Can I have too much rigid foam on the outside of my knee wall sheathing?
BrunoF
| Posted in General Questions on
I am in zone 4A (central NC) and need to insulate my MDF slab attic knee wall doors and figured that while I was doing that I would also add foam board to the knee wall sheathing. I want to be sure that I don’t trap moisture so I would like to know if there is a maximum thickness of rigid foam that I should respect.
the knee wall construction is OSB (taped and sealed), 2×6 studs, R21 fiberglass, then drywall.
thx!
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Replies
More rigid foam on the exterior, or "cold side", of a wall makes the assembly safer. There is no point where you could have "too much" exterior rigid foam in terms of moisture safety for the wall. The problem you run into, besides cost, is that when you get up over 2", trim details can become more complex. With an attic kneewall, you probably won't have that issue though. I have 3" polyiso on the attic side of the upper level attic walls in my own home, as an example.
Bill
Bruno,
Like any exterior wall, it’s having too little, not too much foam that can cause problems:
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
Thx folks!