The purpose of this study was to measure the long-term moisture build-up in the walls of a house which had been retrofitted with Internal Wall Insulation. This was done by embedding moisture sensors in the walls and then monitoring values over an eight-year period. The results showed that the wall moisture of embedded wood piece sensors had increased to a value of 18-20% over 8 years. In a comparable wall, which had been treated with a masonry water repellent in the form of a cream, the build-up of moisture was significantly reduced to 9-10% over the same period. This wall was examined after the 8-year period and no visible mould was evident on the internal face of the wall, behind the insulation.