XPS foam on exterior side of window and door headers
I think have seen a similar question before but wanted to refresh.
Assume building a 2×6 wall. Using Mineral Wool for insulation (which means a smart membrane on the interior side of the wall). When framing headers for doors and windows almost ALL of them only need double 2×8, typically with ply sandwiched to make them flush with the stud depth. In 2×6, simply 3 pieces of header lumber are nailed together to get the 5.5″ thickness of the stud.
Would it be a good idea to actually install 1.5″ of XPS (or EPS) Foam instead of the 3rd additional piece of wood to help with thermal bridging of the header lumber? Would placing it on the interior side warrant any benefit over the outside?
I can’t come up with a reason not to do this. The third 2×8 typically installed in a header is just there for filler and something to nail into, considering most headers only need to be doubled up.
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Nicholas,
Builders install insulated headers all the time, using rigid foam to make up the thickness not needed for structural purposes. I like to put the rigid foam on the outside of the assembly, but it can go wherever you want -- outside, inside, or in the middle of the sandwich.
Most green builders prefer to use EPS or polyiso instead of XPS (since XPS is manufactured with a blowing agent that has a high global warming potential), but if you are using up scraps, any type of rigid foam is fine.
-- Martin Holladay
Would foil faced poly iso be okay to use, or will there be condensation on the foil?
Nicholas,
A three ply header (at 4 1/2") still leaves you about a 7/8" space on a 2"x6" wall, so if you substitute foam you are looking at 2 1/2".
The third ply is usually there for structural reasons when the window head-height precludes deeper dimensional lumber. Whenever possible using a deeper, narrower header is a better idea. 2 2"x10"s are stronger than 3 2"x8"s, and as you say leave more room to insulate. A single ply LVL leaves the most room, but is expensive.
There are advantages and disadvantages to where you place the foam. The outside and middle of the header means placing the foam while you frame. Having it on the inside means you don't end up with solid material above the window for anchoring window coverings. Sandwiching it in the middle can make inspectors nervous as the pieces of the header aren't nailed together and can't act as one structural unit.