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Community and Q&A

Renovated foundation wall detail

devinmccarthy | Posted in General Questions on

I am renovating a duplex in Montreal, Canada (Ashrae climate zone 6: hot, humid summers & cold, dry winters). 

To get the minimum required ceiling height for a basement rental unit, we are going to chip the existing footing back and install a new basement slab level with the bottom of the footing (see pic). This is a common reno detail but it seems there are different practices out there, esp. with regards to drainage.

I would like general feedback on my approach witj regards to 1. basement drainage and 2. insulation and vapour measures. I have also noted some specific questions below.

1. For drainage I plan to install a waterproof membrane out front with a piece of rigid insulation or dimpled drainage mat over top to protect it. Given the underside of slab will be below the top of footing, I am not considering french drains (they would be above the slab); rather I am proposing to install a dimpled membrane underneath the slab, on top of a few inches of gravel, that will slope to a central drainage trench (course gravel covered with filter fabric) that will be in turn emptied by a sump pump when necessary.

2. For insulation, I plan to run horizontal rigid boards just beneath the new window wells 4′ from the foundation wall to keep the bottom of the foundations warm. On the interior of the basement I am showing spray insulation with the studs installed 1″ from the face of the foundation to avoid thermal bridging and moisture build-up in the wood.


Should I avoid closed-cell foam insulation given there is a waterproof membrane on the front of the foundation (will it trap moisture within the wall?) and chose open-cell?

Should I avoid installing vinyl flooring (effectively a vapour barrier?) on the basement slab if the dimpled membrane (lapped and taped) is acting as a vapour barrier underneath? If so, any recommendations on thin vapour-permeable flooring finishes?

Any tips on capillary break details under the studs and drywall where they meet the existing concrete?

Any additional tips are much welcome. Thanks!


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  1. onslow | | #1


    No experience north of our border, but I would hazard a guess that your footing must remain on undisturbed soil. How much excavating are you allowed right next to the footing edge? For your drainage plans, use washed STONE (not gravel). Gravel in my mind is not sufficiently open to water movement and will act more like a sponge. You will need a few inches of stone under the slab and possibly more if you have radon issues to address. It all could add up to enough depth to be disallowed. Cutting into the corner of the footing would be questionable as well. If you say this is a common approach then be sure to confirm before permits are pulled. Do you have support columns to deal with as well?

    If there are no footing drains outside the wall, I would very much urge you to at least consider putting them in a foot or so out from the footing edge, so as to not compromise the bearing soil. The indicated dimple mat, cant strip and membrane to footing bottom won't mean much if all the water you direct down to the footing bottom is not carried away.

    The four feet of foam sheet might be better integrated with the dimple mat by sealing the foam to the foundation with the waterproof membrane, then having the dimple mat drop any water onto the foam, where it will be carried out away from the footing. Placing the perimeter drain below the edge of the foam sheet would make sense then. However, ensuring the water path is continuous down the wall and out the sheet will mean taping all joints. And some prayer for the future.

    The footing bottom will be sitting on soil that will see whatever water resides at that level. Do you already have water issues? You are planning on a sump which is concerning. The footing will transport water up to the wall based on many things. I would guess that there is not a capillary break under the wall. Either way the footing will be doing its best to be as damp as the ground it sits on. Applied wood nailers will have to be rot proof and done with rustproof fasteners. The little 3/4 strips of foam are going to be of little significance thermally, the slab will be essentially in contact with the footing. Whatever temps your ground and footing get to will be reflected in the slab. I would suggest insulation under the slab but you are skating on depth allowance already.

    Fabric over the stone under the slab is not needed. A thick poly sheet is. You are correct to question putting sheet vinyl or similar over a slab that may have water issues under it. If you can get a good 2" of clean stone under all of it and poly over the stone, you might be okay. I would be doubtful though.

    If you use small bits of pvc flat stock under the baseplate to raise it above the concrete footing, you will go a long way toward avoiding a moldy mess. Still use pressure treated on the baseplate. The studs can probably be untreated. Keep the dry wall up off the floor and use a taller baseboard profile to hid the gap and make nailing easier. Float the baseboard off the floor level an 1/8th and do not use MDF stock. Use the PVC trims or solid wood that is back painted.

    I think closed cell spray foam is generally recommended for problem walls like field stone and such, so I will defer to others if they think the dimple mat sandwich is really and issue. I think the more relevant concern is keeping water away from footing and wall in the first place.

    Overall it sounds like a lot of questions could be coming your way from inspectors, so best to check on your plans and get cleared before you spend a pile of money.

  2. devinmccarthy | | #2


    Thanks for the feedback!

    Re: structure, I am working with an engineer with a lot of experience on this type of reno so for the concrete work I am confident what we are proposing is safe and has precedent. We plan to start the slope for the slab drainage mat right at the point of the footing and not disturb anything underneath.

    I am hoping the 4ft rigid insulation drainage board below grade will take care of most of the rain water run off away from the foundation. And if the water level builds up then the underslab drainage trench will fill and be emptied by the sump pump, which is there to deal with this scenario. The previous owner (we bought the building last year and it was built in 1946) didn't have any water leakage aside from when his pipe backed up once due to a clog.

    To install french drains would require underpinning which is $$$. Of course so is a future water issue but given there has not been a history of flooding, I'm trying to find a reasonable measure of improvement on a tight budget.

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