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Community and Q&A

Insulating on Inside of Foundation Walls

advice_yesplease | Posted in Energy Efficiency and Durability on

Hello there

This is kind of a follow-up to another post (insulating and sealing rim joist in house with no sill plate) but I thought the two issues were disconnected enough to warrant two discussions.

My (fairly) newly built house (build around 2000) in Atlantic Canada (Zone 6) has a wrapped foundation (brown dimpled membrane) that was installed afterwards to remediate some moisture ingress issues observed after a pretty heavy spring thaw – I don’t know the exact details but that was what was told to me by the past owners. They also improved the drainage around the house when doing this. I can only assume it’s all-good as we had record rainfall this fall and the sump barely ran and I did not observe moisture on the foundation walls (basement was still humid in the summertime though!)

Anyhow, past improvements aside – with the foundation being wrapped with what I assume is a mostly vapour-impermeable membrane, I was not sure if all of the advice on installing a vapour barrier on the inside of the foundation is still applicable.

My main worry if I install a vapour barrier on the interior is that whatever moisture makes it past the exterior barrier might then get trapped in the foundation walls/concrete and without the ability to dry at a reasonable rate and may cause issues with the foundation itself. Is there any logic to that?

Since there doesn’t seem to be any interior basement moisture issues at the moment anymore, I was thinking that I should leave well-enough alone and just use mineral wool board to insulate the exposed foundation walls, allowing any moisture in the concrete to dry to the inside.

I know this would result in condensation forming on the foundation walls when they are cooler due to the insulation, but the mineral wool is not affected by this I understand, and I would think the water would eventually run to the floor and dry. There will also soon be a heat-pump water heater in the room presumably dehumidifying the space (though I don’t want to rely on this, just secondary).

Any thoughts on this?

I’ll add furhter that half of the existing basement is already finished and insulated – I didn’t intend to redo this part with new insulation/air sealing. I believe (99% sure) that the finished part was done before the foundation was wrapped where interior moisture would have been a bigger issue. The finished part has walls framed about 6 inches away from the foundation walls with vapour barrier and insulated with fibreglass batts – in the parts I can see there is no continuous direct contact between the fibreglass and foundation walls. I see no signs of moisture issues in the finished materials – hopefully this is OK as I didn’t want to demolish that area since it is pretty nicely done.

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  1. Charlie Sullivan | | #1

    The concrete itself isn't damaged by moisture. There's no reason to need to allow it to dry, other than to keep the wood that sits on top of it dry. So you should still follow the usual advice to use vapor impermeable insulation on the interior.

  2. Expert Member
    Malcolm Taylor | | #2

    If the existing insulated walls show no moisture problems I'd leave them well enough alone.

  3. advice_yesplease | | #3

    Hello there - thanks for this clarification Charlie - I don't know where I got it into my head that the concrete cannot stay wet. I'm sure I read somewhere that it degrades it in multiple places, but who knows what I was reading!

    Alrighty I will consider what I need to do to make the inside of the unfinished portion vapour impermeable along the foundation wall and (per my other thread about my weird joist bay area), make that portion vapour-permeable but air sealed. Still need to think about the best materials for those to avoid causing issues with moisture in the joists.

    Malcolm, thanks yes I think I will leave the existing finished space on the inside for now. That section is actually a bit easier to add some insulation on the outside (and the foundation wall is shorter), so I can only imagine that will be "better" since the concrete behind the walls will be a bit warmer. May have to be a future project!

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