Can truefoam foundation plus type two rigid foam be used as a vapor barrier?
I am finishing the 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 r20 “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 two foundation plus by trueform 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.
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This article by Rick Arnold should clarify the questions you have about using your Truefoam systems:
https://www.finehomebuilding.com/item/4822/which-rigid-insulation-should-i-choose
I haven't used this specific system, but in looking at the material data sheets, it seems the FoundationP lus type II is meant to be used on the exterior of foundation walls and the Truewall system is meant to be used on the interior. Both materials rate about an R-4 per inch, but both have perm rating in excess of 1 (FP @2.8 and TW @ 4)
Since your EPS isconsidered vapor permeable and your wood framing "cares" about moisture, I think you could have a potential moisture related problem with your wood framing over time. Alot depends on the type of foundation wall system and exterior waterproofing and drainage systems you have in place. Those issues are outside the scope of information you provided. However, one thing is for sure, your basement framing will not be able to dry to the inside with the polyethylene vapor barrier just under the drywall. I don't see that your finished interior wall system has any choice to dry to the exterior since it is assumed your basement wall are mostly below grade where any moisture would be migrating from in the first place. In other words, there is no opportuity for "breathability" to take place. Thus, any vapor drive coming from the exterior can adversely affect your wood framing material as it looks to 'soak up' or equilibrate any moisture imbalance.
I believe that if you choose to use galvanized metal studing rather than 2x4's and kraft face isulation batts w/o the polyethylene sheeting, then any vapor drive related issues (like mold or mildew) would be largely mitigated. However, I wouldn't be married to using the Trufoam system in the first place unless you know you have an excellent exterior foundation drainage and waterproofing job in place.
The drainage in the house seems to be great. There is no visable moisture getting through the walls however we live in a very humid climate in the summer months. I would say about 7 feet of the 8 foot foundation is below grade. It's 8 inch poured. Anyways although there is no water penetration there is definitely vapor down there. It's very humid and my cold water pipes have been sweating at times. My goal is a dry living space in the basement of this house. Will the true foam foundation plus provide this? Again it is the type two EPS, not the type one. Provided my seams will all be tuck taped and the gaps either caulked with acoustic caulking or filled in with expandable foam. If I have this insulation system against the foundation and omit the vapor barrier between the studs and the drywall will that be a better way to go about this? Just a side note I am a DIYer tackling this for the first time. I want to make sure I get it right. Again my goal is a dry livable basement. Also I think I mentioned dricore flooring wil be down against the rigid foam so that should take care of any moisture vapor from the floor. If there is any more information you need to answer this properly let me know. Thanks again for the help.
perform an easy DIY test for moisture migration
"seems to be" great is not a particularly definitive assessment.
I recommend that you duct tape a 2' square piece of polyetheylene on the inside of the concrete wall (about 1/3 height) and on the floor (about 3' aways from the wall). Leave it in place for at a week or so. Let us know it any water vapor collects on the sealed side of either piece.