Energy consumption has become a significant global issue due to climatic and environmental challenges, a lack of energy resources, and rising energy prices. The building stock accounts for 40% of the total energy consumption. Therefore, efforts should be made to explore possibilities to reduce energy consumption and related CO2-emissions, not only by the construction of modern and energy-efficient buildings but also through energy retrofitting existing buildings. Historic buildings often have thermal discomfort, cold draughts close to the exterior walls, and high energy consumption due to heat losses. Hence, the improvement of energy efficiency of historic buildings is important. Moreover, preserving architectural heritage can reduce our reliance on new materials for new buildings and reduce energy use in manufacturing processes. The potential for energy efficiency of historic buildings depends on an appropriate compromise between the need to conserve cultural heritage inherited in the building and energy efficiency measures.

Reducing energy consumption and raising thermal comfort is possible by adding thermal insulation to the building envelope. Most historic buildings have facades with cultural, historic and aesthetic value, therefore, internal insulation is proposed as a suitable measure. The application of internal insulation changes the hygrothermal behaviour of a facade significantly and might result in hygrothermal risks such as frost damage, interstitial condensation and mould growth. The change in hygrothermal behaviour depends on the type of internal insulation system. Nowadays, vapour-tight and vapour-open, capillary active, internal insulation systems are mostly used for historic buildings.There is no consensus about the third internal insulation system: a vapour-open, non-capillary active due to a lack of knowledge about the hygrothermal behaviour of this system from theoretical and practical perspectives.

The aim of this research is to gain insight into the most influential hygrothermal properties of vapour-open, non-capillary active, internally insulated historic solid brick masonry. Through literature review, background information is obtained on the hygrothermal behaviour of vapour-open, non-capillary active internal insulation of historic solid brick masonry by exploring the factors that influence this behaviour. Prerequisites for the risk-free hygrothermal performance of a vapour-open, non-capillary active, internally insulated historic solid brick masonry wall are defined based on the risk of mould growth due to interstitial condensation and taking moisture-sensitive wooden elements into account. This approach is sufficient for one-dimensional and two- or three-dimensional situations.

The influence of hygrothermal properties on the hygrothermal behaviour of vapour-open, non-capillary active, internally insulated solid brick masonry of historic residential buildings is studied by a parameter study, which is carried out through Heat, Air and Moisture simulation software WUFI 2D.4. To gain insight into the influence of the hygrothermal properties on the hygrothermal performance, the boundary condition of the outdoor environment is simplified by excluding solar radiation and wind-driven rain. In the parameter study, four hygrothermal properties were studied: the μd-value of the finishing layer, the μd-value of the insulation layer, the thermal performance of the insulation layer and the moisture storage capacity of solid brick masonry. Finally, a prediction method of the hygrothermal performance of a building component is explored, which can be a useful tool to quickly assess the hygrothermal performance of a building component, without conducting advanced Heat, Air and Moisture simulations which might not be available.

From the research, it can be concluded that for the studied hygrothermal properties and boundary conditions, the moisture storage capacity of historic solid brick masonry has the most influence on the hygrothermal behaviour of a vapour-open, non-capillary active internally insulated historic solid brick masonry facade. The outcome of this research shows that a high moisture storage capacity of solid brick masonry has a lower risk on mould growth due to interstitial condensation at the interface between the solid brick masonry and insulation layer. Furthermore, the explored prediction method shows quite adequate predictions for the single variation of the parameters.