Open Access

Naturally ventilated educational buildings frequently struggle to maintain adequate indoor air quality (IAQ) under variable occupancy and user-driven ventilation behavior. This exploratory case study evaluates IAQ using the CO2-concentration as an IAQ-indicator in 14 naturally ventilated rooms of six different educational facilities during the heating season in Germany, examining the effects of room design, occupancy, and ventilation practices. After evaluation against current German design recommendations, all investigated rooms reveal the conditional necessity of a mechanical ventilation system. However, all investigated rooms are exclusively ventilated naturally. Continuous monitoring of occupied rooms revealed mean indoor CO2-concentrations – grouped into categories (I to IV) according to EN 16798–1 – ranging between 233 ppm and 383 ppm (category I), 654 ppm and 691 ppm (category II), 910 ppm and 1097 ppm (category III), and 1409 ppm and 2553 ppm (category IV) above the outdoor CO2-concentration. The corresponding frequency of the prevailing CO2-concentrations during the room occupancy periods in the categories I to IV range between 6% and 63% (category I), 9% and 38% (category II), 10% and 42% (category III), and 0% and 59% (category IV). Occupant-specific ventilation rates – determined based on the CO2-decay method – varied substantially (4–188 m³ h⁻¹ occupant⁻¹). Seven of the 14 investigated rooms failed to meet the German Federal Environment Agency’s minimum ventilation rate of 25–30 m³ h⁻¹ occupant⁻¹ under typical operating conditions. Corresponding area-specific ventilation energy losses during the German heating period were estimated to range between 1 kWh m−2 a−1 to 40 kWh m−2 a−1. To translate exposure patterns into management guidance, a CO₂-based 4 × 4 risk matrix was developed, integrating exceedance severity (based on EN 16798–1 categories) and occurrence frequency. While most rooms showed only moderate CO₂ elevations for limited occupied periods, two of the 14 investigated rooms (14%) were classified as high IAQ risk in the proposed matrix, indicating severe and frequent exceedances of hygienically acceptable conditions. The proposed risk-matrix framework provides a reproducible decision-support tool for prioritizing ventilation improvements in naturally ventilated educational sector buildings. By linking exposure severity, likelihood, and energy implications, the approach supports IAQ management under operational and structural constraints.