Moisture storage and transport are critical factors in the degradation of architectural heritage. To better formulate conservation and energy measures, and prevent moisture damage, hygrothermal modelling of building components is a valuable and frequently applied approach. However, hygrothermal modelling requires input properties, among which reliable material properties. Obtaining these material properties often necessitates destructive testing, which is typically unsuitable for historical structures. Consequently, material properties from databases are frequently used as proxies for real materials in modelling, thereby introducing significant uncertainties into the results. In this context, the Karsten tube is a valuable device for assessing the water absorption behaviour of porous materials by monitoring the water level decrease in a graduated tube attached to the material’s surface. As an in-situ and non-destructive measurement technique, it holds great promise in heritage conservation. To enhance the potential of Karsten tube measurements and to identify and mitigate possible sources of error, within this study a variety of bricks have been investigated. The focus is put to the potential impact of personnel operation and to experimental uncertainty. Experimental results obtained by different operators on the same type of brick were compared, revealing noticeable deviations. This prompted an analysis of the causes, and improvements to the experimental procedure were proposed to minimize operator-related variability. Finally, the correlation between results obtained by Karsten tube measurements and those determined by standard water uptake tests is examined, offering insight into the method’s reliability.