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MEMBER Wood Products;

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MEMBER Forest Operations; Pulp, Paper and Bioproducts;

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Summary

Fifteen structural composite lumber (SCL) products including laminated-veneer lumber (LVL), laminated strand lumber (LSL), oriented strand lumber (OSL), and parallel strand lumber (PSL) provided by Boise Cascade, LP, West Fraser, and Weyerhaeuser were tested for moisture-related properties in this study, also covering four reference materials: 16-mm Oriented Strand Board (OSB), 19-mm Canadian Softwood Plywood (plywood), 38-mm Douglas-fir and lodgepole pine solid wood. Water absorption, vapour permeance, vapour sorption, and dimensional stability were measured with limited replication by following relevant standards for a purpose of assisting in improving building design and construction, such as hygrothermal modelling of building envelope assemblies, design for vertical differential movement, and on-site moisture management. The preliminary testing found that the water absorption coefficients in the thickness direction of these fifteen SCL products were all lower than that of the reference 19-mm plywood based on the short-term partial immersion test following the ISO standard 15148 (ISO 2002). Their water absorption coefficients in the longitudinal direction were in general about 10 times higher than their water absorption coefficients in the thickness direction. Two LVL products showed lower water absorption than the lodgepole pine or even the Douglas fir solid wood reference specimens. Vapour permeance of these 19 products were tested, using both the desiccant method (“dry cup”) and the water method (“wet cup”) as provided in ASTM Standard E96 (ASTM 2010). Except for the 19-mm plywood and the 38-mm thick lodgepole pine solid wood references, the other 17 products all had a vapour permeance below 60 ng/(Pa•s•m2) based on the dry cup test. The 15 SCL products covered in this study each had a vapour permeance lower than that of the 16-mm OSB reference specimens. The vapour permeance measured using the wet cup test is in general several times greater than when measured using the dry cup test. In terms of dimensional stability in the thickness direction, the seven LVL products tested tended to have a shrinkage amount close to or slightly higher than those of the 19-mm plywood and the two solid wood references (in the radial direction); by comparison, the LSL and PSL products had in general a slightly higher shrinkage amount, being closer to that of OSB. In the width direction, the seven LSL products generally showed lower shrinkage amounts than LVL and PSL products, being higher than the 16-mm OSB and the 19-mm plywood references and lower than lodgepole pine and Douglas fir solid wood references (in the tangential direction). This study provided a number of implications related to moisture protection. First, construction moisture mitigation measures appropriate for solid wood elements may need to be adapted to account for different water absorption characteristics of SCL. Not surprisingly, all end grain is much more absorptive, requiring better moisture protection, compared to their wider surfaces. Second, SCL member used in a building envelope assembly should be treated as a vapour barrier. Adding other materials with low vapour permeance may eliminate the drying capacity and should be avoided.

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