Cut and Cobble Air Gap?
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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