In order to respond to global warming and natural resources depletion challenges, industrials from the building sector need to propose an adequate offer. Energy simulation tools can support this process. Various studies and real cases show that a high performance level, e.g. primary energy consumption below 50 kWh.m-2 per year (including heating, cooling, domestic hot water, lighting and ventilation), can be reached by appropriate architecture combined with high insulation, free cooling and heat recovery on exhaust air. This last technology is particularly affected by airflows across the building envelope caused by a low air tightness. Thermal modelling tools need therefore to deal with this issue precisely. A multizone model has been developed to compute building airflows in order to evaluate them with a higher degree of precision in the frame of a simplified simulation tool that can be used in early design phases. This model is based on the well-mixed zones assumption and mass conservation principles. The air flow rate between two zones is expressed as a function of the pressure drop between those two zones. Wind pressure and buoyancy effects are the causes of pressure drops. Several types of connections are implemented: cracks, ventilation inlets, large openings. More types of connection will be added. This model has been implemented in the thermal building simulation tool COMFIE developed by Peuportier and Blanc Sommereux (1990). The airflow model uses the zone temperatures as an input and reciprocally the thermal model requires the airflows . Both models run therefore at each time step until convergence is reached using a synchronous coupling method. An algorithm has been developed to ensure the convergence for each time step (from 1/10 to 1 hour). One case study is presented; it is a dwelling building where the influence of airtightness on heating loads is being studied for several types of mechanical ventilation systems.