Performance evaluation of internal insulation systems with smart vapour retarders in European climates

Smart vapour retarders (SVRs) have the potential to reduce the moisture risk of internal insulation systems due to the capability of varying their vapour permeability depending on the moisture content. Despite their introduction over four decades ago, their application in internal wall retrofitting requires extensive research depending on various parameters. This study examines the influence of key parameters (insulation material, thickness, climate zone, wall orientation, and indoor humidity) on the effectiveness of SVRs in solid brick masonry retrofits. Different types of SVRs were investigated: well-established vapour retarders in form of foil and innovative paint-based prototypes. Hygrothermal simulations were conducted to evaluate relative humidity and the mould growth index at the critical point in the construction. This work contributes to a more nuanced understanding of the hygrothermal performance of internally insulated walls and provides practical guidance for selecting and optimising insulation systems with SVRs across diverse climatic and boundary conditions. The study shows that smart vapour retarders generally reduce interstitial relative humidity and the risk of mould growth, with the most significant benefits observed in mineral wool and cellulose systems under Continental and Northern climate conditions. Their effectiveness depends strongly on indoor moisture loads, insulation material and wall orientation, while vapour-tight or capillary-active materials are less sensitive to SVR application. These results emphasise that, when informed by accurate indoor climate data, careful selection and placement of SVRs can improve the moisture safety of internally insulated walls in relation to having no vapour barrier and, in some specific climates and wall configurations, may perform comparably or better than traditional vapour barriers.