From experience, the author’s firm suggests placing special emphasis on accountability in this moisture management plan. Mass timber components can absorb water quickly if exposed to standing water. A discussion about responsibility while standing in a pool of water onsite is the likely result of a poorly developed plan. The plan should include assigned responsibility to specific people and automatic responses to events like high rainfall. It should be made clear failure to attend to moisture protection measures—both installation of passive elements and active responses to wetting events—will result in construction delays as wet wood is dried to safe moisture content levels, and possibly remedial work to remove staining or mould growth on mass timber elements meant to be exposed as an interior finish.
Conclusion
It is true there are many challenges to overcome as more tall mass timber buildings are constructed. Protection from moisture before and during construction, and then through the operational life of the building, is an important part of realizing the long-term benefits of a mass timber structure. The industry has good processes for recognizing and mitigating risk through building enclosure design and then co-ordination between design and construction teams. At a minimum, the author’s firm recommends the following steps for the building enclosure design and construction:
∞ enclose quickly – prefabrication of wall elements allows enclosure at the same speed as erection of
the structure;
∞ protect horizontal surfaces – methods should be chosen based on the type of mass timber and the risk of exposure during construction;
∞ design assemblies for long-term protection – use a rainscreen approach, continuous insulation (ci) to the exterior of the mass timber, and provide for drying; and
∞ develop a construction moisture management plan – many people will be involved; assign responsibilities to specific people and identify triggers for automatic response.
For further information, consult the CLT Design Guide published by FP Innovations, the NLT Canada Design and Construction Guide available at ThinkWood.com, and the forthcoming 2021 edition of the Technical Guide for the Design and Construction of Tall Wood Buildings in Canada. Visit the technical library of RDH Building Science (the author’s firm) for current research, case studies, and related presentations.
Author
Alex Lukachko is a principal at RDH Building Science, Inc., and a lecturer at the John H. Daniels Faculty of Architecture, Landscape, and Design at the University of Toronto. He works on the research and development of future building enclosure systems, and the design of high performance, low-carbon mass timber buildings. He can be reached at alex@rdh.com.
