Buildings are important energy consumers and one of the main contributors to global warming. Energy audits play a key role in achieving energy efficiency and reducing greenhouse gas emissions in old buildings. Traditional energy assessments often rely on analytical models with limited precision. The 3D models used for dynamic energy simulations offer a potentially more accurate and transparent approach to understanding energy flows and identifying potential areas for improvement. This contribution presents a case study of an existing house, tracking its performance during winter using measured data. The measured parameters were used to determine the compliance of indoor temperatures with standards. Subsequently, a 3D model of the house was created in dynamic simulation software to calculate the energy balance of the building. A comparison was made between the results of the dynamic simulation and a single step calculation performed in MS Excel. The case study illustrated the possibility of proposing energy efficiency measures using energy simulations instead of single-step calculations and discussed some of the main differences and implications.