When nailing together built-up window and door headers, I use 1/2-in. foam insulation between the 2x stock instead of 1/2-in. plywood (see drawing). The sheathing — whether foam board or plywood — is acting only as a packing material to adjust the thickness of the header, so it makes sense to use something with a high R-value.
Todd S. Pettinger, Springwater, NY
Edited and Illustrated by Charles Miller
From Fine Homebuilding #79
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Great idea, but why include that solid piece of wood at the center of this "sandwich?' It certainly not need for structure, and you're giving up some serious R-value. Instead, fill the entire 2.5 inch space with insulation.
Btw, that detail -- filling the whole cavity -- was the way we were framing houses way back in 1982 when I was just a minimum wage framer working in North Carolina.
Most headers need to be glued and nailed together for strength. So if your making them the way described above wouldn’t it be stronger/better to put the foam on the inside of the house instead of sandwiched between.
For most headers, this is fine. But on rare occasions that extra layer of plywood serves a structural purpose. Plywood on edge is quite strong, particularly when prevented from buckling by the 2x material on each side (or in between). Properly jointed, I have designed plywood box beams to span openings in excess of 16 feet. I have also used plywood side plates to stiffen double-2x members to reduce deflection.
Energy issues aside, sometimes strength becomes the real issue.
If that foam fails or deteriorates over time, those 2x's could become loose and twist. This doesn't seem like the best solution to me. Plywood, glued and screwed, makes a much stronger member (if necessary). Though for just window and door headers3-4' wide, I doubt strength is an issue at all.