Hydrophobic (or water-repellent) treatments have been proposed to mitigate moisture risks associated with Internal Wall Insulation when applied to solid masonry walls. This can reduce risks associated with moisture accumulation within the structure such as mould growth or the deterioration of joist ends and other embedded timber. However, such treatments slow down drying processes, and therefore may result in a net increase in moisture if the treatment is bypassed by e.g. cracks. Furthermore, such treatments have been linked with damage to external masonry surfaces. Freeze-thaw and salt crystallisation are the two main causes. Hygrothermal simulations may give some indication of risks but techniques to assess surface damage are either simplistic, impractical outside of the research environment, or both.