Airtightness of School Classrooms in Spain: Field Measurements and Retrofit Implications

This study characterises the airtightness of school classrooms in several Spanish educational centres using blower-door measurements (ISO 9972:2015). Building envelope airtightness governs uncontrolled air infiltration and plays a central role in indoor air quality (IAQ): excessively permeable envelopes increase the ingress of outdoor pollutants such as PM2.5, while highly airtight classrooms without adequate mechanical ventilation may accumulate indoor CO2 and thermal discomfort. In Mediterranean climates, where mechanical ventilation is not systematically installed in schools, uncontrolled infiltration frequently becomes the dominant air renewal mechanism during the heating season. The measured classrooms show a mean n50 of 9.08 h−1 (range: 4.32–22.67 h−1), placing them at an intermediate level in the international literature. High within-school variability was identified, indicating that single-classroom measurements may be insufficient to represent building-level airtightness. External façade area showed the strongest exploratory association with n50 (Spearman ρ = 0.663, p = 0.037), but the small, non-random sample does not support generalising this relationship beyond the measured classrooms. The paired comparison in Centre C4 did not suggest an airtightness improvement beyond measurement uncertainty after the specific ETICS intervention, while the C5 case illustrates the potential influence of unsealed service penetrations. Overall, the observations point to windows, junctions and service penetrations as priority locations for diagnosis and targeted sealing, to be verified in larger and more diverse samples.