A field measurement study of the airtightness of 73 - less than 5 year old - French dwellingswas led between 1999 and 2000. Buildings have been selected and classified according to theconstruction structure, the thermal insulation and the occupancy mode. Using a fandepressurizationtechnique, we assessed the air leakage rate of each dwelling with twodepressurization tests. Meanwhile quantifying air leakage rates, we observed qualitatively themost frequent locations of air leakage paths using a smoke detection method and infraredthermography. We assessed the ratio of the air leakage rate weighted by intrinsic dimensionsof each construction, namely : the unheated surfaces and the heated volume. From our results,we compare the performances of the different types of dwellings and we assess the impact ofthe envelope airtightness on the building ventilation efficiency. We show that thermalperformances of buildings can be dramatically affected according to the dwelling constructioncharacteristics. Finally, we discuss the potentials for reducing indoor air infiltration with aview to improve the indoor air quality and the energy efficiency of buildings.