The air tightness of building has been a serious problem over the last 30 years. In 1979 the international Air Infiltration Centre (AIC) was erected within the International Energy Agency (IEA) platform. Infiltration of cold air into buildings needs to be heated to reach to a comfortable indoor climate. But the energy penalty due to that should be minimized. The AIC (later AIVC) had as one of their tasks to find solutions for good air tight buildings and to promote the knowledge about building construction to reach acceptable level of air tightness of buildings. Many publications were produced. The slogan was and is “Built tight and Ventilate right” The problem of air tightness is still important in the building practice of these days. Last year in 2010 TightVent Europe was formed to cope with the problem of air infiltration due to building leakages in low energy buildings. The big question is are buildings still too leak ? To find an answer on the above question the reasons for airtight buildings should be considered.