## 3 Results and Discussion
### Ventilation Strategies  
In order to carry out a detailed analysis of the ventilation strategies implemented in the different case studies, a rough estimation of ventilation rates was conducted using the steady-state method explained in Section 2.4. Additionally, Table 3 compiles some of the morphological characteristics of the classrooms that directly affect their natural ventilation capacity. Figure 6: IAQ analysis based on measured CO\({}_{2}\) levels, classified as follows: optimal air quality (CO\({}_{2}\) < 750 ppm), good air quality (750 ppm < CO\({}_{2}\) < 1200 ppm), and poor air quality (CO\({}_{2}\) > 1200 ppm). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**School** & \multicolumn{2}{c}{**1**} & \multicolumn{2}{c}{**2**} & \multicolumn{2}{c}{**3**} & \multicolumn{2}{c}{**4**} \\
**Classroom** & **1.1** & **1.2** & **2.1** & **2.2** & **3.1** & **3.2** & **4.1** & **4.2** \\ \hline
**Max. window opening surface area (m\({}^{2}\))** & 1.95 & 1.95 & 3.8 & 3.8 & 2.5 & 2.5 & 9.9 & 7.0 \\
**N. doors** & 2 & 2 & 2 & 2 & 1 & 1 & 1 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Case study characteristics linked to natural ventilation. As shown in the table, the classrooms with the largest operable window area are those in case 4, located in northern Italy, followed by those in case 2, in southern Spain. However, while in cases 1 and 2, it is customary to open 100% of the operable window area, this does not apply to cases 3 and 4, where typically only 10-40% of the total operable area is used. Another aspect contributing to higher air exchange rates in the case studies in southern Spain is that the classrooms have two access doors from the hallway, which tend to be left open in accordance with post-COVID recommendations. In most schools in southern Spain, the layout of classrooms typically includes a central corridor providing access to classrooms on both sides [27]. Post-pandemic, windows and doors tend to be kept open in classrooms on either side of the corridor, significantly promoting cross-ventilation. This effect is less pronounced in the classrooms monitored in northern Italy, which only have one door, usually kept closed according to the teachers surveyed. Figure 7 presents the results obtained from the ventilation rate estimation using a boxplot graph. Generally, the highest ventilation rates were estimated in the cases in southern Spain, frequently exceeding 5 air changes per hour (ACH) in all instances. Overventilation was notably evident in classrooms such as 1.2, where values often exceeded 15 ACH. In contrast, ventilation rates were consistent in the cases of northern Italy, ranging from about 2.5 ACH in case 3 to between 3 and 7 ACH in case 4. It should be noted that in Spain, the ventilation rate required in newly built schools is about 9 ACH, while in Italy, about 4 ACH is considered sufficient. In order to analyse the impact of natural ventilation strategies on thermal comfort, Table 4 summarises the maximum and minimum air temperature results for the measured and estimated ventilation rates based on CO\({}_{2}\) values. It is observed that in classrooms with higher ventilation rates, such as 1.2, internal loads are adequately dissipated, and indoor temperatures of around 22 \({}^{\circ}\)C are maintained, even with higher outdoor temperatures (exceeding 25 \({}^{\circ}\)C). Conversely, classrooms with lower ventilation rates, such as 3.1, 3.2, and 4.2, experience higher maximum indoor temperatures than outdoors due to insufficient dissipation of internal loads. Therefore, with moderate external climates, a high air exchange rate favours suitable indoor thermal conditions. Figure 7: Estimated natural ventilation rates. Similar results were found by [PERSON] et al. [39] when comparing factors influencing thermal comfort among students at two universities in Spain and Portugal after COVID-19 mitigation protocols were no longer mandatory.