Around 30% of the European building stock was built before 1950, when no regulations about energy efficiency were in force. Since only a small part of them has been renovated by now, the energy performance of this building stock is on average quite poor, resulting in a significant impact on the energy balance of European countries, as confirmed by data published by ISTAT (Italian National Statistical Institute). However, energy retrofit in historic edifices is a quite demanding issue as any intervention must take into account the need to preserve existing building materials and appearances while also allowing reversibility and low invasiveness. As an example, in these buildings it is not possible to apply an ETICS (External Thermal Insulation Composite System), since this would alter the historic and architectural value of the façade. On the other hand, internal insulation would have the drawback of reducing the net useful floor area, which also implies a loss of economic value. Moreover, internal insulation may induce overheating risks and mold formation. In this paper, all these issues are investigated with reference to an existing historic building located in southern Italy, showing that a retrofit strategy aimed at energy savings and cost-effectiveness is still possible if suitable materials and solutions are adopted.