The theoretical part of J. Zemīša works takes a look at low energy houses and the current practise of designing of their ventilation system as well as the determination of necessary air volume. The paper defines what parameters describe the general indoor air quality and what the acceptable limits of them are in order to maintain safe living environment. In the practical part of the work a new calculation method for dynamically changing indoor air quality parameters is developed. It is resulting from the mass balance and is based on the characteristics of outdoor air, indoor pollutants, building volume and the rate of air exchange. Measurements of indoor air quality parameters in office and residential buildings are done as part of the job to determine the accuracy of established methodology by comparing the theoretical expectations with those actually measured. The obtained results showed that the proposed method is usable for accurate prediction of CO2 concentration while for determining the dynamically changing relative humidity level the method needs to be adjusted taking into account the humidity absorption/desorption in the building elements. This method can be used to determine the necessary ventilation volume in design process.