I would like to thank the two peer reviewers as well as Eric Rirch and Bill Bordas for their feedback. Their comments have been invaluable in revising this paper.
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. Where treatments perform well there is a net reduction of moisture content and risk. 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. Some treatments have may lead to damage to external masonry surfaces in some situations. Freeze-thaw and salt crystallisation are the two main causes Hygrothermal simulations may give some indication of risks but techniques to assess the risk of surface damage are either simplistic, impractical outside of the research environment, or both. This paper reviews the state of the art of assessing and predicting the risk of surface damage associated with surface treatments.