The urgent need to improve the energy efficiency of buildings, in line with the European Union’s climate directives, has increased focus on innovative and sustainable materials for building retrofitting, particularly within the context of Modern social housing. This study assesses the potential of eco-sustainable thermal insulation mortars, with a particular focus on cork-based solutions. Through energy simulations applied to public residential buildings constructed in Italy between 1945 and 1976, the impact of these mortars on the energy demand for winter and summer climate control is analyzed. Results indicate up to a 17% reduction in annual energy consumption, depending on the structural characteristics of the buildings, demonstrating the effectiveness of thermal insulating mortars in enhancing envelope performance without compromising the aesthetic integrity of architectural heritage. While these mortars do not yet fully meet the transmittance requirements for new or retrofitted buildings, their application opens research avenues for the continuous improvement of insulating performance, including the integration of eco-sustainable aggregates in mix design. The ultimate goal is to contribute to a large-scale retrofit of social housing that adheres to environmental sustainability criteria and greenhouse gas emission reduction. This study provides a foundation for developing higher-performance solutions capable of supporting European environmental targets and facilitating retrofitting interventions that respect the original architectural and structural quality of buildings.