Hygrothermal Modelling of Sorption-Effects in Vapour Permeable Wood-Based Materials

Wood is an important building material in Finland, both historically and still at the present day. However, wood-based materials are vulnerable to moisture-induced damages, and the reliable hygrothermal behaviour of building envelopes with wooden materials near the exterior surface require careful design. Numerical modelling has been used for decades in research of moisture-safe buildings and structural solutions, but it is known for having several sources of uncertainty despite the continuous development of the modelling software. In this research, certain details regarding the hygrothermal modelling of wooden materials was studied with the goal of improving the accuracy and reliability of currently popular computational models.

The main topic of the thesis concerns the modelling of hygroscopic materials’ ability to store and release moisture due to water vapour sorption phenomena. Especially in Nordic climate, building envelopes are stressed by the seasonally changing temperature and moisture conditions of the environment. Wood-based materials and their moisture capacity are known to have intriguing characteristics, and it has been unclear how significant these are regarding the reliability of typically used simulation software, which are based on several simplifications. The work focused on studying vapour permeable wood-based materials such as low-density fibreboard, which is often used in Finland as the protective sheathing layer near the exterior surface of exterior walls. Previous studies have shown that the agreement between modelling results and hygrothermal test assembly measurements is often unsatisfactory especially in the interface between fibreboard sheathings and wall frames, which is typically also the critical point in the hygrothermal behaviour of exterior walls.

At first, dynamic sorption measurements by automated sorption balance equipment were conducted and analysed by using inverse problem techniques with the ultimate goal of implementing the effects of pore-scale local non-equilibrium in modelling of the fibreboard in building assemblies. However, this phenomenon was found to be relatively insignificant in modelling of building assemblies with fibreboard. More important factors were the temperature-dependence and hysteresis of sorption in wood. Applying these properties in modelling improved noticeably the agreement between the modelling and test assembly measurements. For studying these properties, a material test in temperature chamber was conducted and effective parameters related to hysteresis were determined by utilising measurements of previous laboratory experiments of assemblies.

Large number of new hygrothermal performance analyses based on long-term simulations in critical conditions were conducted for assemblies with fibreboard sheathings. Results of theses were compared in order to investigate how significantly the conclusions may change depending on whether the temperature-dependent and hysteretic effects are included or not in the model. This comparison resulted in a relieving conclusion, since it was seen that the typically used and more simplified model in general yields more critical evaluations for the hygrothermal performance of assemblies, which especially are stressed by severe moisture loads.

The properties obtained by experimenting the fibreboard were also applied in modelling of exterior walls with loose-fill wood shavings as thermal insulation. These computational studies contained also similar performance analyses for comparisons of different models, and analyses of exterior wall test assemblies with wood shavings insulation installed in test building. The results reinforced the earlier conclusions related to the criticality and reliability of different models.

According to the literature review, the temperature-dependency of wood’s moisture capacity is especially poorly known in cold temperatures. The temperature chamber experiment conducted in this work showed that the moisture capacity of wood seems to increase as the temperature decreases throughout the temperature range, where the conditions are gradually cooled from room temperature to very cold temperatures (-20 °C). This property may have essential role in the conditions inside structures, e.g., during spring, when the moisture possibly accumulated during winter starts to dry as the outdoor temperatures and amount of solar radiation increase. In order to advance the understanding of the hygrothermal behaviour of structures and accuracy of modelling, more research is obviously needed.