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Community and Q&A

How to Insulate an Existing Basement Foundation

Devin_McCarthy | Posted in General Questions on

I am evaluating how to insulate an existing basement that has a rather peculiar condition along the concrete foundation wall. The original owner was a mason, and therefore he decided to install a block wall in front of the concrete foundation the full perimeter of the basement, on top of which he plastered directly and eventually added an additional layer of gypse. There is about a 1-2 inch void between the concrete wall and block. The original slab (very) likely has no vapour barrier underneath it.

In the fall the current homeowner paid for a new exterior drainage system which included a french drain at the footing, liquid-applied waterproofing membrane, delta drainage membrane, and 1″ of Durafoam (moisture-resistant EPS) insulation. The latter 3 elements extend only up to grade (see attached section detail. They have also added new stud walls about 1/4″ in front of the existing gypse/plaster finish, which has been demolished in most places but remains here and there. This leaves about an inch in front of the block. They also added a layer of self-levelling screed on top of the existing slab.

In late November, after the self-levelling layer was added they noticed a fair amount of condensation showing up on the painted gypse/plaster layer behind where they had installed fiberglass batt between the studs. The moisture in the latter in combination with the colder temperatures is what caused the condensation in my mind. But it was only – at least to the eye – on the upper portion of the gypse/plaster finish, starting from just below grade level up towards the joists.

I would like to know suggestions for how to proceed and insulate this assembly, assuming condensation will remian an issue if the concrete and block remains cold. I assume the pink batt between studs was not enough to keep these elements warm, and obviously not airtight enough to prevent indoor air from hitting them.

I am thinking of seeing if a spray-foam installer thinks they can spray the gap between the block and concrete if enough holes are drilled in the latter, and fill it with open-cell foam to permit inward drying. The upper portion of the wall would around the rim joists would be done in closed-cell (or open if that is acceptable?).

If that is not feasable, perhaps open-cell infront of the block, eventually forming a continuous chain with the closed-cell up top if used there. I’m just concerned if leaving the void between the block and concrete will be problematic. If not, I would also consider installing roxul batt between the studs, with a continuous layer of semi-rigid infront of them to remove thermal bridging, followed by an air barrier for air tightness, then gypse.


The building is located in Montreal, Canada. Hot, humid summers; cold, dry winters. Ashrae climate zone 6.


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  1. user-2310254 | | #1

    Hi Devin,

    I will give your post a bump and provide some feedback. If you haven't already, be sure to read this article:

    I'm sure the experts will weigh in, but I would suggest using closed cell foam. You don't want air permeable insulation -- fiberglass batt or open cell -- next to a cold condensing surface. You also should buy an inexpensive hydrometer and begin tracking the humidity level in your basic. If levels are high over time, you probably need to use mechanical dehumidification to avoid turning the space into a mold factory.

  2. Devin_McCarthy | | #2

    Hi Steve,

    Thanks for the link! Your suggesttion re: closed-cell, does that apply if it was installed on the interior side of the existing block wall? I had always followed J. Stiburek's advice about letting basements try to the interior but based on his updated recommendation in the link you sent it seems this would be OK. The block, concrete, and the void between them would remain cold, and in theory the concrete could dry near the top if needed, as noted in the article. I'm still a bit unsure about what moisture will do if it enters the void and block, but if the top of the foundation wall is not sealed, can we expect this elements to dry slowly when needed?

  3. user-2310254 | | #3

    Hi Devin,

    Maybe your reply will generate more input. FWIW. Concrete doesn't need to dry. If you don't have any bulk water issues, I would install enough closed cell foam to meet code and then monitor the humidity levels in the basement. It's possible you will need mechanical dehumidification to keep moisture in check (but maybe not).

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