## 4 Conclusions  
The particular environmental and climatic conditions that apply in the Mediterranean zone, a region rich in tangible cultural heritage with millions of visitors every year, makes the promotion of the deep energy retrofitting of historic buildings converted to museums a significant challenge, mainly due to vulnerable but delicate nature of the exhibits in those museums. When retrofitting measures are to be applied in such buildings, the pre-intervention evaluation of the indoor environmental quality, a critical factor for the preservation and conservation of exhibits and a determinant for the well-being of the users of the building, is crucial for decision makers regarding the optimal retrofitting strategy to be followed. Therefore, the assessment of the quality of the indoor environmental is a key point in the deep energy retrofitting approaches recommended for the Mediterranean reality. As a result, in this work, the evaluation of the indoor microclimatic conditions in a historic building converted to a museum is presented as part of the greater MedZEB (Mediterranean Zero Energy Building) approach, a concept developed within the HAPPEN Project, in which the concept of nZEB (nearly Zero Energy Building) is adapted to the specific characteristics of the Mediterranean area. For this reason, an integrated 23-month experimental campaign to evaluate the indoor environmental quality of a museum housed in a historic building located in Athens took place from February 2019 to April 2021, which was divided into two periods and aimed to contribute to the development of an energy retrofitting protocol for museums hosted in historic buildings by providing useful data. The key findings of this campaign can be summarized as follows:  
The mean monthly T values were found to be lower than the proposed limit range (20-23 \({}^{\circ}\)C) for the cold months of December, January, February, and March in all rooms under investigation, whereas the highest mean monthly T during the cold months were recorded in the 2 nd floor offices, which was mainly due to the frequent use of HVAC split units by the museum's personnel. On the other hand, during the summer months, the average monthly T within the Applied Sciences and Philosophy exhibition rooms (2 nd floor) demonstrated high values, surpassing the proposed limit range (23-26 \({}^{\circ}\)C). As a whole, the museum demonstrated low acceptable T value percentages, which were within limits ranging from 7 to 33% for the entire experimental period, indicating the building's poor thermal behaviour. Moreover, the significant T differences observed inside the museum during the year are likely to have negative effects to the exhibits. Therefore, renovation actions related to the thermal protection and/or the upgrading of the heating \begin{table}
\begin{tabular}{c c c} \hline \hline
**Experimental Point** & **2019 vs. 2020** & **2019 vs. 2021** \\ \hline Auditorium & \(-4\%\) & \(-4\%\) \\ Entrance & \(-1\%\) & \(-1\%\) \\ Law & \(-1\%\) & \(-2\%\) \\ Medicine & \(-2\%\) & \(-4\%\) \\ Pr. Room & \(-4\%\) & \(-7\%\) \\ Dentistry & \(-3\%\) & \(-6\%\) \\ Offices & \(-3\%\) & \(-6\%\) \\ Philosophy & \(-5\%\) & \(-4\%\) \\ App. Sciences & \(-2\%\) & \(-3\%\) \\ Museum (in total) & \(-2.8\%\) & \(-4.1\%\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Percentages of differences for CO2 levels within the museum compared to 2019.  
cooling system, such as the increase of its HVAC power to avoid T levels out of the comfort range, are considered necessary. With regard to the CO2 levels, they were found to be within their respective limits, with the offices (472 ppm) and the Auditorium Hall (472 ppm) at ground floor recording the highest average CO2 concentrations. In addition, the TVOC concentration, representing the usage of cleaning materials such as detergents and antiseptic hand gels, was higher in 2020 and doubled in 2021 as a result of COVID-19 pandemic protection measures, compared to 2019 levels in almost all areas under investigation.