Human performance and health are one of the most relevant topics in modern society. Especially at young ages, when academic performance is indispensable. Thus, as the human being spends most of its lifetime inside a building, thermal comfort has become an essential aspect of a room. The aim of the present research is to measure and evaluate the main thermal comfort parameters such as CO2 levels, relative humidity and indoor temperature so the variation in them can be seen in the study rooms of the library of the University of Gävle as there is student use. For it, Rotroninc Measurement Solutions CL11 sensors and a Testo hot wire probe sensor have been used, as well as IDA ICE software simulations for the result validation. From the research, has been seen that even though the VAV air renewal system works as it should, the CO2 level rises up to 1000 ppm, which is not recommended by different thermal comfort ruling institutions. This way, a modification to the ventilation system control is recommended, changing it from temperature control to CO2 level and temperature control. Moreover, it is seen that during the non-opening hours of the library the ventilation systems are disconnected, generating an important energy-saving without altering the thermal comfort of the rooms at the beginning of the day.