The UK has pledged to reduce its carbon emissions by 80% by 2050. This translates into improving, among other measures, the energy efficiency of solid wall buildings, which requires the use of challenging measures such as internal wall insulation. Internal wall insulation can lead to moisture accumulation and mould growth at the interface between the insulation and the existing wall, if incorrectly designed and installed. Consequently, a thorough risk assessment is crucial for the design and specification of measures such as internal wall insulation. This paper presents the evaluation of mould growth models for the risk assessment of woodfibre internal wall insulation. The hygrothermal conditions within internally insulated solid walls were monitored and three models compared. The suitability of the models was assessed in relation to the risk of mould growth at the interface.