Energy efficiency of building heritage derived from pre-regulation period is one of the most debated topics in Europe. Building façades, through opaque walls and thermal bridges, are a major source of transmission heat losses and require sustainable and consistent solutions. Aiming to achieve an energy demand reduction, thermal features of building façades were evaluated by field measurements and simulations for one INA-Casa listed apartment building built in the 1950s. Non-destructive insulating solutions have been proposed and a comparison between transmission heat loss coefficient in the current situation and the designed intervention was made. Results show that before the renovation, opaque walls and thermal bridges respectively contributed to 25% and 44% of total transmission heat loss in the case-study building façade. After the renovation, total impact was reduced up to 70% depending on whether high performance windows were replaced; in particular, the impact of opaque walls and thermal bridges were reduced by 66% and 82%, respectively. Interventions performed primarily on the internal layer of the façade with insulation elements, when appropriately designed, strengthen the negative effects of thermal bridges in junctions. Findings show that an accurate insulation design allows for both more efficient conditions and the preservation of the heritage-listed buildings.