I've been looking into adding rigid foam sheathing to the attic side of my bonus room knee walls. The house was built in 1996 with 2x4 walls and R19 craft paper faced fiberglass insulation. My question is: do I need to remove the paper facing before installing foam panels? I know you typically do not want two forms of vapor barrier sealing a wall cavity. Or are these fiberglass bats not an issue at all? From what I can find online there do exist perforated foam panels, but these are not commonly available at the home stores. My problem is mainly hot temps in summer, so want to get a panel with reflective foil. Winter is not an issue. Located in zone 7, central NC, if it matters. Thanks, Michael

I am not an expert on this and someone else may have a better opinion but I thing the problem with duel vapor barriers is when they are not against each other. If there is a space they can trap moisture between them.

Posted By newbostonconst on 16 Jan 2021 11:29 AM
I am not an expert on this and someone else may have a better opinion but I thing the problem with duel vapor barriers is when they are not against each other. If there is a space they can trap moisture between them.

Yes, that's the problem I'm concerned about. The craft paper is against the drywall, and the foam board would be on the opposite side of the stud bay. So it would be drywall -> craft paper -> fiberglass bat -> foam panel.

I believe craft paper is vapor permeable enough to allow drying to the interior.

The only other thing I've realized is that the R19 bats are not intended to be compressed into a 2x4 wall. They should have used R13 in a 2x4 cavity. Not sure if they were trying to offset some of the losses of being open to the attic or what. I guess I either replace it with R13 or I fir out the studs to make them 2x6.

What is on the attic side of the 2 x 4 walls now? Adding 2" x 1 1/2" strips of ridged foam then sheet of foam will make the cavity the same as 2 x 6. The foam will be like sheathing.

Posted By ChrisJ on 22 Jan 2021 04:23 PM
What is on the attic side of the 2 x 4 walls now? Adding 2" x 1 1/2" strips of ridged foam then sheet of foam will make the cavity the same as 2 x 6. The foam will be like sheathing.

Nothing on the attic side. They just built 2x4 walls and stuck in R19 bats, so the unfaced fiberglass sticks out past the studs. Firing the studs out with foam is a great idea--that would give me a really well insulated wall. I'd get the full value of the R19 + the R3-ish value of the foam.