## 3 Results
### Definition of Corrective Strategies Based on Simulations  
The church has a master plan for its conservation. Among the actions included in this document is the drafting of a preventative maintenance plan that, since 2018, has featured an HBIM (Figure 10a) made in Autodesk Revit as a centralised repository of information that has proven its value in the process of conservation of the building [31; 32; 33]. From this HBIM, it was possible to generate a DT for the simulation of corrective strategies and to support decision-making processes. The DT was generated in DesignBuilder and exported to EnergyPlus (Figure 10b). Figure 8: Comparison between indoor temperature, outdoor temperature, indoor RH, outdoor temperature, CO2 concentration, and capacity (21–27 August). Figure 9: Comparison between indoor temperature, outdoor temperature, indoor RH, outdoor temperature, CO2 concentration, and capacity (26 August). The DT was validated with the data from the monitoring programme and the capacity monitoring. It provides a building performance model that is very close to the real building with minimum errors. The validation of the digital twin was carried out in accordance with the provisions of the ASHRAE Guideline 14 standard for the calibration of simulation models of building behaviour through monitoring data [79]. This regulation is designed to, among other things, validate relative humidity and temperature data, but in this research, it was also used to calibrate the CO2 concentration. In accordance with what is indicated in this regulation, the normalised mean bias error (NMBE) and the coefficient of variation of the root mean square error (CV(RMSE)) were calculated hourly for each of the monitored parameters, all of them being below the limits set by the regulation. In this way, 4320 data taken hourly in situ for each of the parameters under monitoring could be compared with their DT counterparts, exhibiting the errors defined in Table 2 for each of the monitored parameters. Research is currently underway improve the validation of the digital twin to reduce the error between simulation and reality. However, the researchers are confident that these errors do not negatively impact the results reported in this article. The simulations carried out with the DT focused on detecting which measures would be the most appropriate to improve the indoor environmental conditions of the church without resorting to mechanical conditioning systems. The first simulation focused on a reduction in the visitor capacity in the church. As can be seen in Figure 11, to keep the church below 500 ppm, it is necessary to limit the capacity to below 5% of its maximum capacity. Considering that the church has a capacity of 230 people, this means that there could only be 11 people inside at any one time. This would mean that the church in its current condition would not be able to host large celebrations, putting the calendar of activities that it currently has at risk. As can be seen in the figure, the limitation of capacity has little impact on humidity or temperature. It is, therefore, necessary to concentrate on improving ventilation, given that even limiting the capacity to 5% of its maximum capacity, the church is at concentrations close to 500 ppm CO2. Two new scenarios were designed in which ventilation is improved by (a) opening the three \begin{table}
\begin{tabular}{l c c c c} \hline  & **Limit** & **CO2, ppm** & **Temperature, \({}^{\circ}\)C** & **RH, \%** \\ \hline Normalised Mean Bias Error (NMBE), \% & \(\pm 10\) & 9.32 & 8.58 & \(-8.76\) \\ Coefficient of Variation of the Root Mean & 30 & 15.30 & 9.89 & 25.87 \\ Square Error (CV(RMSE)), \% & & & & \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of NMBE and RMSE according to ASHRAE 14. Figure 10: Digital building models: (**a**) HBIM; (**b**) digital twin. doors during the day and (b) opening the three doors at night (recognising that this option may have security implications).