## 1 Introduction  
Improvement of energy efficiency is one of the priority research areas for the construction engineers/scientists towards the mitigation of the socioeconomic challenges like climate change nowadays. Most countries have committed to the Paris Agreement by United Nations (UN) to pursue efforts to limit the extent of global average rise in temperature [1]. European Union (EU) aims to achieve climate-neutrality by 2050 and wants to lead global efforts in this way. Buildings and the building sector as a whole are responsible for over 30% of the world's total energy demand, and 40% of direct and indirect CO\({}_{2}\) emissions. The International Energy Agency (IEA) describes this sector to be a source of the enormous untapped potential of energy efficiency [2]. A key part of the strategy is to maximize the energy efficiency of buildings [3]. The increasing amount of research findings shows that the incorporation of PCMs in building/construction materials is an effective solution in new constructions and in retrofit solutions [4]. PCMs are a promising passive cooling technology which can increase a building's thermal mass in an effective way [5, 6, 7, 8]. The use of thermal mass in the building envelope have been widely used historically as a thermal energy storage system (TES), and when utilized beneficially it can reduce heating and cooling loads, shift peak load hours as well as dampen the diurnal indoor temperature swings throughout the day [9]. The chemical composition of the PCM can be tailored to achieve suitable melting temperatures for a wide range of climates and environments, and this has been identified by numerous studies as the key design parameter for successful application of the material [10; 11; 12; 13]. Indeed, PCMs are widely utilized in building materials. Among these, PCM enhanced wallboards/gypsum boards are one of the common techniques [14; 15; 16; 17; 18; 19; 20; 21] used in the interior of the building. However, several key challenges are to improve the thermo-physical properties, e.g., to increase thermal conductivity and latent heat of a PCM incorporated composite. Other important issues are to ensure complete melting-solidification cycle and to ensure the fire safety requirements. The benefits of heat storage in the PCM can be completely lost if incomplete charging occurs repeatedly [10; 22]. Application of NV with PCM-integrated gypsum boards was investigated by [PERSON] et al. [23] and a weekly electricity saving of 73% was reported in their study. The solidifying process can be assisted with enhanced convection, and a commonly proposed strategy is NV where air change rates are increased during night time to remove heat from the building. There is a great interest in coupling NV with free cooling by allowing cool ambient air into the building, which can enhance the heat transfer with the PCM considerably [11; 12]. This is dependent on climatic conditions, and works best in locations with diurnal temperature variations of 12-15 \({}^{\circ}\)C [24]. NV strategies utilizing free cooling can consist of mechanical ventilation solutions, natural solutions with window openings or hybrid systems which is a combination of both. While all variations can be equally adequate in priming the PCM for good performance, the highest energy savings benefit can be achieved in the natural strategy as ventilation fans can be shut off [25]. Depending upon the building type, pure natural ventilation can often not be utilized efficiently. There are benefits by combining natural and mechanical ventilation strategies into hybrid NV and should be explored. In their work, [PERSON] et al. [26] revealed three key parameters affecting the efficiency of strategies by experimental measurements in three office buildings in Greece, namely the indoor-outdoor air temperature differential, the useful air flow rate applied during night time and the thermal capacity of the building.