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:
- How do I put fiberglass batts on a wall over the studs (again, no more room in the stud bay).
- 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?
- 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.