Field evaluation of winter indoor environmental performance of a Passive House–level detached house in Mongolia

In Mongolia, residential detached houses are predominantly heated using solid-fuel-fired stoves, resulting in severe air pollution throughout the long winter heating period. Although interest in energy-efficient housing has increased in recent years, there is limited field-based evidence demonstrating the actual in-use performance of such dwellings under real occupancy conditions in Mongolia's extreme cold climate. This study presents a field evaluation of the winter indoor environmental performance of a newly constructed detached house designed to meet Passive House criteria in Ulaanbaatar, Mongolia. Multi-period field measurements were conducted during the winter seasons of 2024 and 2025, under normal living conditions to assess indoor air temperature, carbon dioxide (CO2) concentration, relative humidity, and household energy consumption. Airtightness was evaluated using a blower door test, while envelope thermal performance was examined through on-site U-value measurements and thermal imaging. The results show that the building achieved very high airtightness (n50 = 0.18 h–1) and low envelope heat transfer, enabling stable indoor temperatures despite outdoor temperatures frequently falling below -20 °C. However, intermittent overheating, room-to-room temperature differences, low winter indoor humidity, and elevated CO2 concentrations were observed, mainly associated with south-facing open-plan spaces and reduced ventilation operation. Overall, the findings demonstrate that Passive House-level residential buildings can substantially improve winter indoor environmental conditions in extreme cold climates, while highlighting the importance of appropriate ventilation control, operational strategies, and system design to achieve balanced indoor environmental quality.