Buildings account for nearly 40% of global energy consumption and carbon emissions. Early 20th-century historic buildings in particular, while part of our architectural heritage, have high energy demands and often exhibit poor efficiency and comfort. Modernization approaches typically involve the installation or renovation of complex systems, which require consensus and shared financing within the residential community. Interior insulation offers an exception, as it can be implemented individually while preserving historic ornamental facades. In this study, we analyzed the thermal and vapor-technical performance, application potential and associated risks of various interior insulation systems with differing vapor permeability. The study also included the analysis of junction points and other critical areas, such as box-type windows and bathrooms. To assess performance, we used advanced simulation software DELPHIN to accurately evaluate hygrothermal behavior over a ten-year period. Our findings indicate that interior insulation can reduce the annual heating energy demand by approximately 20%, while also improving surface temperatures—keeping them warmer in winter and cooler in summer. However, applying internal insulation may also result in cold spots in adjacent structures, increasing the risk of condensation and mould growth. These results underscore the potential of internal insulation for improving energy efficiency in historic buildings, provided that condensation issues are carefully managed.