Moisture risks with CLT-panels subjected to outdoor climate during construction : focus on mould and wetting processes

Abstract: When going through relevant research, moisture safety guidelines and talking to builders, moisture experts and architects it is clear - and not surprising - that water and wood make no easy combination. The experiences from building with cross laminated timber (CLT) differ from building sites and there are good and bad examples building without weather protection. In this study the moisture influence on CLT is analyzed. CLT is a type of massive wood with glued lamellas, increasing usage worldwide as structural elements in buildings. The bulk of the work is performed in the hygrothermal calculation tool WUFI(™). Focus is on the wetting process and the evaluation of mould risk from rain loads during production in Nordic climates. Subsequent drying after built into walls and floors is also evaluated. A vast literature survey is performed in order to compare and select material data for modelling CLT. Following the simulation work, moisture content, mould growth and volumetric distortion are judged both with and without weather protection. Results are also compared to measurement data from field tests. It is found that short building times are crucial, some weather protection is required all year around and early planning and constructing for moisture safety are crucial. The benefits of prefabrication and short building times using CLT should be exploited. If there is a risk of rainfall exceeding 10-20 mm, arrangements to divert rain loads should be undertaken. If the expected rain loads are above 40 mm or if the building time exceeds 2 weeks, a roof cover will be required. At air humidities averaging 80% and yearly rain exceeding 1200 mm, a complete building cover is recommended. A controlled environment may be expensive, but it speeds up production and shortens drying time.