In recent years, the building regulations regarding buildings’ thermal performance have been steadily tightened. Such regulations target primarily new buildings. However, construction of new and more energy-efficient buildings alone cannot bring about a significant reduction of the building sector’s negative environmental impact. This implies the importance of thermally retrofitting the existing building stock. Thermal retrofit of buildings typically addresses building envelope as well as building systems for heating, cooling, and ventilation. Both normative calculations and dynamic simulation can be used to estimate the impact of retrofit measures on buildings’ performance. However, past studies have revealed discrepancies between the computationally projected and the actually monitored post-retrofit energy performance of buildings. Such discrepancies, commonly referred to as energy performance gap, must be better understood, if the potential of thermal retrofit measures toward energy conservation and reduction of greenhouse gas emissions is to be more reliably realized. In this context, this paper compares the energy demand of a number of apartments in a multi-unit residential building in Vienna, Austria before and after thermal retrofit. In the course of the study multiple sources of information were considered, including original building plans, details of the retrofit measures, calculated post-retrofit energy demand, and energy bills over different years. Based on this data, the actual pre-retrofit energy performance of the selected units could be compared with the respective post-retrofit values. Moreover, the units’ actual performance could be compared with computed values (obtained via energy certificates and dynamic simulation). The gap between expected and monitored energy performance was analyzed and potential contributors to this gap were discussed.