Hygrothermal performance of a light weight timber wall assembly with an exterior air barrier

Examensarbete för masterexamen
Master Thesis
Structural engineering and building technology (MPSEB), MSc
Watt, Duncan
Sjöberg, Staffan
In Norway, an emerging method in ensuring the air and vapour tightness of light weight timber framed walls, positions a sealed wind barrier at the exterior and a sealed vapour barrier to the interior. It has been found that the exterior layer alone is often sufficient in ensuring an acceptable level of air tightness. In terms of the overall airtightness requirements set on the building, it is thought that the interior vapour barrier could be left unsealed as the wall possesses a certain degree of redundancy. It is then important to assess whether the durability of the envelope is compromised due to moisture ingress from the interior driven by convection and diffusion. This project investigates the effect of convection on moisture accumulation, and mould growth potential, in a light-weight timber frame wall system where the air barrier is situated at the exterior and the unsealed vapour retarder at the interior. A two dimensional numerical HAM (Heat, Air and Moisture) model is used. The model is constructed and verified in COMSOL Multiphysics to evaluate the hygrothermal behaviour of a light-weight timber frame wall system with the previously described attributes. The studied wall is subject to climate conditions representative of the Gothenburg region of Sweden over one year. The light-weight timber wall system assessed is comprised of bitumen impregnated soft fibreboard with a treated exterior surface as the exterior air barrier, mineral wool as the cavity insulation and orientated strand board type 3 (OSB/3) as the interior vapour retarder. The unsealed nature of the interior vapour retarder is represented by the introduction of two 1 mm gaps in the layer. As means of comparison a wall assembly possessing a sealed interior OSB layer is simulated separately (no gaps). The mould growth potential of the wall system is assessed and compared utilizing the data output from the numerical model simulations and a material specific mould growth index. The results suggest that the joints of the interior vapour retarder need not be sealed considering the exterior air barrier, wall geometry, material properties and imposed boundary conditions. While the degree of moisture accumulation is comparatively larger behind the exterior air-tight layer of the simulated wall assembly possessing an unsealed interior OSB layer, especially at the top of the wall section, the influence on mould growth potential is limited. This is because the disparities in moisture accumulation between the two wall assemblies is only apparent during the winter months when low exterior temperatures prevent any kind of mould growth regardless of critical humidity values.
Building Futures , Byggnadsteknik , Building Futures , Building engineering
Arkitekt (konstruktör)
Geografisk plats
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Teknik / material