## 1 Introduction  
Following the effective ratification of the Paris Agreement (official version is avaiable at [[https://unfccc.int/sites/default/files/english_paris_agreement.pdf](https://unfccc.int/sites/default/files/english_paris_agreement.pdf)]([https://unfccc.int/sites/default/files/english_paris_agreement.pdf](https://unfccc.int/sites/default/files/english_paris_agreement.pdf))) by an overwhelming majority, with 185 countries over 197 parties to the Convention (as the date of February 2019), it is clear that the recent trends towards economy decarbonation and energy consumption reductions shall be pursued, specifically in Europe. Among the possible avenues of research, the lowering of the impacts of the buildings sector on energy needs appears to be a promising way. Practically, many optionsare nowadays available and many more practical and multi-disciplinary research projects are under development. When considering only the energy requirements for the heating and cooling in residential and commercial buildings, it is already a third of the total global use and, more importantly, it should increase by approximatively 79% and 84% between 2010 and 2050 [1; 2]. To overcome the use of heating and cooling devices, the use of phase change materials (PCM) has been proposed. They can authorize to benefit from free heating or cooling or to delay the thermal stress, to name but a few examples for passive applications. As a rule of thumb, the topic of PCM has greatly focused the attention of numerous groups in the past decades, with a clear increase during the last twenty years. Thus, when considering the literature, and if one goes from the most general to the more specific, a good (subjective) starting point could be found in [3; 4; 5; 6; 7; 8] or in the reviews summary proposed by [9; 10]. [PERSON] et al. [3] focused on the thermal dynamic behaviors and the energy performance of buildings with PCM. Several important aspects on the use of PCM are withdrawn. Concerning the free cooling and the passive applications (Section 2 pp. 3170-3171 & Section 5 pp. 3173-3179), it is shown that thermal comfort can be increased yet the design process is a keyfactor. Moreover, in the former case, the concept is more interesting if a large difference in temperature between day and night exists. Then, the peak load shifting applications (Section 3 pp. 3171-3173) seem to be promising. Unfortunately, the main results usually only rely on simulations or prototypes; it is consequently difficult to have a detailed view of the energy savings. Finally, let us mention the interesting summary table of the major results (Table 1, p. 3178) and the study of the relationship between the payback time and the price of energy and the price of PCMs (Figure 1 p. 3171). In another way, a very generic and interesting discussion concerning the overall properties of PCM when applied in buildings is proposed by [PERSON] et al. [9]. Generally speaking, this review presents very practical informations. A useful classification of PCMs depending on their use (Tables 3 to 8) is first given. On one hand, one can distinguish the cooling (\(T_{M}<21\,^{\circ}\)C) and comfort (\(23<T_{M}<28\,^{\circ}\)C) applications, and the heating of water (\(29<T_{M}<60\,^{\circ}\)C). On the other hand, some real available commercial products are given. Classification relies on the temperature range one is interested in: \(61<T_{M}<120\,^{\circ}\)C, \(T_{M}<21\,^{\circ}\)C and \(22<T_{M}<28\,^{\circ}\)C. In the same way, a detailed analysis of the PCMs basic requirements is given. Added to this, a classification is propposed in function of their respective melting temperatures and mass and volume latent heat. It separates PCMs in four common groups: paraffins, organics, inorganics, and eutectics (see Figure 3 p. 1686). Finally, let us mention the very enlightening discussions on stability, under thermal cycling or due to corrosion and the possible interactions between the PCMs and their containers.