Numerous classrooms at the University of Warmia and Mazury in Olsztyn, built in the years 1960−1980, require deep modernization to implement the idea of the green university there. The paper presents an energy analysis of possible scenarios of modernization of a selected lecture hall and an indication of the optimal variant in terms of the user’s thermal comfort. The conditions of the room were experimentally measured. Authors compared the energy effects of typical thermorenovation activities, such as increased thermal resistance of external walls (reduction of the primary energy (EP) demand by about 45%), with more advanced measures, such as replacement of natural ventilation with mechanical with heat recovery (indicated in the course of numerical studies as necessary for health and comfort of users). The authors also compared the effectiveness of the heat network and individual sources that could be implemented in the building. The heat pump, in combination with photovoltaic panels, turned out to be optimal (reduction of the EP by 97%, utility energy by 69%). As the problem of modernizing facilities to a similar standard is also faced by many campuses, school facilities, and assembly rooms in Central and Eastern Europe, the presented results can be a reference point for similar activities in other locations.