The European Union’s issued directive no. 2010/31/ES on the energy performance of buildings dictates that starting from year 2015 all new buildings will need to consume less energy for heating. This will cause for the necessity to implement mechanical ventilation to provide healthy indoor environment because natural ventilation through cracks in building envelope be impossible. In the scope of this paper the main problems of indoor air quality in airtight buildings were identified and calculation methods of predicting dynamically changing indoor air contaminant levels as well as relative humidity is provided. The proposed calculation method takes into account parameters of outdoor air, internal CO2 and moisture production, buildings volume and air exchange rate. The paper also presents the results of monitoring of indoor air quality specifics of apartments and offices in Latvia and compares the obtained results with the calculated ones. The results have shown that proposed method is accurate in predicting CO2 level and could be used for evaluation of CO2 level.