The impact of wooden studs on the moisture risk of timber frame constructions

Since timber frame constructions can help to reduce CO2-emissions and lower the embodied energy of buildings, the market share of timber-based buildings is growing across Europe. Unfortunately, timber frame constructions are found to be susceptible to moisture damage, such as interstitial condensation, mould growth and wood rot. To avoid moisture damage, a correct design of the wall composition is crucial, with special emphasis on the ratio between vapour resistance of wind and vapour barrier. Given that experimental investigations are time-consuming and expensive, numerical tools are common to assess the hygrothermal behaviour of building components. And although timber frame constructions are inherently two- or even three-dimensional due to the embedded wooden elements, most often, 1D-simulations focussing on the basic configuration with insulation between wind and vapour barrier are conducted. This paper investigates to what extent neglecting the embedded wooden elements influences the risk assessment of the wall. Three different wall configurations have been considered and their hygrothermal response, as predicted by 1D- and 2D-numerical simulations, are compared. Variability of the exterior climate is included by using four distinct different climate regions. Contrary to common assumptions, buffering of moisture in wooden elements does not always lower the risk on moisture damage, but might even increase it. While the predicted risk on mould growth was found to be similar between 1D and 2D-simulations, the opposite was found for the risk on interstitial condensation. Mainly for cold climates and wall configurations with hardly any other hygric buffering capacity, levels of interstitial condensation were found to be significantly higher when taking the wooden elements into account in the numerical simulations. Hence, care should be taken when assessing the reliability of timber frame walls based on 1D-simulations only.