Ventilation plays an important role in the RT 2000 regulation. This new French thermal regulation takes into account energy for heating and lighting through a C coefficient (primary energy in kWh), as well as summer comfort for non air-conditioned buildings. The paper focuses on the calculation of the C Coefficient. For the ventilation side of it, the calculation method aims both to have simple input data and physically based algorithms. The fact that it was chosen to have a computer based approach made this possible. The algorithms are based on an implicit method as the TC 156 WG2 (prEN 13465) extended to non-residential buildings. Different ventilation or airing system can be taken, into account, based on the possibilities offered by the hygienic regulation: Mechanical system and passive duct systems for residential building (additional window airing is also considered), Windows opening, mechanical system in non-residential building (passive duct is also possible but the algorithms focuses on the use for residential buildings). For each situation of climate and system behaviour the implicit method calculates the different airflows through the ventilation system and the building envelope in a first step and the energy impact in a second step. The paper describes the physical basis of the method and it uses in the RT 2000 approach, and gives some examples of results.