Cross-Laminated Timber (CLT) is the trendy new building material that has received a lot of attention in the northwest this year. It is by definition a series of thick boards (usually 2×4 or 2×6) glued together in an alternating pattern and Cross-to form a solid wall, post or beam. (A similar concept is glue laminated (glue lam) timber that has been used for beam construction for many years.)
To understand how water affects wood construction, one must understand its function in a tree. Wood is designed to provide structural support for the tree, move water from roots to leaves and store the chemical results of photosynthesis (primarily sugars in the form of cellulose). These properties of wood create some challenges for its longevity when exposed to moisture. Thus, while wood can be structurally strong, it naturally absorbs and retains water.
Depending on climatic conditions, it can take some time for CLT panels to fully dry after getting wet. So, moisture control is of utmost importance when building from wood, such as cross-laminated timber.
The first source of moisture concern is during manufacturing. While manufacturers try to have all of their boards have the same moisture content, wood is an organic material and each board reacts to moisture differently. Some take water in more quickly and some take longer to dry. The result is that boards swell and shrink at differing rates which can affect the performance of a CLT panel.
Think of the hardwood floors in older buildings where the floor buckles in places. That is what happens when one board expands against another. As the wood dries again, it shrinks. This expansion and contraction process can damage the wood causing new cracks and gaps. On a floor, one can just refinish it. But, CLT is an integrated structural component and any deformity can weaken its life-safety purposes.
Another source of moisture is environmental moisture during construction. Studies indicate that construction moisture wetting is a serious problem in rainy climates like the Pacific Northwest. It can take several years for the wood to fully dry after construction. With rainfall likely in any month of the year, it is vital that proper precautions be taken to prevent the wood from getting and staying wet.
It is not just rainfall that is a concern. Other sources of moisture must also be accounted for, such as sprinklers, groundwater, pressure washing of siding, sidewalks, driveways, etc. decorative ponds, swimming pools, or other sources of moisture.
Wet CLT panels built with wrappings that do not allow vapor to pass through (both interior and exterior) are much more likely to develop bio-deterioration. There are vapor permeable wrapping on the market that can mitigate this concern to some extent.
Exterior cladding also plays a significant role in preventing moisture penetration. Not long ago, there were significant problems with wood frame buildings experience the negative effects of prolonged moisture, especially condominiums. The resulting lawsuits bankrupted many developers and contractors. With CLT, the stakes are even higher. With wood frame, the walls can be exposed, dried and cleaned. Since CLT is a structural system, that process is much more difficult and expensive.
In comparison, concrete, masonry and block all dry very quickly when wetted. Any remaining moisture can be removed easily (similar to using your shop-vac and a floor-squeegee in your garage after a big rain). Thus, construction moisture is of much less concern. And, there is no concern with rot with concrete, masonry and block construction.
One solution is to cover the CLT panels during construction until they are clad with the exterior components (be it siding, brick or stone). This is complicated, expensive and time consuming. Thus, covering panels during construction can undermine the presumed time and cost savings of CLT construction.
Another possible solution is to install water and air barrier membranes when the panels are manufactured. To date, no manufacturer is doing this, presumably because it would be cost prohibitive.
Certainly, more research needs to be done before the public can be assured of the safety and environmental impacts of cross-laminated timber panels used in construction. Also, new additional building code standards and construction practices would need to be developed to mitigate the health, life-safety and structural concerns posed by our wet climate on CLT buildings. Current building codes allow wood structures to be built up to five stories. The talk is some would like to go up to as many as 20 stories with CLT.
The risk of fire in a wood building has historically been the primary concern with high-rise wood construction, but, water and climatic conditions can have an equally and perhaps more insidious impact on the construction.
CLT may have the potential to be a sustainable building product that lowers the carbon impact of building. But so do proven products like concrete, steel and masonry.
While fire is an immediate and known risk, water’s impact is subtler, but no less threatening. This is particularly true in the Northwest from the Pacific Coast to the Cascades.