The main aim of ventilation systems in dwelling buildings is to ensure optimal comfort conditions and indoor air quality for inhabitants. While temperature and humidity level are crucial indicators for human comfort, CO2 concentration is significant factor for human health. The air-exchange rates in dwelling buildings commonly are calculated by the same methods irrespective of the apartment’s level. This paper is devoted to modeling of CO2 concentration in indoor air and air distribution in multi-storey apartment buildings. A stack effect could be observed in the multi-storey buildings during the winter. When internal walls, ceilings and HVAC shafts are not airtight, in addition to heat circulation, CO2 and moisture circulation from building’s lower levels to upper levels is also taking place. In such case the amount of air-exchange rate and ventilation scheme should differ for upper and lower building’s levels. The paper provides the results of IAQ features in multi-storey dwelling buildings paying particular attention to the influence of building’s level and airtightness of buildings’ internal elements on IAQ parameters.