The airflows through a one family prototype building have been simulated. Supply openings in living room and bedrooms have a size of 200 and 400 cm2 respectively. The ventilation system is a passive stack ventilation system, with ventilation chimneys from kitchen, WC and bathroom. The following parameters have been studied: the supply opening areas and heights, the overflow opening areas between bedrooms and hall and between WC/bathroom and hall, the height of the ventilation chimneys and the opening and closing of living room and bedroom doors. In this study a supply efficiency is calculated for each room and set of parameters. It is defined as the relative number of hours, during the heating season, when the actual airflow is equal to or higher than the design airflow. For living room and bedrooms the outside airflow is used while the exhaust airflow is used for kitchen, WC and bathroom. With the basic configuration, the supply efficiency is as low as ca 2 % for the bedrooms and below 1 % for WC and bathroom. By modifying the parameters it is possible to increase the efficiency. Without fans it is however impossible to reach the design values during the whole period.