Today I sent this e-mail to our energy guru Marc:
The joists in question are 24-inch roof joists from Nordic Engineered Wood in Quebec. These bad boys will allow us to have a clear span roof (no internal bearing walls) that's stuffed with 24 inches of insulation (mostly if not entirely cellulose).
The reason Marc wins this round is that Eli (the builder/architect) and Ben (the structural engineer) were agitating for flat trusses. Trusses have some structural and workflow advantages, but joists have the virtue of being extremely easy to insulate.
For those of you not in the know, trusses are made of dimensional lumber (usually 2x4s or 2x6s) joined together with metal plates. They are exceedingly strong and relatively inexpensive. The problem is that it's hard to properly insulate all the gaps, and the wood and metal turns into a thermal bridge when it traverses the building envelope.
As an alternative, Marc has long wanted us to build the roof with I-joists. I-joists are an engineered product, which means they aren't made from old-fashioned wood like the Pilgrims used. Instead, they're an
unholy adhesive-bound combination of solid wood flanges (the top and bottom bits) and OSB (particle board's stronger cousin). They're called I-joists because the cross-section resembles a capital I.
Unlike trusses, I-joists are very easy to insulate, and the relative lack of material means less thermal bridging. Our I-joists will be a flabbergastingly-deep 24 inches, which is practically unheard of for residential construction. We want this depth for insulation and also for strength.
It was initially hard to find joists this deep. In residential construction they usually max out at 16 inches, and although Weyerhaueser's commercial line includes 24" joists, they aren't available in New England. Fortunately I discovered that our French-speaking neighbors to the north make deeper joists that are relatively easy to obtain.
Ted promises to write a post explaining the technical challenges with using I-joists and why Eli and Ben weren't initially on board.