Airtight construction lies at the heart of achieving high energy performance in dwellings. But how well does it apply in new construction? This paper presents results from airtightness measurements on 44 randomly selected, standard new built single family houses in Belgium and from 4 case studies including 78 additional measurements. The houses were randomly selected after completion, to assure that standard workmanship was used during construction. Where applicable, the effect of incorporating the attic and garage in the building volume was measured by performing a series of tests in different configurations. The results are compared with those from a previous study in the early 1990's, with a database that was compiled with results from 161 air tightness reports executed on newly built dwellings by private party consultants and with the governmental EPBD-database (1884 measurements). The results show that the mean leakage rate is about 6 ACH(50) for the randomly selected houses and 3 ACH(50) for the houses in the databases. The houses in the databases are measured upon the initiative of the owner. Therefore, the attention to airtight workmanship is substantially higher for these cases than in the randomly selected houses. This clearly demonstrates the difference between 'mainstream' workmanship and results obtained by the 'engaged' market.