Energy optimization of historic buildings is often perceived as a threat to their cultural values. This paper examines this assumption. Firstly, we assess the actual energy performance of historic apartment buildings in Denmark using a register-based approach with 20,000 historic apartment buildings of high preservation value. Secondly, we explore the potential for energy improvements in historic buildings by analyzing typical interventions that have been implemented to achieve an EPC label C. Thirdly, we discuss whether these interventions pose a threat to the preservation values of historic buildings and how local authorities might address this challenge. We present three examples of energy optimization in historic apartment buildings, ranging from step-by-step renovations to larger renovations and transformations of historic buildings. The study finds that historic apartment buildings perform at least as well as traditional buildings from the same period in terms of energy efficiency, and there is significant potential for further improvements. However, there are large variations across different types of municipalities. Finally, we observe that most interventions are based on conventional solutions with limited impact on preservation values. We discuss how more radical interventions can be managed through dialogue between local authorities, building owners, and consultants.