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Can TrueFoam Foundation Plus Type 2 rigid foam be used as a vapor barrier?

I am finishing the exterior basement walls in my new home. My plan is adhere rigid to the foundation walls, after that I'll be putting down dricore subfloor, then studding up 2X4 walls on top of the dricore. There will be an air gap between the rigid and the 2X4's as I am laying in R-20 "2X6" batts. Plastic vapor barrier will complete the system before the drywall.

My question is about the rigid foam. I've been told that as long as it's not type 1 EPS, it will act as a vapor barrier and protect against moisture. My local building supplier ensured me the Type 2 Foundation Plus by TrueFoam was the stuff I was looking for, and no vapor barrier between it and the foundation would be necessary.

I'm skeptical of this after looking further into the product. It claims "breathability" and the perfect rigid foam for EXTERIOR applications. However mentions nothing about its use in finishing interior basement walls.

Any help would be greatly appreciated. It's in my garage already purchased and I just have a feeling like it may not be what I need. Thanks for the help.

Asked by Mark McKenzie
Posted Mon, 09/01/2014 - 16:49
Edited Tue, 09/02/2014 - 06:13

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Mark,
First of all, the vapor permeance of the rigid foam installed in this location isn't that important. A wide range of rigid foam products can be successfully installed on the interior side of a basement wall -- ranging from rigid foam that is somewhat vapor-permeable to rigid foam that is vapor-impermeable. All of these products are in an acceptable permeance range. There is no need to have what you call a vapor barrier at this location.

For more information on this issue, see How to Insulate a Basement Wall.

The product under discussion -- TrueFoam Foundation Plus -- is simply one brand of EPS insulation. It will work just fine in your application.

I strongly advise you to omit the plastic vapor barrier that you plan to install on the interior side of the 2x4 studs. This polyethylene layer can potentially trap moisture, leading to mold and rot. When you install insulation on the interior side of a basement wall, the assembly should never include polyethylene.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 09/02/2014 - 06:20

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