**5. Conclusions**

Despite the limitations that a case study supposes, the proposed methodology and proceedings can serve as guidelines for a post-COVID-19 scenario.

Considering IAQ, thermal comfort and the practicality of the scenarios, scenario C is the most effective for this case study. Thus, the mixing model, which ensures the removal of pollutants, is guaranteed by continuous cross-ventilation. It can be reached using different opening levels, depending on the use and occupation of the classroom, which contributes to acquiring an adequate air quality for the use of the space. Air flow regulation reduces the thermal effects of the new air coming from outdoors.

A similar air flow can be obtained by fully opening doors and windows in designated moments of the day. However, there is both a thermal loss and an absence of crossventilation while teaching. These conditions promote an air quality fluctuation value, while a constant value is preferred for these kinds of spaces. This can be observed in the application of Scenario B, with similar ventilation values but 82% lower cross-ventilation, where adequate thermal comfort results were not achieved.

As demonstrated in Scenario B and Scenario D, having specific openings set was only effective if there was cross-ventilation. As a result, if the layout of the classrooms require coordination between them, it cannot be considered functional since its application over time would be compromised.

Overall, two types of protocols are suggested to be applied in naturally ventilated classrooms: one for health emergency scenarios, in which higher ventilation rates are required, and another for non-emergency situations, in which ventilation rates are not that demanding.

Finally, all these natural ventilation scenarios are influenced by the pollutants present outdoors. This aspect should contribute to defining criteria to operate the openings by the constant monitoring of those pollutants. In this case study, high-pollutant values were not detected, neither in PMs nor TVOC. For instance, it was possible to verify that the indoor concentration of pollutants is ruled by a proportional relationship with the values registered outdoors.

**Author Contributions:** Conceptualisation, A.M., H.J.-M. and I.P.-C.; methodology, A.M., H.J.-M. and I.P.-C.; formal analysis, A.M., A.L.- Á. and M. Á.P.-M.; investigation, H.J.-M.; resources, A.L.- Á.; data curation, H.J.-M.; writing—original draft preparation, H.J.-M.; writing—review and editing, A.M. and I.P.-C.; visualisation, A.M.; supervision, I.P.-C.; project administration, A.L.- Á.; funding acquisition, M. Á.P.-M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research "Metodologías de estudio y estrategias de mejora de eficiencia energética, confort y salubridad de centros educativos en Castilla y León. Investigación básica" was funded by JUNTA DE CASTILLA Y LE ÓN. CONSEJERÍA DE EDUCACI ÓN, gran<sup>t</sup> number VA026G19 (2019–2021).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
