Who says you've got to add framing to include insulation? Why not cut your profile from 1/2" or 3/4" plywood, cut a bunch of lightening holes where you don't need strength for windows, doors, or cabinetry ... panel the inside with 1/8" plywood paneling ... stuff 1/2" or 3/4" foamboard insulation in the holes ... then cover with 1/8" or 1/4" plywood (I prefer 1/4" if I'm gluing the aluminum to the sides) ...

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You'll get all the benefits of plywood walls, all the benefits of insulated walls, less work, and a weight that is closer to a stick-framed construction! Notice that I didn't say the
same as stick-frame ... my plywood-framed trailers run 75-100# heavier than my old stick-framed ones, but they're 100-150# lighter than if they were solid plywood.
Food for thought ...

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CHEERS!
Grant