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Summary

Practical solutions are needed for on-site moisture management of mass timber construction. Six groups of cross-laminated timber (CLT) specimens, together with reference specimens including plywood, OSB, and nail-laminated timber were assessed for their wetting and drying behaviour. The focus of this study was to assess the effectiveness of water repellents and membranes installed on CLT in preventing the wetting that can be caused by, for example, rain during outdoor exposure, installation of wet concrete topping, or sitting on a damp concrete slab. Seven water repellent products covering a range of formulations and three membranes including a self-adhered vapour-permeable membrane, a self-adhered vapour-impermeable membrane, and a lumber wrap were assessed as potential temporary moisture protection measures. Three groups of CLT specimens were exposed to the rainy wintertime weather in the rear yard of FPInnovations’ Vancouver laboratory, from late October 2017 to the end of January 2018. This work tracked the wetting and drying behaviour of horizontal wide surfaces with a butt joint covered with plywood spline or without any joint; and horizontally exposed CLT edges. Each exposed test surface was pre-treated with a water repellent or covered with a membrane. Another three groups were set up in a shed, simulating sheltered exterior conditions, also typical of new construction. One group was used to assess the impact of pouring wet light-weight concrete topping on CLT floors. One group was set up to assess moisture uptake through an edge of CLT, which sat on damp concrete. The other group of CLT specimens was initially wetted through exposure to weather for one month, and then installed with one layer (12 mm) or three layers of drywall, or rigid mineral wool insulation (38 mm thick) to assess the impact of a potential fire safety measure on drying. This study confirmed that moisture was generally slow to penetrate into the wide surface of CLT; by comparison, CLT edges (with exposed end grain) are much more susceptible to moisture absorption and penetration and are therefore a higher priority for moisture protection. Wetted CLT dried slowly under the damp and cold conditions of the winter months. Regarding the effectiveness of various treatments, when a horizontal wide surface of CLT without joint was exposed to intense rain for three months, none of the six water repellents tested was found to considerably reduce water absorption or penetration. The lumber wrap tested was slightly better than the water repellants studied. However, the two self-adhered membrane products (vapour-permeable, vapour-impermeable) assessed were able to effectively prevent wetting by liquid water. When a horizontal surface of CLT with a butt joint covered with plywood spline was exposed outdoors for three months, not only was the plywood soaked with water, moisture also penetrated deeply into the CLT mostly through exposed end grain at the joint. The self-adhered vapour permeable membrane tested showed good effectiveness in preventing wetting. It is expected that a self-adhered vapour-impermeable membrane would provide similar or better protection, if the assembly needs to be vapour impermeable on the top, such as a conventional low-slope roof. When a CLT edge was directly exposed to intense rain for three months, an end grain sealer was found to be the most effective among the three water repellents assessed to prevent wetting. For more severe wetting conditions, it is expected that a self-adhered vapour-permeable membrane would be best to prevent water penetration from exposed end grain but also to maintain the drying capacity. When uncured wet concrete topping (a product of self-leveling floor screed) was placed on CLT, negligible moisture from the concrete penetrated in the CLT below. Although the two types of membranes tested reduced moisture interaction between the wood and the wet concrete, the most practical protection is probably to keep the wood reasonably dry (e.g., with a moisture content of CLT below 16%) by sheltering it before and after installing concrete topping to minimize the moisture risk. When an edge of untreated CLT sat on damp concrete for three months, the moisture uptake from the concrete caused considerable wetting. A damp-proof treatment is needed to reduce the moisture risk. Among the two water repellents tested, the end grain sealer kept the moisture content at two measurement locations largely below 15%. Regarding the impact of a fire-proof covering on the drying performance of CLT, it was found that 25 mm thick mineral wool insulation did not appear to impede drying, while one layer of drywall reduced the drying rates by about 20%, and three layers of drywall reduced the drying rates by about 50%, compared to the CLT control specimen without any surface covering. However, only 40% of the moisture gained during the initial wetting period evaporated from the uncovered CLT control specimen over a period of two months under sheltered conditions. This suggests that natural drying cannot be relied upon and that the wood should be kept dry before and after being covered with other materials, particularly in the winter in coastal areas. Implications for moisture protection practices based on the test were summarized at the end of this report.

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