This research addresses the challenge of enhancing energy efficiency in public buildings while maintaining or improving occupant comfort. With stricter modern energy regulations, many older facilities, such as sports halls built between 1960 and 1980, face the need for renovation to meet current standards. The central research question investigates what measures can be implemented to improve the energy efficiency of sports halls without compromising comfort for the occupants. This study examines strategies, techniques, and possibilities for optimizing energy performance during the rehabilitation of universal sports halls within sports centers. It includes a theoretical and analytical evaluation of various measures in line with existing regulations and thermal comfort requirements. This research uses simulation software, the Integrated Environmental Solutions Virtual Environment, to model different Passive House measures applied to a case study of a sports center built in 1976 in Belgrade. This study provides practical guidelines for enhancing thermal insulation on the building’s envelope to achieve energy savings. The application of these measures demonstrates that significant energy savings can be realized by focusing on specific sections of the building, such as the administrative areas, rather than the entire facility. The findings offer valuable insights into energy-optimization strategies for existing sports facilities, highlighting the practical application of measures to improve energy performance in a real-world context. The results contribute to the development of effective renovation practices for older sports buildings, ensuring they meet modern energy efficiency standards while maintaining optimal comfort for users.