A year -long investigation of air change rates in an occupied house was undertaken to establish the effects of temperature, wind velocity, use of exhaust fans, and window -opening behavior. Air change rates were calculated by periodically injecting a tracer gas ( SF 6 ) into the return air duct and measuring the concentration in 10 indoor locations sequentially every minute by a gas chromatograph equipped with an electron capture detector. Temperatures were also measured outdoors and in the 10 indoor locations. Relative humidity ( RH ) was measured outdoors and in five indoor locations every 5 min. Wind speed and direction in the horizontal plane were measured using a portable meteorological station mounted on the rooftop. Use of the thermostat -controlled attic fan was recorded automatically. Indoor temperatures increased from 218C in winter to 278C in summer. Indoor RH increased from 20% to 70% in the same time period. Windows were open only a few percent of the time in winter but more than half the time in summer. About 4600 hour -long average air change rates were calculated from the measured tracer gas decay rates. The mean ( SD ) rate was 0.65 ( 0.56 ) h À 1 . Tracer gas decay rates in different rooms were very similar, ranging only from 0.62 to 0.67 h À 1 , suggesting that conditions were well mixed throughout the year. The strongest influence on air change rates was opening windows, which could increase the rate to as much as 2 h À 1 for extended periods, and up to 3 h À 1 for short periods of a few hours. The use of the attic fan also increased air change rates by amounts up to 1 h À 1 . Use of the furnace fan had no effect on air change rates. Although a clear effect of indoor -outdoor temperature difference could be discerned, its magnitude was relatively small, with a very large temperature difference of 308C (548F ) accounting for an increase in the air change rate of about 0.6 h À 1 . Wind speed and direction were found to have very little influence on air change rates at this house.