An airtight building envelope ensures not only the energy-efficiency of a building, but also a damage free construction. Important to achieve optimal airtightness are the planning, implementation and materials. Long-term airtightness requires efforts in all three aspects. Airtightness products are being tested under lab conditions but these results cannot be transferred one-on-one onto buildings. To gather more information regarding the durability of the airtightness 17 passive houses were re-measured as part of a research project of the International Energy Agency, “IEA Task 28, Annex 38”. The re-measured buildings included various construction types, different building types, e.g. terrace houses and single family houses, and buildings aging from 1.4 to 10.5 years. As all buildings are passive houses, airtightness was a key aspect during the planning phase and the initially measured n50-values were very low. For 16 out of 17 buildings the re-measured results were good to very good with an average n50-value of 0.42 1/h. The re-measurements have shown, that it is rather the planning than the type of construction that has a significant impact on the long-term durability of the airtightness of buildings. In 2016 the first passive house in Darmstadt-Kranichstein, was re-measured again 25 years after it has been built. The two terrace buildings originally reached n50-values ranging between 0.2 and 0.4 1/h, which experts thought not possible at the time. During the first re-measurement in 1999 the level of airtightness has not dropped significantly. After 25 years the sealants for windows and doors, that can be opened were identified as a possible cause for leakage and were replaced. Measurements before and after the replacement presented an improvement due to this very simple measure. Even though many materials were not yet available in 1991, when the passive house in Darmstadt-Kranichstein was built, it could still obtain a high airtightness. This proves the importance and impact of airtight design in early stages of the planning phase onto the long-time durability of passive house building airtightness.