A longitudinal study of CO₂ and air exchange rate variations in naturally ventilated airtight energy-efficient dwellings

Background: In energy-efficient dwellings, the relationship between airtightness and natural ventilation is critical but underexplored. As airtightness increases, the ability of natural ventilation to provide adequate ventilation may diminish, potentially compromising Indoor Environmental Quality (IEQ). Aim: This study provides a unique assessment of ventilation performance in energy-efficient, naturally ventilated, airtight dwellings, with a particular focus on the impact of long-term variations on indoor CO₂ levels and Air Exchange Rate (AER). Methodology: A novel longitudinal approach used a decay method based on occupant-generated CO₂ to estimate AER. CO₂ concentrations were monitored in nine Irish homes over one year, followed by annual and seasonal assessments. Results: The findings revealed clear long-term and seasonal variations in ventilation performance. Winter CO₂ concentrations were consistently higher, indicating reduced ventilation due to lower window use. The seasonal median AER range was 0.15 h⁻¹ (winter) to 0.27 h⁻¹ (summer), reflecting poor ventilation during colder periods. Over 51 % of AER values fell within 0.1–0.3 h⁻¹, and 15 % were below 0.1 h⁻¹, suggesting persistent under-ventilation. Only 0.4 % of values exceeded 1.0 h⁻¹. The mean AER (0.28 h⁻¹) was 42–70 % lower than in conventional dwellings, reinforcing the inadequacy of natural ventilation strategies over time. Purge ventilation events were infrequent and short-lived, with transient rates of 0.4–2.7 h⁻¹. Conclusion: The study underscores the need for adaptive ventilation strategies in energy-efficient buildings, particularly in colder months. While natural ventilation can provide adequate rates, improvements such as hybrid systems and occupant education are recommended for optimising ventilation.