In practice, therefore, we want to keep mass timber relatively dry. Components are expected to arrive at the building site at less than 12 percent moisture content. The goal then is to limit moisture content to below 16 percent until the building is fully enclosed and the environmental control systems are operating. Designers are encouraged to use these limits as general guides, but it is important to note each species of wood—and each type of engineered wood component—has a different response to moisture. Additionally, the actual moisture content should be expected to change over time in response to changing environmental conditions (See the sidebar below “Changing Moisture Conditions for Mass Timber Elements from Construction to Occupancy”).
| Changing Moisture Conditions for Mass Timber Elements from Construction to Occupancy |
| Figure 1 (page 24) plots the equilibrium moisture content (m.c.) of a mass timber element against the relative humidity (RH) of the surrounding environment. Overlaid on this plot is a storyline showing the movement of wood through different moisture regimes during construction and operation. Dot #1 is the approximate starting point: perhaps 12 percent moisture content as the component leaves the manufacturer. As the element is transported to the construction site, outdoor conditions (in the blue range) are at a higher RH. With weeks of exposure, the mass timber element’s moisture content moves higher, maybe as high as 20 percent m.c. between dots #2 and #3. Construction is completed, the building closed in, and operating conditions in the building are established (the light orange range). The mass timber moisture content moves downward as it comes into equilibrium with the interior conditions, at dot #4. Afterward, during normal operation, seasonal changes in RH will move the mass timber m.c. upward and downward within the operating range, between dots #4 and #5. All of this is normal and will occur without moisture-related problems. In fact, the response of wood to the environmental conditions capacity represents an ability to accommodate changes—a built-in safety—as long as the conditions do not range too high or too low.
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| Figure 1 Curtain wall (left) and insulated light metal frame wall (right) concept enclosures. Images © RDH Building Science Inc |
Mass timber structures are subject to moisture in the following ways:
- Before or during construction:
◦ manufacturing◦ wetting during storage◦ wetting during transportation◦ after erection of the structure and
before enclosure
- During operation:
◦ bulk water leakage through the enclosure (a significant design consideration for tall buildings with higher wind-driven rain loads)
◦ accidents (plumbing, sprinklers)
◦ changes to interior environmental conditions (temperature, humidity) and potential accumulation of moisture via air leakage or vapor diffusion.
Most of the sources listed above can be controlled through careful design of the building enclosure to minimize bulk water leakage, avoid accumulation by air leakage or vapor diffusion, and allow for some drying to the interior or exterior as environmental conditions allow. The period of time the structure is unenclosed during construction is the highest risk of moisture exposure for the structure. Although tall buildings present a smaller horizontal “footprint” to rain, wind-driven rain has access to a greater number of floors and, given the longer construction time for tall structures, the exposure may be significant until the building enclosure is in place.To address the concerns above, the author’s firm has the following recommendations for tall mass timber buildings.
Enclose the structure quickly
Mass timber structures assemble and are loadbearing more quickly than those that are concrete-framed—similar in speed to steel. Speed of building enclosure construction is, therefore, a crucial factor in the protection of wood from moisture, reducing the amount of time structural elements are exposed to the weather. To fully protect the mass timber elements, construction of exterior wall and roof elements should be planned to enclose each floor level quickly as the erection of the primary structure proceeds. This can be done with temporary protection (scaffolding and tarping, for example) but the costs may be significant. Prefabrication of wall components is recommended to meet this “fast enclosure” objective.
Unitized curtain wall or insulated precast concrete systems are good choices with robust technical details, good design support, and a mature procurement process. These systems, however, may not easily meet increasingly stringent project requirements for low operating and embodied carbon. Other options, such as light steel frame, light wood frame, or cross-laminated timber (CLT) panels (Figure 1), are possible, but may require additional design work and planning, including:
- detailing of joints and interfaces with other enclosure systems.
- coordination of multiple suppliers offsite (frame, insulation, cladding, glazing systems).
- some level of work onsite to complete critical seals and finishes.
| Figure 2 Installation process (left) and installed wall panels (right) in an 18-story mass timber building. Photos © RDH Building Science Inc. |
The challenges listed above have been successfully addressed for several tall mass timber projects in North America. For example, Figure 2 shows the installation of large frame panels with windows and cladding pre-installed offsite. No finishing work was required on the exterior of this building, but final air and water seals, additional thermal insulation, and interior finishes were installed from the interior after panels created a watertight exterior enclosure.
