Acceptable indoor air quality in homes is largely governed by mechanical ventilation. Maintaining it with limited energy consumption requires balancing contaminant removal with space-heating demand. Brief window airing can refresh air yet increase air exchange and heat loss; impacts depend on climate, airtightness, wind exposure, and outdoor pollution. Although common, these behaviors are rarely evaluated with comparable metrics. This study fills this gap by comparing closed vs. scheduled window openings under different climates. The IAQ-energy trade-off is evaluated in a two-story multifamily building with (1) single-flow ventilation and (2) balanced ventilation with heat recovery and PM2.5 filtration. Each system is tested with (a) all windows closed and (b) a scheduled opening scenario for windows under eight boundary conditions combining airtightness (tight/permeable), wind intensity (weak/strong), and outdoor PM2.5 level (low/high). Heating operates from 1 October to 30 May, with a setpoint of 20°C and 16°C for day and night, respectively. Energy performance is based on fan electricity and air-renewal heat losses, while IAQ evaluation accounts for indoor concentration of CO2, formaldehyde, PM2.5, and relative humidity levels. Overall, scheduled window openings lead to a substantial increase in heat losses, while delivering only slight improvements in IAQ levels.
Kiani et al. (2026) studied this question.