In this paper the effects of atrium and other similar architectural design features (e.g. shafts) on ventilation efficiency are examined in a multi-storey office building. Attention has been given to simulate the use of the main entrances, the vestibules and the various shafts. An atrium and an non atrium solution were compared for the examined building. Stack effect was the dominating force and wind effect was present yet not significant, but enough to produce negative pressures at the area of the atrium, mainly due to the form of the atriums roof. Among others, it was concluded that the implementation of the atrium increased ventilation efficiency especially at the upper floors, generating incoming flows from the atrium and increasing the infiltration and ventilation rates, even for high density urban areas. The outcomes are useful for IAQ control during winter and for passive cooling during the summer. Special attention is needed to evaluate the users behavior, since the effects proved to be intense and often overturn initial ventilation and energy design.