Abstract
In historic masonry buildings, wood can be embedded in the walls as storey partition beams, or as supportive wall plates. Half-timbered masonry constructions, or wooden frames, e.g. combined with internal insulation, are other examples of wooden elements. Wood decaying fungi can cause serious damage to wood, which may lose mass and strength, ultimately yielding the risk of collapse. In addition, some fungal species may even be hazardous for occupants. All wood decaying fungi depend on favorable moisture and temperature conditions, although the threshold conditions may vary with various fungal species and types, and state of the wood. To predict the risk of occurrence of wood rot, several models have been developed, however most of these are based on a limited number of experiments, or very specific cases. For these reasons, the applicability of the models to other scenarios (fungal species, wood species) may not be appropriate. Furthermore, another failure mode for wood and moisture, is mold growth, which is initiated at lower moisture levels. An indication of risk of mold growth would indicate problems or risks before the initiation of wood rot. Mold growth does not deteriorate the wood, but is usually equally undesired due to health concerns of occupants. For this reason, there might be places where some mold growth would be acceptable, e.g. embedded beam ends if there is no transfer of air from the moldy area to the indoor air. Therefore, risk of rot could be important. The paper investigates models for mass loss due to wood decay and mold growth based on exposure time to favorable hygrothermal conditions. The investigation is based on inspection of wood samples (wall plates) from a full-scale experimental setup of masonry with embedded wood and monitored conditions, to which the prediction models will be applied. Monitored hygrothermal conditions were implemented in mold and wood decay models, and samples were removed from the test setup. The implemented models yielded high mold index and mass loss, whereas neither mold nor decay was observed in the physical samples. Results indicate that the implemented models, in these cases appear to overestimate the risks of mold and rot in the supportive lath behind the insulation.
Highlights
In older masonry buildings, wood is often embedded in the walls as storey partitioning beams, supportive wall plates, or wooden frames in half-timbered masonry
In-situ measurements of hygrothermal conditions in wall plates were implemented in prediction models for mold and decay, and using a destructive method, wood samples were removed from the test setup and studied with regard to the predicted risks
That the moisture content has an upper limit in this case, due to the limits of the sorption isotherm as was implemented [15], the magnitude of the values are compatible with on-site measurements
Summary
Wood is often embedded in the walls as storey partitioning beams, supportive wall plates, or wooden frames in half-timbered masonry. Biological degradation of wood is a naturally occurring process of fungal growth given favorable conditions of both e.g., moisture and temperature. Most wood decaying fungi have optimum conditions at 20-30°C, and a wood moisture content of 2030 % (wt.). Some species are able to decay wood at lower moisture levels than 20 % (wt.) and still other species prefer much higher moisture contents for optimal growth. There are more than 30 known wood decaying fungi species as may be found in buildings [3], and they have different preferences and optimal growth conditions. Other fungi can cause serious damage within short time frames given optimal conditions. The optimal moisture conditions for fungal growth depends on other factors, such as the time of wetness, temperature and the type and condition of the wood. Attacked wood is more prone to new attacks
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