Laboratory measurements have shown that when pressure differences are applied across wall and roof elements, the majority of the pressure drop takes place across the vapour barrier . Similarly , field measurements have shown that the majority of the leakage in Norwegian buildings occurs at the joints in the vapour barrier , at wall / floor joints , around penetrations of the vapour barrier and through holes in the vapour barrier . Prior to 1980, the standard vapour barrier in Norway was 0.06 mm thick polyethylene sheeting. However it was suggested that the use of thicker material could reduce the number of holes in the vapour barrier created during construction so in 1980 the use of 0.15 mm thick sheet was initiated and this is now the standard. This paper reports on a comparative study of the air leakage performance o f these two vapour barrier thicknesses. The study considered 10 identical single family houses, five of which were constructed using 0.06 mm polyethylene and five of which used 0.15 mm. Air infiltration measurements were carried out on completion of the houses and repeated four years after construction. The results show that the houses with 0.06 mm film were on average about 17% more leaky than the houses with the 0.15 mm film when they were new.