## 4 Conclusions  
Indoor air quality in natural ventilating buildings is strictly dependent on windows frame airtightness. The more and more pressing norms aiming at energy saving in buildings played an important role in making airtightness in buildings more effective: it contributes to energy saving but, on the other hand, it can also cause indoor air quality deterioration. In this context, in the present paper, two case studies were investigated: two natural ventilated rooms with different airtightness class of the window frame. The first one, with a volume of about \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{3}{*}{**Process**} & \multirow{3}{*}{**Configuration**} & \multicolumn{2}{c}{**MAA [Min]**} & \multicolumn{2}{c}{**MAA [Min]**} & \multicolumn{2}{c}{**Relative Error**} \\  & & **Experimental Data** & \multicolumn{2}{c}{**ANN**} & \multicolumn{2}{c}{**[\%]**} \\ \cline{3-8}  & & **Case Study 1** & **Case Study 2** & **Case Study 1** & **Case Study 2** & **Case Study 1** & **Case Study 2** \\ \hline \multirow{3}{*}{Training} & OD\_OW & 4.92 & 5.65 & 7.14 & 6.64 & \(-45.0\) & \(-17.5\) \\ \cline{2-8}  & CD\_OW & 15.91 & 21.91 & 16.75 & 21.98 & \(-5.3\) & \(-0.3\) \\ \cline{2-8}  & OD\_CW & 31.54 & 31.37 & 32.97 & 31.12 & \(-4.5\) & \(0.8\) \\ \cline{2-8}  & CD\_CW & 144.09 & 168.05 & 145.40 & 167.71 & \(-0.9\) & \(0.2\) \\ \hline \multirow{3}{*}{Testing} & OD\_OW & 4.26 & 4.92 & 7.06 & 3.76 & \(-65.8\) & \(23.7\) \\ \cline{2-8}  & CD\_OW & 19.16 & 22.33 & 21.79 & 23.21 & \(-13.7\) & \(-3.9\) \\ \cline{2-8}  & OD\_CW & 30.48 & 36.44 & 32.41 & 32.82 & \(-6.3\) & \(9.9\) \\ \cline{2-8}  & CD\_CW & 142.85 & 177.00 & 148.63 & 166.67 & \(-4.0\) & \(5.8\) \\ \hline \multirow{3}{*}{Total} & OD\_OW & 4.66 & 5.28 & 7.11 & 5.20 & \(-52.6\) & \(1.7\) \\ \cline{2-8}  & CD\_OW & 17.21 & 22.12 & 18.77 & 22.60 & \(-9.0\) & \(-2.2\) \\ \cline{1-1} \cline{2-8}  & OD\_CW & 31.12 & 33.06 & 32.75 & 31.69 & \(-5.2\) & \(4.2\) \\ \cline{1-1} \cline{2-8}  & CD\_CW & 143.59 & 172.52 & 146.69 & 167.19 & \(-2.2\) & \(3.1\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: MAA mean values comparison: experimental data vs. ANN outputs. 32 m\({}^{3}\), is characterized by a window with airtightness Class 1/2 (corresponding to 50-27 m\({}^{3}\)/hm\({}^{2}\) of air of infiltration). The airtightness class of the second one, with a volume of about 44 m\({}^{3}\), is 4 (3 m\({}^{3}\)/hm\({}^{2}\) of air of air of infiltration). The mean age of air (MAA) was experimentally evaluated in compliance with ISO 16000-8:2007 [16], in thirty-three different experimental campaigns, twenty for case study 1 (situated in Terni) and thirteen for case study 2 (situated in Perugia). During each experimental campaign, four different room configurations were investigated: open door-open window (OD_OW), closed door-open window (CD_OW), open door-closed window (OD_CW), and closed door-closed window (CD_CW). During the experimental campaigns, the main indoor and outdoor environmental parameters were also monitored. The CO\({}_{2}\) concentration and the MAA values obtained for the two case studies were then correlated. Results show that:  
1. MAA values increase from the configuration OD_OW to the configuration CD_CW, with an exponential trend and with a very significant gap between the configurations with open and closed window, while the configuration of the door has a less influence;
2. MAA calculated for case study 2 is substantially higher with respect to the ones related to case study 1. The difference found in the configurations with closed window is mainly due to the airtightness of the frame, while the one in the configurations with open window is probably due to the volume of the room;
3. A similar behaviour is observed when comparing the MAA with air flow rate per room volume (G/V), where G represents the air change and V the volume of the room;
4.