Greenhouse systems in the form of glazed balconies may be accomplished both in the newly designed buildings and in the existing ones, raising their energy standard in a quick and inexpensive way. However, basic parameters influencing the efficiency of the systems are often chosen intuitively, not allowing to fully benefit from the sunspaces or causing overheating of the rooms and discomfort for the users. These issues are common drawbacks of passive systems and may become especially important in the aspect of anthropogenic climate changes, including temperature rise and summer heatwaves. The paper presents results of a long-term summer temperature monitoring in flats with glazed balconies of different construction. They were located in prefabricated multi-family buildings, in residential districts of Lublin and Zamość. The cities are situated in the south-eastern part of Poland, belonging to the warm-summer humid continental climate area. The monitoring enabled overheating assessment according to the concept of adaptive comfort and connecting it with the sunspace construction and the inhabitants’ behaviour. These issues are new aspects in the research area, and the work is a part of extensive studies including monitoring and dynamic simulations of dwellings with passive greenhouse systems in Poland. Practical application: Glazed balconies raise the energy standard of buildings in a quick and inexpensive way, but it is important to consider their function not only during the heating season but also in the summer. Recording of temperatures enabled the monitoring of thermal conditions in the flats and the overheating assessment. It is possible to keep the internal temperature in the rooms within the desired range thanks to the sunspace ventilation and occupants’ behaviour.