Historic buildings are a vital part of Europe’s architectural heritage, representing cultural identity and uniqueness. Internal insulation is a common solution for improving winter thermal performance. However, climate change is leading to rising temperatures and more intense rainfall, raising concerns about the long-term effectiveness of current retrofitting solutions. Internal insulation must be assessed for its hygrothermal performance before implementation, yet this is typically done under present climate conditions. Another common critique is that internal insulation may exacerbate summer overheating. The novelty of this study lies in investigating these two research questions under future scenarios: (i) how internal insulation performs, and (ii) how different insulation strategies—internal, external, and hybrid—compare in mitigating summer overheating. Understanding these factors is essential to ensure energy-efficient and climate-resilient retrofitting of historic buildings. In the simulated case study, internal insulation performs well for present and future climate with regards to moisture management, showing that the future climate is not worsening the situation. On the contrary, rising temperatures lead to a slight improvement in the wall’s performance. When comparing different insulation methods for overheating, the first key finding is that any type of insulation helps reduce overheating compared to having no insulation at all. Additionally, internal and external insulation perform similarly. Internal insulation slightly reduces the number of hours when temperatures exceed 25 ℃, while external insulation is more effective in lowering the overall intensity of overheating.