## 1 Introduction  
Building envelopes have a crucial impact on building energy use for space heating and cooling systems and their designs are crucial to reduce these loads while maintaining the occupants thermal comfort [1]. Currently, both heating and cooling demands represent 50% of the total global energy consumed in European buildings and this demand is increasing with the comfort requirements in developing countries [2]. Within this context, the use of phase change materials (PCM) for passive heating and cooling systems in buildings has attracted the attention of the scientific community as an appropriate technology which can promote net-zero energy buildings [3]. These materials, when integrated into building walls or roofs, can provide thermal benefits due to its high thermal energy storage capacity, which can prevent or delay the cooling peak load during summer and can absorb part of the solar radiation in winter. In building envelope applications, PCM stores latent heat as the ambient temperature rises to the melting point (most PCM changes from a solid to a liquid state) and as the temperature cools down, the PCM returns to a solid phase, releasing the stored latent heat [4]. Several researchers have investigated and developed different ways to incorporate PCM into building envelopes [5], such as microencapsulated [6; 7; 8], impregnated [9], or installed in macro-encapsulated panels [10]. Numerical and experimental research have emphasized the benefits of using passive PCM technology in buildings: (1) reduction of maximum room air temperature during the cooling period [11], (2) reduction of thermal oscillation during the cooling period, (3) cooling load reduction [12], and (4) possible solar absorption and heating load reduction [13]. However, there exist specific technical barriers that limit the benefits of both PCM heating and cooling passive technologies and do not allow them to become an attractive technology able to justify their initial economic investment. These technical barriers are blocking the implementation of this technology into the market. In case of PCM passive cooling, the solidification process of PCM is limited as the PCM layer is not directly exposed outdoors but thermally protected by insulation (Figure 1), which makes the technology completely useless in periods when solidification cannot be ensured. Within this context, [PERSON] et al. [14] experimentally tested passive PCM in two-storey buildings and showed that PCM did not provide benefits from July to August (when cooling is highly demanded) as solidification was not achieved during night time. Moreover, in PCM passive cooling applications, the building peak cooling load is accumulated in the PCM and therefore its effect indoors is delayed, however, that latent heat is mainly discharged indoors because the insulation strongly limits its dissipation to outdoors. Finally, passive cooling PCM technology only interferes in outdoor cooling loads transmitted by conduction but it cannot deal with internal heat loads or solar gains from openings. In fact, this technology provides higher energy consumption for space cooling in the presence of significant internal heating loads [15]. On the other hand, the use of PCM for heating purposes is based on its capability to absorb solar gains coming through the glazing and to release stored solar energy during the night [16]. This limited activation conditions limits: (1) its energy benefits (quantified around 4-5% [17]), (2) its implementation mainly to internal solar exposed surfaces (internal walls or floor), and (3) its use in climatic regions with non-severe winter conditions (otherwise, only operative during early and late winter as well as spring and autumn seasons) [13]. The idea behind the system presented in this paper relies on changing the age-old concept that building envelopes are static and fixed during its use. Research has been developed in the frame of developing climatic adaptive building shells (CABS) [18; 19; 20].