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Short version: If I install 3" of rigid foam over a poured concrete foundation wall that has been sealed with Xypex, will I have any moisture issues if I install plywood directly over the rigid?

Longer version: I'm finishing a basement space as an eventual bedroom. To be code-compliant, I'll need to bring the walls up to R-15 (and address egress as well, but that's another subject). My basement walls are comprised of 4 feet of poured concrete with 3.5 feet of framing on top. I'd like to save as much square footage as possible, and therefore not build a 2x4 interior wall after installing rigid.

The plan I've devised is to tackle the upper, framed portion of the wall and the lower, poured concrete portion of the wall separately (see image).

  • Upper, framed portion: air seal the stud bays (exterior sheathing is butted board), stuff bays with rock wool, and install 1/2" drywall.
  • Lower, concrete portion: seal concrete with Xypex, install 3" rigid foam (polyiso?) with adhesive, install 3/4" plywood

Electrical will be installed with surface-mounted conduit to the plywood.

No foundation sealer is perfect, but will my wall still breathe enough that I won't experience any moisture issues?

My contingency plan would be to add 1" furring strips to the rigid, then install the plywood over that.

enter image description here

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    I have seen many basement walls that were sealed fail. Since you are asking about possible moisture problems how old is the home? If I it have any moisture problems prior to sealing? Are there foundation drains or sump pumps to keep the basement dry. – Ed Beal Aug 28 '17 at 1:59
  • @EdBeal Yes, that is often a big problem...plus, another issue is “Dew Point” developing “in” the wall between the concrete wall and rigid insulation due to the location of the existing sealer. – Lee Sam Jan 3 '18 at 7:12
  • Don't cover the top of the wall. You want access to the sill plate. It's important for various kinds of inspections. For example, a common route for termite entry is up the exterior of the concrete and under the sill plate. Seeing their mud tunnels there is a valuable early warning. – fixer1234 May 21 '18 at 23:35
  • The joints in the foam layer should be taped to seal. There is a Building Science report on construction and retrofit approaches to basement moisture control. – DaveM Aug 28 '18 at 2:53
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The wall is already sealed. You won't be introducing any more moisture restriction than what's already there. Build on.

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If you have water problems in your basement, look to the exterior of the house for solutions. Mainly, get water to flow away from or around the foundation. If water is building pressure behind the concrete, it's only a matter of time before it finds a way in. In a high-precipitation area, painting the inside of the basement is unlikely to stop the water for long. If the sealing fails, then you have water trapped behind the foam and that's going to lead to other (smelly) problems. Previous owners of my home tried to seal the basement from the inside and it simply led to the deterioration of the concrete at various points.

If you have an old drain-tile setup with actual ceramic drain-tile, get it updated. The whole strategy of those systems was often inherently flawed.

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