We investigate the airtightness of 12 French non-residential buildings, by means of experimental fan-depressurization tests. For this study, 12 recent large (volume > 500 m 3 ) buildings have been chosen according to the construction structure and the activity. Four categories of buildings have been selected : hotels, schools, offices and polyvalent halls. We assessed the air leakage rate of each building, with a fan-depressurization equipment, following the recommendations of the international norm project ISO 9972. The airtightness of the sole envelope is measured by sealing intentionally the openings provided to the buildings for natural or mechanical ventilation. Meanwhile quantifying air leakage rates, we also observe the locations of air leakage paths using a smoke detection method and infrared thermography. We assess the ratio of the air leakage rates weighted by intrinsic dimensions of each construction, namely : the unheated surfaces and the heated volume. We analyze the infiltration air exchange contributions according to the types of constructions, and we compare the results to the requirements applicable in France since June 2001 for the Thermal Regulation 2000.