The National Energy and Climate Plan for Ireland (2021–2030) commits to retrofitting 500,000 homes to B2 BER or cost-optimal level by 2030. However, there is widespread international evidence of inappropriate retrofit works causing long-term damage to traditional building fabric (Gemmell 2014; Webb 2017). The FabTrads project intends to improve BER (EPC) assessments by measuring the in situ U-value of a wide range of traditional Irish walls. Additionally, it shall measure the hygrothermal properties for a range of construction materials in use in Ireland between 1700 and 1940 through in situ and laboratory testing. The findings will support hygrothermal modelling in accordance with IS EN 15026:2007. Default U-values in the National Calculation Methodology are often used by Irish building professionals when considering thermal upgrading (wall insulation) of traditional buildings; however, it has long been questioned how these defaults were derived and whether they are truly indicative of the thermal performance of traditional solid masonry construction. Working with the Sustainable Energy Authority of Ireland and the National Standard Authority of Ireland, the FabTrads project, therefore, set out to measure up to 50 in situ U-values of a wide variety of typical traditional Irish solid walls to determine the accuracy of the existing defaults and whether a greater range of defaults are required. This paper presents initial results from the first year of testing traditional (pre-1940) Irish buildings including two granite walls in Co. Carlow and Dublin, a variety of limestone walls across the country and a wide range of brick walls from the nineteenth and early twentieth century. These initial tests have begun to present results, which may be representative of the wider traditional building stock. U-values have been shown to vary for different materials, by thickness, by moisture content and repair condition. Two in situ U-value tests were run on each wall, which in many cases revealed slightly different results. As the in situ testing is non-destructive in nature, a number of case study walls will be modelled using numerical simulation software once all hygrothermal properties of the stone and brick are measured in the laboratory. This research will contribute to a greater understanding of the range of thermal performance of traditional building materials in situ, thus informing the development of better retrofit guidance for historic buildings.