Abstract: This paper investigates the optimal combination of thermal insulation and phase change materials (PCMs) to retrofit the inside of external walls. Multiple variants of office and residential buildings located in Brussels are analysed by means of dynamic building performance simulations. Findings reveal that, for external wall retrofitting, the building’s insulation performance has the greatest impact on energy savings, with PCMs having a more significant effect in lightweight structures. The results substantiate the utility of PCMs as a retrofit measure, particularly in well-insulated office buildings, outperforming adding thermal insulation due to the high internal loads. For interior retrofitting with a maximum thickness of 5 cm, the combination of PCMs and thermal insulation is more effective than insulation alone in most scenarios. This combination effectively balances heating and cooling energy savings, reducing overall energy demand up to 4.8% in the office building and up to 26.7% in the residential building.