Post-Insulating traditional massive walls in Southern Europe: A moderate thermal resistance can be more effective than you think

Post-insulating existing buildings is a promising solution for reducing operational CO2 emissions from the European built environment. Nonetheless, its efficacy is unclear when traditional and historic massive walls are considered, especially in Southern Europe. This study employs a validated and calibrated dynamic hygrothermal simulation model to assess indoor comfort and energy demands in a public library with thick stone masonry walls and intermittent occupation, considering three Southern European climates: Porto, Lisbon, and Bologna. Five insulation materials, including three thermal mortars and two conventional materials (Hydrophobic Mineral Wool and Expanded Polystyrene), are compared using internal and external insulation solutions. Thin insulation systems (4 cm) with moderate thermal resistance (Rt = 0.3-1.0 m2K/W) are studied and found to provide more benefits than drawbacks. One thermal mortar-based system demonstrates comparable performance to conventional insulation materials, indicating that low-conductivity thermal mortars are effective for retrofitting historic and traditional massive walls. Numerical analyses show that optimal reductions of energy demand can be achieved with an insulation Rt of 0.9-1.3 m2K/W, while further increases yield no additional benefits and even counterproductive outcomes. Results support adopting moderate Rt insulation in Southern European climates and highlight the need for future research considering the effect of post-insulation on climate change adaptation.