## Materials and Methods
### Building Retrofitting and Thermal Loads Calculation  
The building that is going to be retrofitted in 2019 is part of an old (completed in 1960) high school building of Feltre, situated in the province of Belluno, northern Italy, in the Dolomiti Mountains area. The climate is rather severe in wintertime (3100 degree-days). The gym and the laboratories will be retrofitted in order to become a nZEB. The retrofitting will be done by the Belluno Province Administration, a public authority charged with the public education service. The main part of the retrofitted building is a large gym (33 m \(\times\) 25 m \(\times\) 8.40 m, expanding on two levels). Changing rooms, bathrooms with toilet and showers, and technical rooms are located on the ground floor; an office, a small gym, and a bar are on the first floor. On the second floor, six laboratories will be retrofitted and made newly available (Figures 5 and 6).  
Figure 4: Temperature profiles (axial node 10 at the top of the tubes) during the unloading process of paraffin/Al-foam composite PCM and pure paraffin wax (simulated and experimental curves). Water temperature was kept at 23 \({}^{\circ}\)C. Experimental temperature was measured in the centre of the section.  
Figure 5: The retrofitting intervention and definition of thermal zones for the ground floor (**a**) and first floor (**b**).  
The building has a total floor area of 2435 m\({}^{2}\), an outward surface area of 2505 m\({}^{2}\), and an enclosed gross heated volume of 11,060 m\({}^{3}\). The design of the retrofit intervention is carried out to obtain a nZEB: the envelope is well insulated, with outer walls and roof allowing for an average thermal transmittance of approximately 0.15 W m\({}^{-2}\) K\({}^{-1}\), floor to the ground of 0.5 W m\({}^{-2}\) K\({}^{-1}\), and the glazing system of 0.7 W m\({}^{-2}\) K\({}^{-1}\). In Italy, such an intervention benefits from the \"condo termico 2.0\" (thermal incentive 2.0) that has been operating since 2016 (see Section 3.2).  
Based on the Trnsys 17 dynamic simulation, each thermal zone of the building is defined by means of scheduling the presence of people, type of activity, lighting and other internal gains, and air temperature set points. For the heating season, temperature is fixed at values between 17 \({}^{\circ}\)C and 20 \({}^{\circ}\)C for the large gym (at ground floor) and the small gym (at first floor), whereas it is fixed at 19-21 \({}^{\circ}\)C for the other rooms, depending on the time of day. The same set point for cooling is fixed at 22-24 \({}^{\circ}\)C and 24-27 \({}^{\circ}\)C, respectively. For ventilation, the set point for inlet air is fixed at 18 \({}^{\circ}\)C for gyms and 19 \({}^{\circ}\)C for other rooms for heating, and 21 \({}^{\circ}\)C and 24 \({}^{\circ}\)C respectively for cooling.  
The HVAC system provides ventilation (by two air handling units, AHU), space heating and cooling, and DHW production. Figure 7 reports the heating, cooling, and DHW monthly energy needs calculated with a 0.25 h simulation time step.  
Figure 6: The retrofitting intervention and definition of thermal zones for the second floor (**a**); building as is and post-intervention (**b**).  
Figure 7: Monthly energy needs in terms of heating, cooling, ventilation (hot and cold coils of air handling units (AHU)), and domestic hot water (DHW).  
The maximum cooling load (21.6 kW) occurs at the beginning of June, when the school is still fully operating, that is, it is open to students and professors, and gyms are open to extracurricular activities as well. The maximum heating load (\(-\)44.6 kW) occurs in the second half of January. However, even during summer months, post heating ventilation requires some heat. During the heating season, ventilation needs are prevailing with respect to heating needs; this is a consequence of the very high thermal insulation of the retrofitted building and of the minimization of thermal bridges. Moreover, DHW needs (2000 L per day at 45 \({}^{\circ}\)C) are a large quota of the total heat request.