Wood is a desirable construction material for many reasons including its low embodied energy. But, until recently, it has not been possible to build tall wooden structures because of the relative weakness of conventional wood stud construction methods. This is starting to change as a new method of fabricating wood panels, called cross-laminated timber, or CLT, is making “massive wood” construction a possibility for mid-rise construction, as well as for other construction uses.
As a building material, cross-laminated timber (CLT) is a rather new development. The first work with CLT was done in Switzerland and Austria in the early to mid 1990s.
Because the panels are lighter than masonry and hollow-core concrete plank, and since the panels can be lifted with lighter equipment, construction can be faster and thereby less expensive. It also requires a smaller crew to install and connect the panels at the job site, which allows for rapid and efficient construction.
In one example, a 9-story building saved an estimated 22 weeks of construction time by using CLT instead of concrete. The panels are prefabricated in a shop, so the benefit of controlled conditions is present, as with structural-insulated panels (SIP) and other pre-fab materials.
CLT is used for both walls and floors. The panels are made as alternating boards are laid perpendicular to one another (the same thing is done with plywood) for greater strength. Since the CLT panels are predominantly solid wood, there is far less glue in them than an equivalent volume of plywood. CLT panels have been tested for VOC off gassing from the glues, and are far below regulatory limits in that regard.
Since they are prefabricated, CLT panels can be installed with tighter tolerances and provide more plumb and true construction. Solid wood is not a great insulator, but wood is better than CMU or steel, and the building can still be insulated on the exterior or interior.
Using a massive wood structure — from a carbon cycle perspective — not only produces far lower emissions during the manufacture of the material, but the wood also sequesters carbon for its lifespan.
Although one might think it would be a fire hazard, massive wood construction is actually fairly fire resistant. For decades, building codes have recognized this for buildings constructed with large solid-sawn post and beam construction. Wood will char on the surface, but it takes a very long time for it to burn through. CLT is the same.
At present, the only manufacturers of CLT panels are in Europe, but two plants in Canada and one in the United States are currently under construction. There are a number of examples of mid-rise apartment buildings constructed with CLT in Europe. The first North American non-residential CLT structure was a bell tower completed in 2010 in North Carolina.
[Ed. Note: In my backyard, the University of Utah's Integrated Technology in Architecture Center is working on an ICLT -- interlocking cross laminated timber -- system with no fasteners or adhesives. It's in development and testing with a 3-5 horizon for commercial availability. -- Preston]
Disclosure: Some information for this article and the sample of CLT wood for some illustrations were included in an Architect’s Toolkit box of promotional materials I received from the BC Forestry Innovation Investment/NaturallyWood.com.
Credit: Forestry Innovation Investment Ltd. and the author.Article tags: CLT, wood