Abstract
The preservation and renovation of heritage buildings in Norway pose challenges in achieving adequate indoor air quality while maintaining thermal comfort and energy efficiency. This study reports the results of a one-year monitoring campaign in two heritage buildings comprising four residential apartments with different renovation levels and ventilation strategies. Indoor environmental conditions were assessed using measurements of temperature, relative humidity, carbon dioxide, formaldehyde, total volatile organic compounds, and fine particulate matter. One building included an extensively renovated apartment with internal insulation and mechanical exhaust ventilation in the kitchen and bathroom, alongside a neighbouring apartment with limited mechanical ventilation and reliance on natural infiltration. In the second building, one apartment remained unaltered, while the other was renovated with internal insulation and mechanical exhaust ventilation. Balanced mechanical ventilation was not implemented in any apartment. The results show clear differences in indoor air quality and thermal comfort between apartments with similar layouts but different renovation and ventilation solutions. Occupant preferences and behaviour strongly influenced achieved thermal conditions. Apartments with wood-burning stoves exhibited higher indoor temperatures and increased concentrations of fine particulate matter and formaldehyde. In addition, internal insulation combined with insufficient ventilation led to elevated carbon dioxide and volatile organic compound levels during occupied periods. These findings demonstrate that renovation strategies in heritage buildings significantly affect indoor environmental quality and emphasize the need for tailored guidance that supports both building conservation and healthy indoor conditions.