Measurement of Indoor Air Quality parameters (IAQ), as CO2 concentration, temperature, and relative humidity, was performed in one classroom at the Faculty of Mechanical Engineering in Sarajevo, with the aim to assess ventilation efficiency. In 2011, several energy efficiency measurements as part of the pilot project, were implemented on the building of the Faculty of Mechanical Engineering in Sarajevo, such as thermal insulation of the building and replacement of existing boiler, but without adequate system for ventilation and cooling. Faculty still continues to use natural ventilation. With implementation of mentioned energy efficiency measures, savings on the heating energy were achieved for about 43 %, but at the same time due to the lack of appropriate ventilation system, there is a significant impact on indoor air quality, especially during the heating season. Beside achieved airtightness of the building, IAQ is affected by other factors, such as the volume of the room, number of users and ventilation frequency. Measurement has lasted for ten days and results were compared with the standard EN16798-1:2019. The measured values showed that in the classroom, during the winter period, the average indoor air temperature was 21 ˚C, relative humidity 35 % and measured CO2 concentrations were up to almost 3 times higher than the recommended by the standard. [ABSTRACT FROM AUTHOR]