The construction industry is a key driver of environmental change due to its extensive use of resources and high levels of emissions, thus placing a significant burden on global efforts towards sustainable development targets. A large portion of the environmental footprint of buildings results from the energy required to sustain indoor comfort levels. Therefore, enhancing the energy efficiency of existing buildings becomes critical in reducing their environmental impact. This study examines the influence of increasing thermal performance with the aim of reducing heating demand. Thus, numerical modelling tools and different energy performance assessment meth-odologies have been used. Specifically, it evaluates two existing residential buildings in Romania and compares heating demand predictions using PHPP and Mc001-2022 methodologies across various climatic data sets and case studies. The results reveal significant variability in heating demand predictions, emphasizing the importance of methodological choice and climatic data. The study highlights that both PHPP and Mc001-2022 methodologies yield varying results depending on the specific conditions and characteristics of each building. This variation in results underscores the necessity for a nuanced and context-specific approach to energy performance assessment, in-corporating multiple energy efficiency measures tailored to the unique features of each building.