The airtightness performances of buildings and ventilation systems can have a major impact on the indoor climate (IAQ, thermal comfort,...) and on the energy performance. Measurement results for Belgian buildings clearly show that the airtightness is often moderate to very poor. As part of the proposed energy performance legislation for the Flemish Region, it is envisaged to pay attention to the airtightness of buildings and ductwork. In the first part of the paper, results found for Belgian buildings and systems are briefly presented and discussed. In the 2 nd part of the paper, specific aspects of the proposed approach for building and ductwork airtightness are explained.