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KS weather (zone 6) is always averaging 5°F in winter at least a few times, and always over 100°F in summer for 20+ days. I bought a manufactured home that apparently has 2×4 exterior walls. I discovered black mold when taking off crown molding, and ended up removing the drywall and R-11 insulation they had, and working on ceiling as well. Roof leaks existed for years before previous owner finally replaced it. Damage was already done, and painted over. I decided to utilize this situation and use 1/2" or 5/8" Sheetrock instead of the previous 1/4", and better insulation which is apparently only R-15. I've considered closed-cell foam spray insulation of 1 inch on outer exterior layer (from within the house), and add rockwool for the other 3 inches for interior insulation(even though spacing is technically 3-1/2" depth). I really want thermal and sound insulation. I have noisy neighbors. Exterior wall is some type of faux vertical wood paneling, that’s merely large squared sheets of fibrous material. It looks it’ll breathe well for moisture evaporation concerns.

Will this concept work well for ceiling and walls, or is it faulty thinking?

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    The usual approach would be rockwool 3.5" between studs, then sheet foam over the studs, tape the joints for vapor barrier, (pick your thickness, keep track of the stud locations) and drywall over the sheetfoam (use screw lengths suited to the thickness of the sheet foam.) That gets you more insulation than you can get in the small stud cavity and takes care of the thermal break at each stud.
    – Ecnerwal
    Commented Oct 8, 2023 at 18:59
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    If possible add 2x2s and have 6(5.5) inch walls. Insulation goes up to R-20.
    – crip659
    Commented Oct 8, 2023 at 19:01

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Your plan is good. However I don't understand how you can spray foam and only get 1 inch of coverage. ( but that's just my misunderstanding of the process.)

I would consider foil faced polyisocyanurate sheets 3/4 inch. Then the rockwool would be less compressed and theoretically more efficient.

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