When assessing moisture risk in an exterior retrofit project, existing conditions are important to understand. Initial moisture in existing materials and assemblies can be quite different than in new construction. At the time of retrofit, the existing materials may have been in-service for 30+ years. Absorptive claddings, such as brick, stucco, and masonry may store significant amounts of water after rain, for example. In the case of overcladding retrofit types, the potential for moisture-related issues can arise if retrofits are carried out without allowing sufficient drying time for the existing claddings. This paper focuses on the development of a hygrothermal modelling protocol to be used during the design stage of exterior energy retrofits. The protocol aims to help determine a safe threshold of built-in moisture that can be accommodated by the retrofit assembly and how long the existing assembly ought to be allowed to dry after rainfall, before being overclad. The protocol uses best modelling practices and conservative assumptions about the existing conditions. The protocol is intended to eventually evolve into a best practice document for modelling methodology and consistency for future ambitious exterior retrofits in Canadian climates. The protocol will be reviewed and iterated with more case studies.