Traditional bricks have a range of properties which may be unknown, thus their impact on moisture risk is uncertain. Selecting an inappropriate material for hygrothermal modelling can produce misleading simulation results. A sensitivity analysis using WUFI-Pro was performed using different ‘library’ bricks to investigate the influence of brick properties in modelling a Victorian terrace house upgraded with IWI. Results were categorised by a seasonal ‘amplitude’, the difference between the highest and lowest monthly average relative humidity at the brick/IWI interface. Moisture diffusivity best explained the differences in the moisture response. Two bricks produced results consistent with brick/IWI interface measurements in the house. The study illustrated the need for guidance in choosing materials and conventions for modelling to assess moisture risk.