Abstract The economic considerations force the building body optimization during its design, as well as during the modernization of existing facilities. The thermal modernization effects are calculated according to the national rules and standards. The theoretical savings from the reduction of heat losses by transmission and ventilation in a secondary school located in Poland were calculated (59–71%) and compared with the real savings (33%), calculated on the base of data from the measurements conducted in several heating seasons before and after modernization. The achieved ecological effect (33%) was also lower that theoretical one (69%). The results of this work are worth taking into account in further schools modernizations and a city planning. The simple model for energy use estimation was proposed. The bigger sample of schools will be analyzed to propose changes in the calculation procedure after the end of the whole study.