Hygrothermal Performance of Timber-Framed External Walls in Finnish Climatic Conditions: A Method for Determining the Sufficient Water Vapour Resistance of the Interior Lining of a Wall Assembly

TLDR: In this article, the authors examined the effects of the diffusion of water vapour in wall assemblies in Finnish climatic conditions by examining the effect of different properties of the assembly layers on the moisture performance of the wall, followed by the setting of minimum values for the water vapor resistance ratio between the interior and exterior linings of different wall types based on selected criteria and study conditions.

Abstract: This study looked into the moisture performance of timber-framed external wall assemblies in Finnish climatic conditions by examining the effects of the diffusion of water vapour in wall assemblies. The study has been preceded by decades of discussion about the need of a vapour barrier in the interior wall lining of external wall assemblies. Much research has been conducted related to this subject worldwide in the last 70 years, and studies have shown that in Nordic climatic conditions the interior wall lining always requires sufficient water vapour resistance against the diffusion of the water vapour emitting from indoor air. On the other hand, it has also been found that external wall assemblies can be implemented without incorporating a tight vapour barrier into the interior wall lining. Furthermore, in more southern climatic conditions the interior wall lining may have to be permeable to water vapour when the direction of the diffusion flow is from the outside in. Yet, despite numerous studies, agreement has not been reached on the minimum water vapour resistance that should be required of the interior wall lining of a timber-framed wall assembly under different conditions. Consequently, different guidelines and regulations for water vapour resistance values of interior wall linings exist also in countries located in highly similar climatic conditions. Comprehensive assessment of the moisture performance of wall assemblies requires establishing the performance criteria and limit values that an assembly must meet, the outdoor and indoor conditions to be used in designing moisture performance, and the assembly solution and used materials as well as their building physical properties. The acceptability of the moisture performance of an external wall assembly depends essentially on all these factors during the service life of a building. The aim of this study has been firstly to create a method which allows examining the moisture performance of timber-framed external wall assemblies in different situations. The first phase involved setting the performance criteria, limit values and reference boundary conditions for analysing wall assemblies under Finnish climatic conditions. Then, the effect of the different properties of the assembly layers on the moisture performance of the wall was examined followed by the setting of minimum values for the water vapour resistance ratio between the interior and exterior linings of different wall types based on selected criteria and study conditions. An attempt has been made to ensure the reliability of the developed analysis method by conducting different laboratory and field tests and calculational analyses in connection with the study. External wall assemblies were examined in the laboratory with building physical research equipment developed during this study, the key building physical properties of the materials used in assembly tests were determined for calculational analyses, external wall assemblies were also analysed in field conditions in a one-family house and in test houses at the test field of Tampere University of Technology, indoor air excess moisture was analysed in field tests of timber-framed one-family and row houses, and the performance of the used heat, air and moisture transfer simulation model (HAM model) was verified by tests conducted in various situations. Moisture condensation and mould growth within were chosen as the performance criteria of the external wall assembly. Limit values were selected according to the following principle: