This chapter investigates the capillary water absorption characteristics of ten traditional Irish bricks. Water absorption is an important property of building materials and strongly influences the response of brick walls to rainfall events. The moisture content in a wall is a critical factor for the safe and effective thermal upgrading of traditional walls with internal wall insulation. It is often not possible to measure the water absorption of traditional brick walls in the laboratory and selecting the most appropriate brick (with a similar A-value) for hygrothermal modelling is challenging. In the absence of laboratory testing, this chapter reviews methods to estimate the water absorption of a brick based on visual inspection and historic provenance, porosity and the results of a Karsten tube test. The capillary absorption of ten traditional Irish bricks tested ranges between 0.05 and 0.51 kg/m2s½ and is broadly within the range observed by other authors of traditional bricks and within the water absorption values of bricks in the WUFI database. The capillary absorption results highlight that water absorption cannot be solely estimated based on visual inspection and provenance of the brick. The chapter further investigates the relationship between porosity and capillary absorption and observes that at commonly found traditional Irish brick porosities (>30%), there is only a weak relationship between increasing porosity and capillary absorption, and therefore, measuring brick porosity is not a reliable method of estimating the water absorption properties of a traditional brick. The Karsten tube test is an easy in-situ water absorption test. The results found a strong linear relationship between water absorption measured in the laboratory (according to ISO15148:2002) and the Karsten tube test. The 3-dimensional water absorption measured during the Karsten tube test was approximately 4 times greater than the water absorption measured in the laboratory when using units of kg/m2s0.5. The bricks were subdivided into three categories of low, medium and high water absorption (Karsten 1–9 ml/hr, 9–16 ml/hr, 16–50 ml/hr, respectively), and an additional category of very high absorption for bricks was added with absorption Karsten > 50 ml/hr. The chapter proposes that the Karsten tube test can be used as part of the repertoire of non-invasive in-situ aids to assist in selecting the most appropriate and closest matched bricks in hygrothermal modelling software.