The paper elaborates the results of measurement of indoor concentrations of carbon dioxide in the building of Mechanical Engineering Faculty Sarajevo (MEF) after implementation of energy efficiency measures. MEF’s building was the first pilot project of energy efficiency in Bosnia and Herzegovina funded by UNDP and USAID in 2011. Among others, within the project, thermal insulation and efficient windows were installed on the building. As results of the measures, consumption of natural gas has been reduced 43% on annual basis. However, due to the lack of mechanical ventilation system in the building, energy efficiency measures have significant impact on indoor air quality (IAQ). In order to investigate this impact, measurements of IAQ parameters were performed in two classrooms in different period of day and week. Measurement of IAQ parameters have lasted for three weeks. The measurement sensor was installed on the wall 1, 1 m above the floor, 1 m far from the occupants, on oposite side of the windows and door. Results of measurements showed that level of relative humidity is in acceptable range, corresponding to the standards for winter period when measurements were performed, but high concentration of carbon dioxide was recorded most of the time. Concentrations were very often high during the measurement periods, with the highest measured concentration of 2.598 ppm during the lecture. In educational institutions, which relies on natural ventilation, daily carbon dioxide concentration should be less than 1500 ppm or 500 ppm above outside carbon dioxide concentration level.