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Vapour barrier on interior concrete wall

Andy2022 | Posted in General Questions on

We have done an addition and part of the existing exterior concrete wall in our basement is now an interior wall. Each end is connected to exterior concrete walls covered in 6” of exterior foam, but it’s footings are directly on the subsoil.

We’re going to insulate the 2×4 wall on either side to stop it being a thermal bridge, but neither I or my builder can work out whether it should also have a vapour barrier.

We’re zone 7, and the footing/base of wall is ~6’ below the frost line, ~ground temperature of 7 degrees C. At that depth I’m thinking it should, as there may still be a risk of condensation on the lower part as it will act more like a slab?

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  1. Expert Member
    Michael Maines | | #1

    Because it's the temperature of the concrete that would cause condensation, a vapor retarder won't help--the condensation will just form on it instead of the concrete. It would be better to insulate the interior side of the concrete with rigid foam. You don't need much; just 1/2" would probably be enough to prevent condensation. The concrete doesn't need to dry to the interior, though hopefully you have a capillary break of some sort at the top of the wall.

    1. Andy2022 | | #2

      With the vapour retarder I’d be installing like a traditional basement wall - I.e concrete wall> 2x4 /studs with R14 batts> vapour retarder (Intello)> drywall. The difference is the other side of the concrete wall, rather than being to the exterior would also have (pre existing) 2x4+R12> vapour barrier (6mil poly)>drywall.

      Vapour retarder would have to be sealed to the old rim joist. The poly under the new slab extends about 2ft up the concrete, but would be sealed separately against it for radon mitigation.

      I hadn’t considered the potential issue of moisture wicking up from the subsoil. We’re using Intello in the rest of the house, so it should allow some moisture movement out of the walls.

      In this scenario concrete>1/2” foam (taped), 2x4 w/ R14 batt > drywall would be a bette r option? Unfortunately we wouldn’t be able to do it to the other side, which is not being touched (and has fibreglass and poly, so more susceptible to moisture than mineral wool and intello which would be on the new side).

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