Are wood headers insulated with rigid XPS as the filler as structurally sound as those padded out with OSB or plywood?
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What about 2×6 walls with 2.5″ of XPS in between?
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Matt,
As long as the size and number of the headers (usually two pieces of 2x lumber, although different types of header material can be specified by an engineer) are adequate, the use of foam will not weaken the header assembly. Of course, the header should be adequately nailed.
I believe he's talking about foam between the wooden plies of the header, and I would ask the engineer or building inspector before doing this. Some specs call for wood members to be fastened directly together.
I know the IRC calls for insulated headers and corners, but is there any research supporting the use of such as a replacement without consulting an engineer every time?
Matt, headers are sized to carry a load. Load programs are available. Books too. You may like to visit both the JLC and Finehomebuilding bookstore sites.
Headers are often way over sized. Old cabins here mostly have no headers and I have not seen one failure though this is not done today.
I use foam in my headers, the least amount of wood and the most amount of foam for the load and span involved.