Designing highly insulated cross-laminated timber external walls in terms of hygrothermal performance: Field measurements and simulations

Abstract

Keeping wood dry both under construction and service is important to prevent possible moisture damages in wooden buildings. However, practice has shown that without weather protection, wooden structures may get soaked during construction. Therefore, our aim was to evaluate the impact of vapour permeable and low permeable highly insulated external wall assemblies, façade orientation, and initial moisture content on the hygrothermal performance of cross-laminated timber (CLT) external walls. We carried out field measurements in the cold and humid Estonian climate, for which five types of test walls were designed and manufactured. Simulation models for future research were also created and validated. Field measurements showed that the CLT external walls were hygrothermally safe in the observed cold and humid climate and the CLT panel provided sufficient control of the water vapour diffusion. The use of low vapour permeable external insulation can be beneficial against vapour permeable insulation if low moisture content of the CLT panel (factory dry) during construction is maintained and insulation is installed airtightly. Cellulose insulation can be a safe choice when selecting water vapour permeable insulation for the external wall in case the construction is planned without weather protection. Mould growth risk calculations based on measured hygrothermal conditions showed that the high moisture content in the thin surface layer of the CLT panel may not be critical when dry-out time is short. The probability of failure must be determined by stochastic analysis using a variety of influencing parameters.