Robust solutions for internal retrofitting solid masonry walls in historic buildings with regards to hygrothermal performance

The built environment is estimated to be responsible for nearly 40% of the world’s total energy usage, and 30-40% of the emissions. Approximately 40% of the European building stock was constructed before 1960, and these buildings often have large heat losses through the building envelope and perhaps also thermal comfort issues.

A large share of the aforementioned buildings have an exterior appearance worthy of preservation due to their historic or aesthetic values which prevents the use of renovation measures that could compromise these values. In terms of retrofitting the façade walls, external insulation is often prohibited leaving internal insulation as the only remaining option.

However, internal thermal retrofitting solid masonry walls is generally regarded as risky since the added insulation causes the existing wall structure to become colder and wetter, which increases the risk of interstitial condensation. In addition, the added insulation reduces vapour diffusion drying to the room side.

These factors contribute to elevated moisture levels in the wall structure and an increased risk of moisture induced damage. Consequently, there exists a need for defining robust methodologies for the installation of internal insulation of historic solid masonry walls, which do not cause hazardous fungal growth or other undesirable issues e.g. decay of embedded wooden elements.

The purpose of this study was, in a large field experiment, to investigate several either diffusion-open or diffusion-tight insulation systems to determine if it is possible to internally retrofit solid masonry walls without causing hazardous fungal growth. The investigated diffusion-open insulation systems were: polyurethane with calcium silicate channels, monolithic calcium silicate, lightweight autoclaved aerated concrete, and lime-cork based insulating plaster.

The diffusion-tight systems were: phenolic resin foam, and traditional mineral wool with a vapour barrier. These systems were examined in combination with exterior hydrophobisation. The field study findings were complemented by calibrated numeric simulations performed to investigate additional parameters and the robustness against changing climate conditions in the future.

The field experiment and the laboratory tests also investigated if controlling the immediate surroundings would prevent the occurrence of fungal growth and the risk of fungal growth affecting the indoor climate. Lastly, several mathematical models for prediction of mould growth and wood decay were assessed and compared to on-site microbiological tests.

The field study showed high relative humidity levels in the internally insulated solid masonry walls without exterior hydrophobisation in the case of high indoor moisture load during winter. In addition, the diffusion tightness of the insulation was found to be of considerable importance in terms of the hygrothermal performance.

Without exterior hydrophobisation the diffusion-open insulation systems were found to perform slightly better than the diffusion-tight systems. However, in the case of test walls with exterior hydrophobisation the diffusion-tight systems performed considerably better. The material and microbiological tests in the field experiment and the laboratory study showed no fungal growth in the interface between the masonry wall and insulation in the case of high alkalinity (pH>12) in the surrounding materials i.e. the internal render and adhesive glue mortar.

This suggests that if high alkalinity could be maintained then fungal growth could probably be prevented despite the presence of unacceptably high moisture levels. In systems where the alkalinity declined rapidly it was found to be of considerable importance to ensure that no organic additives or elements were present in the critical locations.

Furthermore, diffusion tests with VOCs mimicking Microbial Volatile Organic Compounds produced by fungal growth showed that in the case of growth behind the insulation the VOCs were able to penetrate most of the examined systems and potentially affecting the indoor air quality negatively. The rate of diffusion through the systems was found to be relatively large for the highly water vapour diffusion-open insulation systems, especially in the case of highly volatile VOCs such as acetone or ethanol.

The mould growth and wood decay prediction models were found to overestimate the risk, and for the mould growth predictions this was probably due to the high alkalinity and insufficient access to nutrition in the critical locations.