Subjects 11 private dwellings at Taby and 5 at Brunna (all 2-3 yrs old) to repeated airtightness tests over a period of two years. Finds the largest leakage is 2.5 ach and the mean leakage is 1.6 ach. Immediately after its completion, the air tightness of a building undergoes a certain deterioration, after which it stabilizes. Over the 2yr. measuring period the changes are small, and could all be attributed to occupancy effects. Leakage paths occurat the junctions of wall and ceiling and wall and floor, and at service entries. In order that airtightness behaviour should be fully measured,tightness measurements should be repeated after 3-5 yrs.