The indoor air climate of a church, with a total building volume of 9500 m^{3, was investigated from January to October, 2008. During this time, infiltration and ventilation measurements were performed at four periods ranging from 17 to 45 days. The local mean age of air was determined in several locations at different heights using passive samplers and homogeneous emission of PFTs. In addition, the temperature and relative humidity was measured at selected locations. The total infiltration of fresh air into the church was found to be mainly driven by the difference between indoor and outdoor temperature. The rate of infiltration ranges from 780 m3/h (deltat=14.8 degrees C) in January to 240 m3/h (deltat=3.5 degrees C) in August. The local mean age of air in the upper and lower air volumes were found to be roughly equal, both during the heating season and in the summer. This indicates that the air volume in the church is well-mixed throughout the year. Transient measurements to assess the effects of doors opening in the summer were also performed using pumped sampling.}