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Community and Q&A

Cut and Cobble Air Gap?

stolzberg | Posted in General Questions on

I’m planning to insulate 1900 farmhouse in rural MA with no current insulation. The walls are real 2×4 with exterior board sheathing covered in wood shingles which are covered in cement shingles (i.e., two layers of shingle siding). We’re planning to remove lathe and plaster on exterior walls and insulate from the inside.  To save money and environment, thinking about using recycled rigid foam to cut and cobble inside stud bays and then put 1 inch continuous layer of rigid foam over studs, tape, and then drywall. Should I leave 1/2 inch air gap between rigid foam and board sheathing (sort of like internal rain screen) or just try to fill the full 4 inch depth of stud bay with insulation right up against sheathing?

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  1. Expert Member
    ARMANDO COBO | | #1

    After Katrina, I helped a friend do a reno in NOLA, where by historical zoning laws, he couldn't remove the siding, so we installed 1/2" backer rods against the interior of the lath sheathing, then 2" polyiso rigid foam, and the batts.
    I know you live in different climate zone, but it would make sense to me to to follow your ideas, so the wall assembly will dry to the outside.

  2. Expert Member
    Michael Maines | | #2

    Stolzberg, if you are committed to the cut-and-cobble approach, I would do as you and Armando recommend and leave an air gap.

    If you are not committed to foam, I would insulate with borate-treated, dense-packed cellulose and add a variable permeance membrane at the interior. That will limit moisture movement but protect any framing from mold and fungus growth, and also allow drying to the interior if and when you use air conditioning.

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