This study sets out to investigate to what extent the air permeability of a building envelope of a dwelling remains constant over longer periods of time. This was evaluated by executing an air pressurisation test in 30 dwellings located in Belgium and comparing these results to the initial measurement results obtained shortly after the construction of the buildings. The time span between both measurements ranges from 293 days to 4045 days. On average, the air infiltration rate of the building envelope increased with 24%, i.e. an increase of 64 m³/h at a 50 Pa pressure difference. In 9 of the 30 case study buildings, the building envelope appeared more airtight compared to the initial measurements. Most of the buildings in this study have been conceived as relatively airtight, with an initial air infiltration rate n50 of 1.2 h-1. As a result, the projected impact of the change in air tightness on the annual heating energy demand is relatively limited, with an estimated average increase of 4.1%.