In the drive to reduce operational carbon emissions, increased focus is being put on retrofitting the ‘worst-performing’ buildings, often understood to mean the oldest buildings. For these buildings, generally of solid masonry construction, conventional retrofit strategies such as internal insulation systems of capillary inactive and highly vapour resistant materials have been demonstrated to increase the moisture risk of the assembly, sometimes to a point of failure. In the assessment of the moisture risk of such existing structures and their proposed retrofit measures, hygrothermal simulations have become invaluable tools, as they can consider the impact of complex moisture transfer mechanisms. However, unknowns typically remain around actual boundary conditions and hygrothermal properties of the wall materials. There is a knowledge gap regarding the validation of hygrothermal models in a temperate cool maritime climate like Ireland. In the presented case study, a historic solid brick wall of known assembly in Dublin, Ireland, was monitored for a full year for its thermal transmittance and climate conditions, including temperature, relative humidity and weather data from a public meteorological measuring station in close proximity, but excluding moisture monitoring within the wall buildup itself. In addition, the hygrothermal material properties were experimentally characterised, and simulation materials based on these inputs created. This information is used in 1D hygrothermal modelling, and the presented parametric study compares the impact of synthetic and actual external climates, derived and actual internal climates, and existing database and measured materials on simulation outcomes. It highlights the impact of measured climate data, and finds that heat flux can provide insight into the accuracy of the model calibration. Due to the availability of full hygrothermal material parameters, the case study provides rare opportunity to test the effect of different boundary conditions on simulation outcomes, and highlights the importance of refined information for surface parameters for solid wall assemblies.