Nowadays during the construction of new buildings and the renovation ofthe existing ones the mechanical exhaust from kitchens and bathrooms is commonly used and natural air supply is acquired through the simple air inlets in bedrooms. In many cases, after the renovation, the only natural exhausts persist in bedrooms, and the ventilation is possible only by opening the windows. The results of the research have shown that simple mechanical or natural exhaust cannot ensure optimal indoor air quality. In order to get information on indoor air quality in a whole building, the practical measurements of indoor air in different building‘s levels were done. The theoretical evaluation of infiltration and exfiltration influence on indoor air quality should be done in order to find the optimal air exchange rate and workingprincipals of the ventilation system.It is incontestable that during the winter air infiltration is acquired through the lower levels of the building and exfiltration – throughthe upper levels.