Influence of hygrothermal property variability on moisture-risk assessment of traditional solid brick walls

Traditional masonry buildings, particularly those constructed before the 1920s, represent a significant proportion of the existing housing stock and are typically characterised by poor thermal performance. Moisture is a major cause of deterioration in these buildings, contributing to increasing the risk of moisture accumulation, mould growth, and frost damage. Reliable moisture assessment is therefore essential for developing safe retrofit strategies. This study investigates the variability of hygrothermal properties in traditional bricks and evaluates its influence on moisture-related risks. 60 bricks from three groups, dating from the 1850s to the 1910s, were tested for three fundamental hygrothermal material properties, including absorption coefficient, bulk density and free saturation content. The measured data were then used to define material inputs for three-year hygrothermal simulations in DELPHIN under a current climate scenario. The results show clear differences between brick types and between properties. Handmade yellow bricks showed the greatest variability, while later red bricks were more consistent. Within the same wall, inner common bricks were more variable than external facing bricks. The simulations indicated low mould growth potential and no frost-risk events under the assessed conditions. Moisture accumulation differed more between brick groups than within each group, suggesting that brick type had a stronger influence than individual sample variability. The water absorption coefficient showed the closest relationship with moisture accumulation, although its effect varied by brick type. These findings support material-specific sampling and modelling strategies for more reliable moisture-risk assessment in traditional solid wall retrofit.