Abstract The LTEE laboratory of Hydro-Quebec, in collaboration with Canada Mortgage and Housing conducted an indoor air quality study involving 30 single family-detached houses heated with electric baseboard heaters in the vicinity of Trois-Rivieres during the 1993–1994 heating season. The houses were selected according to the measured air leakage at 50 Pa so as to have a sample distribution similar to the distribution of air leakage of houses in the province of Quebec. The ‘source strength’ of several air pollutants were calculated from measurements of ambient pollutant levels and total ventilation during a 1-week test. In addition, the indoor CO 2 and humidity levels were recorded in eight of the houses continuously during the heating season. Model studies using the measured pollutant source strenghts and measured equivalent leakage areas of the buildings indicated that the recommended health guidelines for airborne respirable solid particles (RSP's), CO 2 and formaldehyde are exceeded during periods of low total ventilation, coinciding with mild outdoor temperatures and low wind conditions. With the room occupied and the door closed, CO 2 levels in the bedroom increased steadily during the night until morning, when the door was opened, to levels in excess of 3500 ppm with one person and in excess of 4500 ppm with two persons. Various different methods of ventilating some of the houses were tested, including quiet replacement exhaust fans, mixing fans for indoor air and a fresh air intake and mixing system. The effects of operating various air handling systems were monitored by keeping track of indoor CO 2 and relative humidity in the master bedroom and occupancy in person-hours per day.