The need for sustainability and smaller ecological footprint lead to the construction of more airtight building envelopes with better thermal insulation in order to increase the energy efficiency according to the Energy Performance of Buildings Directive (EPBD 2010/31/EU). However, specific fire risks can be encountered by the occupants since they can be blocked for a long period due to the fire-induced pressure and the inward door opening. In 2015 a full-scale fire setting was built in Bauffe (Belgium) by the Régie Provinciale Autonome du Hainaut RPA on its fire brigade training site for studying the effects related to fire development in a building with high air tightness. These experimental results were used for validating the zone model CFAST 7.3.2 predictive capability for fire induced pressure. Thanks to the very low calculation time, using a fire zone model as first step is a practical way to understand the influence of the major physical phenomena before using much more complex software such as computational fluid dynamics. However, serious limitations were observed with CFAST version 7.2.3. For the tests carried out without mechanical ventilation, CFAST does not consider the effect of pressure on the natural leakages area which increases with pressure. This limitation leads to an overprediction of the fire induced pressure by a factor of 4. For the tests carried out with mechanical ventilation, CFAST version 7.2.3 does not consider the reverse flow of fan supplying fresh air into the building when the fire induced pressure is higher than the stall pressure of the fan neither the extra flow of fumes of the fan extracting air from the setup leading also to an overprediction of the fire induced pressure by a factor of 4. This paper presents the development of a one-zone fire model for the calculation of the fire induced pressure in a multiple rooms airtight building taking into account the effects of the fire induced pressure on both the effective leakage area (according to the ASTM E779) and the characteristic curve of the fans. This new code has been validated thanks to the novel experimental results obtained at large scale. Satisfactory results were obtained for parameters such as fumes temperatures, pressures, major species concentrations and flow rates in the mechanical ventilation. This paper focuses on the results obtained without mechanical ventilation, the ducts being tickly closed.