Wood is good … but strange

ASHRAE Journal, Nov, 2007 by Joseph W. Lstiburek

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The most impressive old guy wood technology was the Colonial door (Photo 3). Colonial doors were designed so that the frames had virtually no cross grains, whereas the panels, which were cross-grained, were allowed to slide into joints (Figure 9).

When we began to use wood trusses in place of rafter roofs, we rediscovered what old guys knew about wood movement. A wood roof truss has an upper chord and a lower chord and they are connected ... duh. The upper chord gets cold in the wintertime in a vented attic, whereas the lower chord stays nice and toasty warm if it is under lots of insulation. The more insulation we add, the colder the attic, and the colder the upper chord because we reduce the heat loss out the top of the building. The more attic or roof ventilation, the colder the upper chord becomes. What did old guys know about wood? The colder we make wood, the wetter it gets. The warmer we make wood, the drier it gets. The top chord becomes wet and grows, and the bottom chord becomes dry and shrinks (Figure 10). Presto! We have truss uplift (Photo 4). You cannot restrain the truss from movement by fastening it to the walls. The wall will move (Photo 5). The only solution is the Zen solution--let it move. Hence, we have floating corners and truss clips (Figures 11 and 12).

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With all its strangeness, we have learned to love and work with wood. Too bad we are running out of the really good old stuff. So is this the end of wood? Nope. We must learn to work with the younger woods, the smaller woods and the composite woods and the "was-woods." Will they work the same as the old stuff? Nope. Any bets on us finding new things to worry about? In a generation, I bet my kids will look back and say we old guys sure knew stuff. We won't tell them about the mistakes we made figuring stuff out. Be prepared to see stuff you have never seen before. Don't panic, it's just wood and we've had to deal with this before. We will figure it out. We always do.

* Wait for it ...wood curtain walls. Treated wood in an assembly performs better in a fire than steel studs, and wood is not thermally conductive. Don't believe me? Visit New Zealand and Australia and check out some of their 10- and 20-story buildings. They use concrete structural frames and treated wood frame wall infill assemblies with gypsum board linings on the inside and outside of the wood frame assembly covered with open rain screen vented fiber cement panels. These structures are exceptionally energy efficient, low cost and sustainable.

By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE

Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Consulting in Westford, Mass.