the first ideas was to use PCM in shading systems like blinds, e.g., a louver system [PERSON] et al. (1993). In such systems, the energy from solar radiation is stored during the period of excessive heat gains (during a day) and released at night [PERSON] et al. (1993). Such solution can not only balance the heat flux through glazing facades effectively, but also prevents overheating during peak hours as well. Alternatively, PCM can be applied directly to the glazing unit as a filling for an air gap between glass sheets [PERSON] et al. (1993) or as a glass block [PERSON] et al. (1993), giving a similar effect as blinds. However, recent reports [PERSON] et al. (2013) have shown that PCM affects not only the direct transmission of solar energy, but also can improve the energy performance of opaque, well-insulated walls [PERSON] et al. (2014) or roofs [PERSON] et al. (2014). In opaque structures, PCM was used to stabilize the temperature of photovoltaic (PV) panels and increase their effectiveness [PERSON] et al. (2014). Such a solution can be attractive for any PV system integrated with building envelope, e.g., building facades (BIPV) characterized by a limited capability to cool down covering elements exposed to solar radiation [PERSON] et al. (2014). The main purpose of this work is to experimentally investigate the effect of external, thin PCM layers on the dynamics of the thermal insulation and the overall performance of the external wall. The full scale experiment was conducted under moderate climatic conditions over the whole calendar year.