Nowadays the energy policy enforces improvements in materials used for building constructions to achieve lower heat transfer coefficients of external barriers and decrease energy usage for heating, especially in East and North Europe. Reduction of heat losses by transfer results in a significant increase of ventilation losses share in the overall energy balance. Looking for further savings, a ventilation air flow is often reduced although it could entail deterioration in the indoor microclimate parameters. The author presented results of measurements and simulations for a lecture hall located in a building of Bialystok University of Technology, Poland. I conducted measurements in the lecture halls during meetings to show actual parameters of the indoor environment as temperature, humidity and carbon dioxide level. Results showed that the temperature was in a range of national rules, whereas the CO2 concentration exceeded recommended limit after 10-18 minutes from the beginning of meetings. Then a few variants of air change rates (ACH) for natural ventilation were analysed to find value allowing to maintain lower level of carbon dioxide concertation in rooms in the end of meetings. In case of natural ventilation it would be necessary to keep ACH as 5 h-1 not to exceed 1000 ppm what would result in 220-260% increase in energy consumption for heating and ventilation, so also costs of maintaining proper indoor temperature. The results of research show significant discrepancy between striving for maintain natural ventilation sufficient to ensure proper IAQ and reducing energy consumption in buildings to decrease costs.