Internal wall insulation is one of the most promising methods of improving the energy efficiency of buildings while maintaining their original facades and construction. However, porous materials or poor construction quality can allow moisture ingress, creating favourable conditions for mould development within building fabric. Currently, there is no established guidance on moisture criteria in the construction industry for the assessment of interstitial mould growth. Some studies have considered relative humidity (RH) criteria that account for the duration of exposure but potentially overestimate risk at interfaces. This study implemented more specific RH criteria, based not only on the duration of exposure but also on temperature and substrate material. Results demonstrated a dramatic decrease in predicted mould risk, with minimal risk to health or structural integrity, in comparison to the present more stringent standards.