## 4 Conclusions  
This work investigates different internal wall insulation strategies for historic buildings. The Monastery of Santa Maria de Monfero in Galicia (Spain) has been adopted as a case study to compare the effectiveness and the hygothermal behavior of different technological solutions based on: (i) insulating plaster layers (P) and (ii) dry counter wall systems (C). The study has led to the drawing up of a methodological approach that could be applied to design the retrofitting of the interior walls of historic buildings as it can be used to assess the impact of different alternatives on indoor thermal comfort, the energy performance and hygothermal compatibility through heat and moisture studies and through a dynamic energy simulation of the whole building. The results obtained from the hygothermal simulations at the wall level show that both dry counter wall systems (C) and the application of thermal insulating plasters (P) have a great impact on the reduction of wall heat losses, even when applied in a thin layer (a 6 cm thick insulation layer was considered in this work). It was possible to observe, by comparing the two internal insulation intervention strategies, that thermal plasters always present a higher level of moisture content in the winter period, which implies a higher thermal transmittance and thus a higher heating energy demand. Nevertheless, it is worth mentioning that, under real conditions, the performance of dry counter wall systems may be reduced by the effect of the thermal bridges of the window frames, which could mitigate the difference in the U-value. As far as the summer thermal behavior is concerned, all retrofitting solutions have determined a deterioration of the indoor summer conditions (higher peak temperatures  
Figure 10: Annual thermal energy demand for heating (Qh) and cooling (Qc) in Monfero (**a**) and Turin (**b**). and cooling needs). Indeed, the inner wall side insulation determined a neutralization of the thermal inertia of the solid stone wall and a mitigation of its interaction with the indoor thermal environment. However, the application of insulating plasters (P) has revealed a better summer performance than dry counter wall systems (C) and shows lower peak temperatures and higher comfort conditions when the building operates under free-running conditions, as well as lower energy demands for cooling when an air-conditioning system is used. An important point should be underlined considering the need for the installation of a cooling system. Two plaster solutions (P3 with cork and P6 with PCM) have demonstrated that a cooling system installation could even be avoided for the Monfero climate (where no hours were above comfort category II), while a very limited number of hours were above comfort category II for the other plaster solutions, with the exception of P5 (aerogel-based plaster). This is of particular interest for all the cases in which the installation of a cooling system presents technical/aesthetical constraints, i.e., a limited space for ducting and for the installation of an external unit. In conclusion, the study points out the importance of performing hygrothermal simulations on a case-by-case basis as they represent an essential step for the design of the most appropriate wall retrofitting strategy. Moreover, this type of simulation should always be followed by a thermal simulation of the whole building in order to fully understand the impact of the retrofitting intervention on the indoor comfort conditions. Conceptualization, S.F., V.S., I.M.L. and D.L. ; methodology, S.F., V.S., I.M.L. and D.L. ; formal analysis, I.M.L., D.L., E.F.; investigation, I.M.L., D.L. and E.F.; data curation, I.M.L., D.L. and S.F. ; writing--original draft preparation, I.M.L. and D.L. ; writing--review and editing, S.F., V.S. and E.F.; visualization, I.M.L., D.L. and S.F. ; supervision, S.F. and V.S. All authors have read and agreed to the published version of the manuscript. This research received no external funding.