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Community and Q&A

Basement / crawl space fix

dirtcraft | Posted in GBA Pro Help on

I’m doing a renovation on a house that is half crawl space and half basement. The crawl space is part of the original house (circa 1950) and involves a concrete footing with a pressure treated knee wall built on top of it to support the floor joists and above wall (and the rest of the single story house). It is skinned with PT plywood and backfilled to a depth of about 4 feet with the local silty dirt. It was insulated with pink fiberglass batts and had an interior vapor barrier. We’ve pulled off the VB, removed the insulation, and beside being full of rodent droppings, the actual wood seems in decent health. This is a very dry area of the country (Okanagan region of Canada, getting about 12″ of rain annually).

The other half of the house was built in 1996 and is on a full concrete basement. The interior walls were 2×4 framed and contained pink batt insulation, a VB, and drywall. This has all been pulled out, save for the framing. There is evidence of water staining along the bottom of the walls, more pronounced in some areas than others. This is odd given the dryness of the climate but it’s there.

Both spaces flow into one another, with no separation, and given that there are not vents, and it was insulated, I would consider it a sealed crawlspace.

So, I’m trying to decide what the best way forward is. The catch is that this house is in a “red zone” and the geotechnical engineers will not let us touch the exterior walls at all. We’re left to live with the PT plywood and wooden knee wall. Does a person simply spray foam the whole thing or is there a better way forward?

Thanks for your help. I could send photos it that would help.


Ashley Lubyk
Penticton, BC. Canada

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  1. GBA Editor
    Martin Holladay | | #1

    In the basement section, it would be preferable if the interior studs didn't touch the concrete. If the studs are tight to the concrete, you might consider dismantling the stud walls and relocating them about 2 inches away from the concrete.

    Otherwise, it sounds like you are ready to call up a spray foam contractor and spray all the walls with closed-cell spray foam.

    For more information, see these two articles:

    How to Insulate a Basement Wall

    Building an Unvented Crawl Space

  2. dirtcraft | | #2

    Thank you Martin. I have 3 quick questions:

    1. It seems they are gapped about 1/2". If we don't move them, would this still be sufficient? I understand 2" though.

    2. In the crawlspace area, can the spray foam be sprayed beyond the 2x4 walls for additional insulation? Does this need to be fire protected?

    3. In the basement area, is drywall simply put up over the spray foamed walls? No vapour barrier necessary?

    Thanks again,


  3. GBA Editor
    Martin Holladay | | #3

    Q. "It seems they are gapped about 1/2 inch. If we don't move them, would this still be sufficient?"

    A. Considering the difficulty of dismantling and rebuilding the stud walls, it probably makes sense to leave the studs where they are, as long as there is a 1/2 inch gap between the studs and the concrete. Just make sure that your spray foam contractor fills the 1/2-inch gap with spray foam.

    Q. "In the crawl space area, can the spray foam be sprayed beyond the 2x4 walls for additional insulation?"

    A. Yes.

    Q. "Does this need to be fire protected?"

    A. This is a matter of code interpretation. The only opinion that matters is the opinion of your local building inspector.

    Q. "In the basement area, is drywall simply put up over the spray foamed walls? No vapour barrier necessary?"

    A. You shouldn't install any interior polyethylene before installing your drywall. Closed-cell spray foam is already a vapor retarder. In basements, many builders prefer to use paperless drywall, water-resistant drywall, or cement backerboard instead of ordinary gypsum drywall. If you do install regular drywall, make sure that you leave a 1/2-inch gap at the bottom, so that the drywall doesn't touch the concrete slab.

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