However, concrete doesn’t have the same advantage when it comes to tensile strength. In building construction, rebar, or reinforcing steel bars, provides the tensile strength lacking in concrete. Concrete has a very low coefficient of thermal expansion, and as it matures, concrete shrinks. All concrete structures will crack to some extent, due to shrinkage and tension.

Wood’s strength is dependent on loading direction—it is strongest in tension along the fibers and weakest in radial and tangential directions. When loaded longitudinally along the grain, wood can have a strength-to-weight ratio advantage relative to steel of 2:1. However, when wood is loaded in other directions, including radial and tangential to the grain, this advantage disappears. Wood’s psi varies among species: western red cedar may have a psi of 7,500, Douglas-fir a psi of 12,400, and mahogany a psi of 25,400.

For decades, the wood industry has been evolving high-strength products in the form of engineered wood—plywood, oriented strand board, glulam beams, I-joists, and laminated veneer lumber, to name a few examples. Generally stronger than traditional lumber, engineered wood is often made from (among other things) chips, particles, fibers and wood from small-diameter trees not suitable for lumber—which is part of the reason the wood industry is able to utilize more than 99 percent of every tree harvested and brought to a mill.²

One innovative engineered wood product is CLT, a material widely used in Europe that is poised to significantly increase the possibilities for North American wood buildings. CLT is comprised of boards stacked together at right angles and glued over their entire surface, creating a product that retains its static strength and shape, and allows the transfer of loads on all sides. Besides being dimensionally stable, it can span long distances and be erected rapidly.

Internationally, CLT has propelled wood construction to new heights, the most recent example of which is the Forté, a 10-story apartment building in Australia. It offers the structural simplicity needed for cost-effective projects, as well as benefits such as rapid installation, reduced waste, energy efficiency and exceptional design versatility.

In North America, CLT is relatively new but quickly gaining momentum. In 2012, the American National Standards Association approved ANSI/APA PRG 320-2012 Standard for Performance-Rated Cross-Laminated Timber, a product standard that details manufacturing and performance requirements for qualification and quality assurance. Due to recently approved code changes, CLT is scheduled to be included in the 2015 International Building Code. In the meantime, a handful of innovative designers have already built CLT structures in the U.S. and Canada, having had them approved under the relevant code through an alternative or innovative solutions path.

Moisture Resistance

“All materials have challenges when it comes to moisture; however, when moisture is managed properly, wood exceeds expectations," says Cheryl Ciecko, ALA, AIA, LEED AP, noting that wood acts as a moisture sink and a thermal break.

Lumber grading rules and many building codes require wood be dried to 19 percent moisture content or below—still substantially below the fiber saturation point of 28 percent, the level at which mold or decay can begin to thrive. Decay fungi feed on wood and require oxygen and moisture to thrive. Because damaging fungi affect wood primarily when the moisture content exceeds the fiber saturation point for a prolonged period of time, adverse effects can be prevented by avoiding direct contact between untreated wood and the ground or other moisture sources. Treating wood with preservatives will also protect it from undesirable fungi and insects.

All materials, including steel and concrete are susceptible to mold, since dirt or dust can be the food source, along with moisture. Bulk water, air infiltration and condensation can be a source of moisture in all types of buildings. While wood acts as a thermal break due to inherent insulating properties, steel, concrete and masonry are thermal bridges which can provide a cold surface on which warm, moist air can condensate, increasing the potential for deterioration or mold. “This moisture due to condensation can be a huge problem," says Ciecko. “Wood can hold some moisture for short periods of time, acting as a moisture sink, without harm. However, steel cannot, making it potentially susceptible to corrosion with even small amounts of water contact."

A study by FPInnovations showed that interior wood paneling can reduce peak moisture loads in a typical Canadian house by 10 to 25 percent—a scenario that leads to both improved user comfort and reduced need for air conditioning.³