## 2 Problem Definition and Research Challenge  
As it was stated in Section 1, the application of PCM in external layers of building envelope becomes more and more popular and attractive solution. One of the first works dedicated to find the optimal location of PCM layer in an external building wall was done experimentally by [PERSON] et al. in 2013 [PERSON] et al. (2013). Authors evaluated and compared thermal performance of building walls with and without PCM. It was concluded that the optimal location of a PCM layer is approximately 1/5 of a wall thickness from the internal surface. Similar analysis of position of macro-encapsulated PCM in concrete walls was investigated in [PERSON] et al. (2013). The authors considered only three possible positions: the inner part, outer part, and the middle of the wall. They found out by experimental investigations that on sunny days, the maximum and minimum temperatures of the case externally bonded with PCM were respectively 2.0-2.5 K lower, and 0.5-1.5 K higher when compared with those of the reference model without PCM. This solution is advantageous to applications in buildings where PCM walls can be used to absorb heat during daytime and release it at night, or when there are no internal heat sources. Similar results were achieved by [PERSON] et al. [PERSON] et al. (2013) who revealed more than 28% of energy savings by the location of a PCM layer at an optimum position in the external wall between the outer side of the brick and the plaster layer. The other parametric study of PCM location was done using computational techniques [PERSON] et al. (2013). Authors revealed that the outer location of a latent heat storage layer could considerably improve thermal performance in summer. For cold climate applications, the recommendations based on numerical simulations were formulated in [PERSON] et al. (2013).  
The effect of an externally positioned PCM layer is much higher when the material is combined with thermal insulation. Initial studies and reports on the enhancement of insulation layer were performed based on the investigation of the effect of microencapsulated PCM dispersed in a cellulose insulation [PERSON] et al. (2014). The material was applied in an attic floor and dynamic thermal characteristics were analyzed numerically. Using a refined computational model, the effect of the external PCM layer in a wall was investigated by [PERSON] in 2016 [PERSON] & [PERSON] (2016). In the paper [PERSON] & [PERSON] (2016), a novel way for testing various kinds of wall constructions, known as plug-and-play walls (PPW), was introduced. The use of the PPW concept was demonstrated by evaluating the thermal performance of a thin PCM layer, herein referred as \"PCM thermal board\" (PCMTB). The results showed that the average daily heat transfer reduction, was 27.4% and 10.5% for south- and west-directed walls respectively. Additionally, the average heat flux reductions when the heat fluxes of the control walls were at their peaks were 67.0% and 80.2% for the same facade orientations, respectively. It was demonstrated that the integration of a thin PCM layer also delayed the peak heat transfer rate per unit of a wall area approximately from two to three hours. Therefore, this solution was classified by some authors as a new building technology towards dynamic thermal insulation [PERSON] (2016); [PERSON] (2016).  
The concept proposed and developed in this paper concerns the case where the PCM layer is located on the external surface of thermal insulation exposed to the external environment. The PCM composite was optimized for summer conditions to decrease the heat gain flux and to reduce overheating risks in the room. Authors assumed that PCM layer stabilized external surface temperature not only during the day (storage period), but also during the night (releasing period). For long term exploitation, it would lead to balancing of heat transfer through the wall during the whole summer season and would generally improve the thermal performance of the wall.