Ventilation systems are absolutely necessary to ensure adequate levels of indoor air renewal. In Portugal, multifamily buildings use natural, mechanical or mixed ventilation systems. However, when using natural ventilation, we can hardly know the correspondence between the ventilation system installed and the ventilation rate achieved with the same system, due to its strong dependence on wind. In this article, through simulation with software CONTAM, and based on experimental data of air permeability of the building envelope and on the performance characteristics of ventilation devices, we carry out a sensitivity analysis by varying some of the most significant parameters in mixed ventilation systems. These parameters are as follows: pressure loss of air inlets, air permeability of window frames, air extraction flows in the kitchen and positioning of interior doors, either closed or open. The annual simulation of air flows takes into account the factors as follows: environmental conditions representative of the Portuguese continental climate (four cities); subdivision for heating and cooling climatic seasons; and the production of indoor pollutants (CO2 and water vapor). The most significant conclusions to be drawn from the simulations are the following: the low air permeability of the envelope leads to reductions in air change rates close to the limit set by the Portuguese regulations; the continuous mechanical ventilation in the kitchen, the installation of an air inlet in the kitchen (opening at the maximum air flow), as well as closing the doors of service compartments (kitchen and bathrooms) significantly reduce the levels of pollutants. (C) 2017 Elsevier B.V. All rights reserved.