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Basement Wall

moose_head27 | Posted in General Questions on

Hi I’m currently finishing my basement. My basement wall is poured concrete and has 3.5 inches of EPS foam on the interior and 1/2 inch drywall cover. The drywall is literally screwed to the foam. The house is about 10 years old. I don’t see evidence of poly on the wall from the exposed perimeter of the drywall. My best guess is the inspector allowed the foam to be counted as a vapour barrier but as from my understanding EPS foam isn’t a vapour barrier but maybe the perm rating was close enough to be acceptable. 

I was originally planning to build a 2×4 wall against the existing wall. Leave it uninsulated and run my electrical through it and drywall it. That way I leave the existing wall untouched.  Some friends are telling me I should insulate the new wall and install a poly vapour barrier. I’m not sure if this is the best approach. I’m unsure if the added insulation could effect the wall assembly in a negative way on a building science standpoint. As for the vapour barrier installation I’m a lot more hesitant for retaining moisture and have have a potential double vapour barrier since I’m in a cold climate.

The reason why I’ve wanted to build a 2×4 wall was that I do not want to cut in the existing foam and drywall to run my electrical wires. I was told by electricians that the wire needs to be stapled to something structural every so many feet and I don’t have any proper supporting elements. The only element I would have is the concrete foundation. Anyways it seemed like a lot of work and was significantly more costly that method on a electrical stand-point from the electrical quotes. Installing the electrical on with the 2×4 wall was something I could complete partially if not all on my own and safe significant amount of money.

Any thoughts? Should I just leave the new 2×4 wall and drywall as originally planned? Add the insulation? Add the poly vapour barrier as well? Or simply scrap the 2×4 wall and install the electrical on the existing wall.


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  1. Expert Member
    BILL WICHERS | | #1

    Don't put in any poly. You don't need a vapor barrier here, and if you add one you're likely to have more chance of problems. 3.5 inches of EPS is pretty much a vapor barrier anyway.

    It's generally OK to have a vapor barrier if it's against the wall. What you generally do NOT want is a vapor barrier spaced out away from the wall, with or without any insulation between it and the wall. That space tends to be a place for mold to grow.

    You can run surface electrical in conduit without any problems, just secure it to the drywall with drywall anchors. If you want it to look a little nicer, look into "wiremold" or other surface-mount electrical raceways. These surface raceways are a little nicer looking than conduit. Note that you need to pull individual wires (usually THHN) into raceways or conduit, do NOT try to run NM cable ("Romex") in raceways.


  2. user-6623302 | | #2

    A lot depends on what level of finish you want. Stud wall if you want it to look like the living room, conduit if it is a basement. In SIP buildings, the electrical is sometimes run behind a deep baseboard. Receptacles also installed Maybe some stud walls for wall switch, doors and the like. and the rest in the baseboard.

    Do not rely on drywall anchors. You need to fasten to the concrete.

  3. wastl | | #3

    why not take the old drywall off and then do insulated stud bays+drywall?
    Not worse in terms of vapor than today and better insulated. A vapor barrier as you asked for will give you a wet outer drywall.
    yes- as above - tie the studs against the concrete.
    If the existing drywall is not painted with a retarder wall you can also keep it, if you insulate with Rockwool and new drywall (and no vapor retarder surface treatment) then this is ok. Will make the whole assembly just that tad thicker.

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