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I have a finished attic that has a kneewall insulated with fiberglass batts, insulation facing in. The batts fill the depth of the wall cavity completely to the end of the stud, there's no more room for batts between the studs.

I want to glue rigid foam board (R-10) to the wall studs in a continuous, tight, taped air barrier to add both additional insulation and address air gaps.

However! Foamboard is a vapor barrier, and since the insulation already has the kraft facing on the inner side, doing this would create two vapor barriers, and I'm worried that this would trap moisture in between them and cause serious problems.

Using additional unfaced fiberglass batts would avoid this problem, but it creates three questions for me:

  1. How do I put fiberglass batts on a wall over the studs (again, no more room in the stud bay).
  2. How can I do so in a way that helps with air gaps since there will be major slices between the batts for air to get in?
  3. How can I do this without an insane amount of effort to install?

Using foam board would be so much better and so much easier than more fiberglass for those reasons. But I don't want to ruin my house with moisture.

One thought I had was to flip the batts so that the kraft paper is facing out instead of in... but there would still be an air gap (though much smaller) between the facing and the foam for moisture to get trapped in.

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  • Use box cutter type knife and slice kraft paper to ribbons. No more vapour barrier.
    – crip659
    Apr 10, 2021 at 14:14

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I have done what you asked when I was an insulator for a few years. Unfaced batts were added horizontally. After each run was set and bedded well over the lower insulation, wires were placed at the top of the horizontal run, using screw eyes, enter image description here there is a tool that zips these right in that you use on your drill, or staples. Then the next horizontal run was added and wire were added above that run as well. These wires were installed to hold "chicken wire", a light weight, large spaced mesh to keep all the insulation in place. This kept the insulation tight to its surroundings. Using R-19 will help you keep it in place better than R-11 or R-13.

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Cut the kraft paper, add the foamboard (and presumably drywall over that.)

If you put enough cuts or slits or holes in the kraft, it's not a vapor barrier anymore.

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