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Community and Q&A

Basement wall insulation

Jonathan Lawrence CZ 4A New Jersey | Posted in General Questions on

I am insulating the interior of my basement walls with 2.5″ EPS panels made by InSoFast.  I like these panels because they are glued directly to the foundation wall and gypsum can be attached directly to the panels secured with screws through plastic studs integrated into the panels every 16″ on center, thus eliminating the need for a framed stud wall.  The manufacturer claims the panels have an R-value of 10, but I think that is a stretch given they have chases cut out for electrical wiring, vertical plastic studs and ridges on both sides.  I was thinking of adding a 1/2″ layer of Tuff-R polyiso between the panel and the gypsum.  The polyiso has a foil faced side that would be in direct contact with the gypsum and I wanted to confirm there is no concern that the foil would become a condensing surface.  The basement will be conditioned, and there is 4″ of EPS under the slab so I don’t see how this could be in an issue, but I wanted to confirm this is ok before proceeding.

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  1. Norman Bunn | | #1

    In CZ 4/5, you should have a minimum of R-10, which you have (more or less). The polyiso will add about R-3, which is gives you a total close to the requirement of CZ 6.

    The foil surface of the polyiso would have to get below the dew point in order to have condensation, which I doubt would happen with R-13 between that surface and the ground outside (I could be wrong, so lean more on someone like Martin or Dana for a better assessment).

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Go ahead. There is no risk to adding polyiso with foil.

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