Developing a construction moisture management plan
The plan to manage exposure to moisture during construction will ultimately be the responsibility of the constructor. However, as discussed above, the plan to protect mass timber components (e.g. using a prefabricated enclosure to quickly enclose the structure) does inform design decisions and should impact construction sequence planning, both at a high level (time of year) and in sequencing trades onsite. The author’s firm suggests the creation of a draft plan early in the detailed design process to help establish requirements that will later become part of the general conditions for the project. At a minimum, the draft moisture management plan should include:
- identifying moisture risks during manufacturing, transportation, storage, and construction, with a statement of impact for each construction schedule and long-term performance of the building.
- specifying measures to mitigate these risks, including identification of parties responsible for each.
- considering both passive (for example, factory-applied coatings, or tenting the structure) and active measures (such as a protocol for removal of ponding water).
- processing for documentation and reporting.
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 mold 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 in place for recognizing and mitigating risk through building enclosure design and then coordination between design and construction teams. At a minimum, the author’s firm recommends the following four steps be taken 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. (Much more information about building enclosure design for mass timber buildings
can be found in the reference provided in the following paragraph.) - 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 2022 Technical Guide for the Design and Construction of Tall Wood Buildings in Canada.1 Visit the technical library of RDH Building Science for current research, case studies, and related presentations.2
Notes
1 The CLT Design Guide can be accessed at web.fpinnovations.ca/clt. The NLT Canada Design and Construction Guide is available at www.thinkwood.com/mass-timber/nlt, and the Technical Guide for the Design and Construction of Tall Wood Buildings in Canada can be found on https://web.fpinnovations.ca/tallwood/.
2 Visit www.rdh.com/technical-library.
Authors
Alex Lukachko is an assistant professor at the John H. Daniels Faculty of Architecture, Landscape, and Design at the University of Toronto. At the time of writing, he was also a principal at RDH Building Science Inc. 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.lukachko@daniels.utoronto.ca.
Graham Finch is a principal at RDH Building Science Inc., based in Victoria, British Columbia. He has led the firm’s building enclosure consulting services on many larger mass timber buildings in North America. Finch leads research related to mass timber moisture management, and has co-authored many industry best practice guides on building enclosures for mass timber buildings. He can be reached at gfinch@rdh.com.
