Thermal renders for traditional and historic masonry walls: Comparative study and recommendations for hygric compatibility. 

To reach European climate neutrality by 2050, the strategic importance of retrofitting the existing building stock is clear. For this scope, thermal rendering systems have emerged as a very feasible solution for historic and traditionally constructed walls. Nonetheless, a definitive guideline for the selection of suitable solutions for the application in this context is not yet available. This research aims at providing recommendations for the choice of hygric-compatible solutions in an early-stage design, for the context of temperate climates with mild winters. In this study, the massive masonry walls of three historic buildings located in Portugal and Italy are considered. Mono-dimensional hygrothermal simulations are validated against the data measured on-site. Simulations are then used to evaluate the impact of thermal renders on the walls, in comparison to more common insulation materials. Two moisture-related risks are considered: moisture accumulation and reduction of drying. This investigation shows that, for traditional porous walls, retrofitted configurations should be simulated not only under typical operational conditions but also considering a very high initial water content in the wall substrate, when the insulation is applied. Otherwise, moisture-related risks may be overlooked. Recommendations on the choice of thermal rendering systems are provided.