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• Indoor environmental quality (IEQ) in low-income flats. • Occupants have an influence on air quality. • Variability in window opening routines and natural ventilation intervals. • Observed outcome was poor quality and often unhealthy indoor air. Thermal comfort and Indoor air quality (IAQ) in residential buildings with different degrees of airtightness was studied in two climates in Spain. Behaviour was compared in the areas occupied by day and by night. The IAQ of the buildings studied, erected before energy efficiency regulations were in place (1939–79) and lacking mechanical ventilation, was compared to their airtightness. The rationale for that approach was that under such circumstances air change depends on uncontrolled natural ventilation (=opening windows) and consequently on the outdoor temperature. Relative humidity was also taken into consideration, given the condensation that may be induced where ventilation is insufficient. In winter in both climates, the CO 2 levels were over 1200 ppm, with means on the order of 1900 ppm in Madrid and 1400 ppm in Seville and higher at night than during the day. Infiltration-mediated air changes/hour appeared to be insufficient to maintain the house under healthy conditions and the risk of surface condensation is higher in the most airtight dwellings. [ABSTRACT FROM AUTHOR]