**5. Conclusions**

The IAQ of two classrooms with a natural ventilation system was assessed during the cold season. PM levels were always within acceptable ranges, while CO2 and TVOC concentration exceeded recommended concentration levels. Their increase was related to the occupation of the classrooms by the students.

The CO2 concentration was within the accepted range only 26% of the time, while TVOC concentration was only acceptable 43% of the time in the most favorable scenario. These results are worrying, especially if it is considered that both classrooms were below their maximum capacity. The opening of doors and windows favors a decrease of the concentration of the pollutants, but users rarely opened them during the teaching period. Even though the acceptable limits recovered every day after the users left the building, it only took a few minutes to exceed them when the classes started the following day.

Therefore, it is undeniable that the air renewal contributed by natural ventilation and air infiltration was not enough to maintain good IAQ. However, the obtained ventilation efficiency of one of the analyzed classrooms (53.25%) was adequate for its current configuration. The ventilation performance was poor due to the randomness of both air infiltration and natural ventilation, which also encompass energy loss during the cold season, whose estimation was not the object of this study.

All things considered, the improvement of the ventilation performance of the classrooms and the airtightness of the envelope should be pursued, maintaining at the same time temperature and RH comfort conditions, to guarantee the health and good academic performance of the students. Implementing a specific controlled ventilation system adapted to the case would improve IAQ and contribute to reducing the energy demand as a consequence of a minoration of the airflow needed.

The best scenario assumes an equilibrated model, in which the inlet air and exhaust air have the same flow. Inlet and exhaust positions should promote cross ventilation, reducing the stagnation and contributing to avoid air infiltration. Nevertheless, it must be acknowledged that any change in the conditions would alter the air pattern affecting the ventilation efficiency, so a further evaluation of the ventilation system would be needed.

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

**Funding:** This research was funded by the University of Valladolid in collaboration with ARCOR, S.L., and Hermanos Rubio Grupo Constructor HERCE, S.L.U., gran<sup>t</sup> number 18IQBC. The APC was funded by the same team.

**Acknowledgments:** This work was supported by the University of Valladolid under the research project "REVEDUVa: Energy recovery through ventilation of university classrooms", framed within the R&D projects on energy efficiency measures and the application of renewable energies in the operation of the university buildings of the University of Valladolid in collaboration with ARCOR, S.L., and Hermanos Rubio Grupo Constructor HERCE, S.L.U. The authors would also like to thank the University of Valladolid for the funding of the doctoral program of one of the authors.

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