Impact of built-in moisture on the design of hygrothermally safe cross-laminated timber external walls: A stochastic approach

In terms of hygrothermal performance, solid wood panels composed of cross-laminated timber (CLT) are sen-sitive to moisture. However, to the best of our knowledge, no clear indications of critical initial moisture con-ditions for CLT envelopes regarding the dry-out of built-in moisture have been provided. Therefore, our main objective was to set critical limit values for CLT external wall design in terms of their initial moisture content (built-in moisture) and dry-out capacity (vapour resistance of wall layers) using a stochastic approach. The focus is on five types of CLT external walls that differ in their dry-out capacity. The key factors in the hygrothermally safe design of the CLT external envelope are sufficient dry-out capacity and control of the CLT initial moisture content level during the construction phase. The results of our stochastic analysis confirmed that a high initial moisture content in CLT causes a high mould growth risk when CLT is covered with vapour tight layers. Therefore, the limit values for critical initial moisture content of CLT and the water vapour resistance of the wall layers that ensure dry-out capacity of different external walls to prevent the risk of mould growth on the CLT surface in a cold and humid climate were determined. These limit values take into consideration the weather conditions during the construction phase and classes of material sensitivity to mould growth.