Climate change, energy consumption, and carbon footprint goals pose challenges for the hygrothermal performance of Nordic building structures. Residential buildings in Finland, designed to last up to 100 years, must perform well under not only the current but also future climate conditions. Realized carbon emissions align with the RCP8.5 scenario predict a temperature rise of over 4 °C by 2100, resulting also in much higher moisture loads for buildings. This study validated numerical models with data from five insulation assemblies to assess their hygrothermal performance and mould growth under future climate conditions. Measurements conducted in a climate cabin over a 70-day period showed strong alignment with simulations. Using Finnish Moisture Design Years (MDYs) for various future climate scenarios, we found the critical point at the exterior side of the interior sheeting gypsum board. Mineral wool insulation inside the vapour barrier performed poorly under increased humidity, while organic insulations showed markedly better performance. Mould growth on tested organic insulations was predicted only in the RCP8.5 2080 scenario. Based on the studied options, optimal structures may combine linen or wood fibre inside and mineral wool outside the vapour barrier, with gypsum board having higher water vapour resistance to mitigate indoor humidity effects.