This study examines the hygrothermal behaviour of CLT external walls enhanced with wood-fibre-based ETICS. The research aims to evaluate the impact of the bio-based insulation approach on the wall’s response to the cold and humid climatic conditions typical of Estonia, with a particular focus on scenarios involving rain penetration. The area most critical to mould growth was identified to be between the rendering system and the insulation. Findings from field measurements suggest that further investigation is needed to determine the mould sensitivity of both the wood-fibre insulation and the rendering system. Accurate determination of material properties and an extended measurement period with prolonged leakage periods are essential for precise future calculation model validation.