Ventilation response and thermal comfort trade-off under winter conditions in junior secondary classroom operation

In winter classrooms without dedicated outdoor-air supply systems, window opening is the main means of maintaining indoor air quality, but may conflict with students’ thermal comfort. We propose a simple evaluation framework linking thermal acceptance with measured indoor environmental quality to guide window-opening strategies. Field observations in one secondary school classroom in severe-cold northeastern China combined timetable-based period monitoring of indoor CO 2 , air temperature, and relative humidity with window-opening observations and questionnaires from three classes. Window opening occurred in only 5.2 % of periods. Mean CO 2 was 1337 ppm and peaked at 2921 ppm; 87.7 %, 23.8 %, and 8.3 % of periods exceeded 1000, 1500, and 2000 ppm, respectively. During class-period opening, CO 2 fell by 173 ppm and temperature by 0.9 ℃ on average. Over 80 % of students did not oppose slightly cooler classrooms for better air; low-disruption ventilation was generally preferred. The framework identifies behavioural bottlenecks and supports low-cost ventilation management.