This paper is devoted to the analysis of indoor air relative humidly in buildings depending on ventilation volume. In the scope of this paper the main problems of indoor air quality in airtight buildings were identified and calculation method of predicting dynamically changing indoor air relative humidity is provided. The proposed calculation method takes into account parameters of outdoor air, internal CO2 and moisture production, volume of building and air exchange rate. The paper also presents the results of monitoring of indoor air quality specifics of apartments, offices and classrooms in Latvia and compares the obtained results with the calculated ones. The results shows that proposed method is accurate in predicting CO2 level and could be used for evaluation of CO2 level while the prediction of relative humidity still opposes some problems.