The climate crisis is widely regarded as one of the biggest challenges of our century, driving the European Union to adopt ambitious climate action policies targeting rapid transitions across all sectors, particularly resource-intensive ones like the building industry. The building sector accounts for 40% of Europe’s energy consumption and 36% of its greenhouse gas emissions, making it a critical sector for achieving the EU’s goal of net-zero emissions by 2050. Despite its potential for improvement, significant amounts of European buildings remain energy inefficient, and only 1% undergo renovations each year. These inefficient buildings consume over 270 kWh/m² annually, suggesting that effective retrofitting could significantly reduce energy use. Insulating external walls alone has the potential to achieve energy savings of up to 60%.

Historic buildings, however, present unique challenges due to the need to preserve their cultural, historical, and aesthetic significance. Internal insulation is often the only viable solution for such structures but brings with it complex hygrothermal risks. Insulating solid masonry walls internally reduces heat flow, lowers the wall’s temperature, and increases the likelihood of interstitial condensation and consequently frost damage, wood decay and mould growth. Moreover, the use of vapour-permeable, capillary-active insulation materials introduces additional complexities, as their ability to absorb and redistribute moisture can result in both benefits and risks depending on the interior and exterior boundary conditions.

The aim of this research is to gain insight into the most influential factors affecting the hygrothermal safety of embedded wooden beams in solid brick masonry with interior capillary active insulation. Through literature review, background information on the subject is provided. Creating an understanding of the hygrothermal changes to the solid brick masonry after the application of interior capillary active insulation and associated hygrothermal risks of the wall assembly, with regard to the biological decay of embedded wooden beams. To find the most influential factors on hygrothermal performance, methods to simulate this performance are assessed and compared and possible high influential material properties, such as temperature and moisture, further researched.

The influence of thermal conductivity, water vapour diffusion resistance and moisture retention of the insulation, as well as the water vapour diffusion resistance and moisture retention of brick on the hygrothermal performance is assessed with a Sobol sensitivity analysis. This sensitivity analysis indicates the most influential material properties, and is based on 1536 simulations using Delphin hygrothermal simulation software. Python scripting is used to create the simulation and material files, control the Delphin software and analyse and assess the generated results. To create a better understanding of these results and assess the hygrothermal safety, wood decay potential is calculated and visualised using the post processor of Delphin.

From this research, it can be concluded that the exterior boundary conditions, such as rain load, solar radiation and orientation, are more dominant on the hygrothermal performance of the geometry in comparison with the material properties. When solely analysing material properties, the sensitivity analysis concludes that the moisture retention of brick is the primary influence on hygrothermal performance. The moisture retention of insulation is of secondary importance within the research boundary conditions. Furthermore, the impact of capillary active insulation is studied by using wood decay assessments. Revealing positive contributes to the prevention of wood decay. However, incapable of reducing moisture content to levels that prevent wood decay completely.