Example of CLT used for aesthetics.

How to Protect CLT in Wet Climates

Cross-laminated timber (CLT), a mass timber product, is becoming increasingly common as a building material. Its ability to sequester carbon helps with controlling global warming while its structural capacities rival steel-reinforced concrete. At the same time, CLT is sensitive to impacts of moisture during construction, requiring specific measures to protect it from damage.

Moisture content of dry mass timber materials should be limited to 16% instead of the recommended 19% for dimensional lumber products.

AIA Seattle, Mass Timber Committee (2022)

In the Pacific Northwest, the rainy climate can lead to high levels of moisture present in unprotected building materials. Dimensional lumber can dry out with heat, fans and dehumidification measures. But the same methods used for CLT yield different results due to the high levels of glue used in CLT and its thickness. The panel’s interior can still retain high levels of moisture, leading to mold growth, decay, and expansion. This is why it is necessary to limit the amount rain that CLT is exposed to before building dry-in.

CLT Moisture Management Plan

One of 4EA’s approaches is to Find A Solution. Creating a CLT moisture management plan solves the issue of coordinating an effective strategy that sets realistic expectations and ensures the design and construction teams have the same understandings. Your plan should include the following:

  1. The methods of protection from delivery until the enclosure is complete.
  2. Criteria for removing water during and after rain events.
  3. Assigning which team member will be taking the moisture readings, when the readings will be taken, what instruments will be used to measure the moisture content, how the data will be recorded, and the parties this data will be shared with.
  4. Defining acceptable moisture levels at varying depths of CLT panels prior to application of finishes or cementitious toppings.
  5. Establishing cleaning methods and the required level of final finish. These should include specifications for cleaning agents, sealers, and other materials to be applied to the CLT.

A Rapidly Evolving Material

Cross-laminated timber offers great benefits to a project, including carbon capture and storage, improved aesthetics, and reduction of construction time. However, successful projects are not without challenges and require careful planning by well-integrated teams of thoughtful designers, consultants, and contractors. Mass timber construction materials and practices are rapidly evolving and as such, staying up to date on the latest guidance and advice on how to use these successfully is a must.

“Mass Timber Moisture Protection: Considerations for CLT Construction” highlights best practices for handling CLT. The reader will find guidelines for creating a moisture management plan and a table of approaches to providing field protection of installed CLT. Read the full paper here.

Jeff Speert, Principal and Director of 4EA’s Seattle office, recently led the Mass Timber Committee of AIA Seattle in an update to a guideline outlining moisture protection considerations for CLT construction. His passion in designing with environmentally sensitive building materials has led Jeff to become a subject matter expert in the area of mass timber construction.

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