Air leakage through the building envelope joints is usually one of the main reasons why airtightness targets are not achieved. The objective of this study was to analyse the air leakage of joints filled with polyurethane foam and its influencing factors. Wooden test specimens (54 in total) with planed, sawn and plastic-coated cavities and two cavity thicknesses were filled with three different polyurethane foams and tested according to standard EN 12114. The surface type and thickness of the joint had a significant effect on the air leakage of joints filled with polyurethane foam. In laboratory conditions, a consistent and very low air leakage rate was obtained with planed timber surfaces. Joints with plastic-coated and sawn timber surfaces performed worse, on average, by a factor of two or more and contributed to very variable airtightness, with up to 28% and 50% of the test specimens failing the airtightness testing. On the basis of the high ‘failure rate’, polyurethane foam may classify as a not completely trustworthy solution in guaranteeing the airtightness of construction joints. A comparison of estimated and previously measured overall airtightness of an entire building envelope showed dependency on ‘failure rate’ rather than on average measured leakage rate.