Dimensional Changes and Distortions in CLT Under Moisture Variations – With and Without Protective Tape : An Experimental Investigation on Moisture Behavior in CLT

Building materials play a key role in reducing carbon emissions from the construction sector. Cross-laminated timber (CLT) has been highlighted as a more sustainable alternative to traditional materials like concrete and steel. Skanska has applied this approach by constructing the new reception building at the New Central Hospital in Karlstad with an eight-story CLT structure, making it the first hospital building of its kind in Sweden. To protect the structure from moisture damage during construction, a full weather protection system is used. For smaller timber buildings, a diffusion-tight and moisture-resistant tape from Siga Swiss is applied to the timber elements instead. The aim of this study is to investigate how CLT is affected by variations of moisture compared to C24 construction timber, while also evaluating the effectiveness of the protective tape. The study is based on an experiment where sample pieces of both materials were exposed to water up to about one-third of their height and then dried. The samples were either unsealed, taped, or taped with a cut to simulate faulty installation. The unsealed samples were also protected at the end grain using silicone sealant. Throughout the test, the samples were measured, weighed, and photographed at different time intervals. The unsealed horizontal samples were also monitored using the ARAMIS optical 3D measurement system to analyze their deformation. The results show that CLT is more sensitive to moisture changes than C24 and that it undergoes greater deformation and dimensional changes. The tape provided some protection during short-term exposure but proved unreliable during longer contact with water, especially at the edges or when poorly applied. The study concludes that the tape should only be used for short-term protection, such as during transport. Therefore, the full-scale weather protection used by Skanska at the new reception building, is a well-justified and necessary solution for larger CLT buildings.