In the last few years there has been a tendency towards the air-tight sealing of buildings. This phenomenon can be seen both in older buildings being refurbished, as well as in those that are new or thermallyupgraded. The use of excessively air-tight windows in educational buildings with gravity (natural) ventilation system limits the inflow of external air to classrooms. The analysis covered 50 educational buildings in Częstochowa, which are used by primary and middle schools. Windows in those buildings were in a bad or very bad technical condition and their air-permeability was very high. Their defects cause excessive heat losses from heated rooms. It turns out, however, that the high air-permeability of windows (rate of outdoor air leakage (a) at the level of 6-7 m3/(hmdaPa2/3) allows for a relatively beneficial level of natural ventilation and sufficient inflow of fresh air in the absence of other inflow of outside air solutions. Unfortunately, as a result of the replacement of old windows with new ones and no changes in the existing system of natural ventilation, a considerable deterioration of microclimate in classrooms can be expected, especially due to non-compliance with the requirements related to the sufficient supply of fresh air. To prevent this, it is necessary to take action to improve existing ventilation systems.