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EPS on basement wall — vapor barrier advisable?

Nick Welch | Posted in Green Building Techniques on

I came across some unfaced 2″ EPS for cheap, so I want to use it to insulate my basement walls. I don’t know its density but I assume it’s whatever is most common. At 4″ total, this puts it at about 2-2.5 perms. The perfectionist in me wants to put poly sheet behind it to have a full vapor barrier, but how much difference will that realistically make in the dryness of the basement? It seems like gluing the poly to the wall, and the EPS to the poly would be awkward, and I don’t really want to sink anchors into the concrete. I already have a bunch of poly just sitting around.

It’s a typical 1950 concrete basement in the Pacific Northwest. It’s currently not conditioned and ranges from about 50F/50RH in the depths of winter to about 70F/70RH in the summer. 3/4 of the rim joist is still not air sealed (though it will be eventually), the floor is part unfinished and part tile. The walls are Drylok’d.

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  1. GBA Editor
    Martin Holladay | | #1

    The EPS will work fine, but you should never include polyethylene as part of your wall assembly when you are insulating a basement wall on the interior.

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

  2. Expert Member
    Dana Dorsett | | #2

    "It seems like gluing the poly to the wall, and the EPS to the poly would be awkward, and I don't really want to sink anchors into the concrete. "

    What Martin said about the poly skip it. Even 2" Type-I EPS is more vapor retardent than standard latex paint, and 2" of Type-II EPS is on the low end of the Class-III vapor retardency scale. Either would be fine with a latex-painted finish on the interior side of the stackup.

    But you still need an interior side thermal barrier for fire safety to meet code, which involves either furring through-screwed to the concrete on which to install the thermal barrier (half-in gypsum is the most common) or a studwall. If the latter, it's worth insulating the studwall cavities with rock wool. unfaced fiberglass would be next-best, but you have to be a bit more careful when using cellulose in sub-grade applications due to it's hygroscopic nature. If studwall, put a strip of EPS under the bottom plate as a thermal & capillary break, and TapCon it to the slab through the foam. The studwall is not structural, not supporting the house, only the wallboard & foam, so 24" o.c. with single top-plates is fine.

  3. Nick Welch | | #3

    Well I'm glad the consensus is to skip the poly. It should be easier to install the EPS without poly in the equation.

    Dana, I will be putting up a stud wall with drywall for a thermal barrier, and was planning to put some EPS under the bottom plate thanks to seeing you recommend that in the past.

    Martin seems to be quite against any fibrous insulation in the stud cavities, so I'm not sure whether I want to add some rock wool or not. Also, Roxul is pretty much the same price as new polyiso (in terms of price/(R*sqft)) for me so it's not very appealing in this case where I don't need its vapor permeability and I don't need its thermal barrier capability. I was thinking 4 inches of EPS, a small gap, then a thin stud wall made of 2x4's turned sideways. That's continuous R-16 which I think is pretty good considering my mild zone 4c (requirement for new construction is R-10 for continuous basement wall insulation) and it is conservative enough to make even Martin happy by not including any fibrous insulation. :-)

  4. GBA Editor
    Martin Holladay | | #4

    I like your attitude!

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