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Basement wall insulation with interior dimple mat and spray foam

RH68 | Posted in Green Building Techniques on

This question has probably been asked a bunch on here, but I’m coming up short with my searches.  Here’s the summary, and see attached drawing:  Our partially submerged, very old basement in Chicago (zone 5) will be furred out with studs and drywall, have new windows installed, and will be insulated with closed cell spray foam to approximately R-20.  The basement will remain unfinished; we’re just trying to clean it up and regulate temperature.  We installed an interior perimeter drain tile and sump, and the system works great.  The foundation has a few vertical cracks typical of a house of this age, some of which are known to let a trickle of water in occasionally during huge storms.  Micro seepage cracks are present in some areas.  We plan to epoxy fill all the main cracks and install a dimple mat at the below-grade walls prior to insulating.  My question is this:  Is there any concern about mold growth behind the dimple mat after everything is in place?  It seems like the right thing to do, rather than rely on the foam to seal micro cracks or pores.

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Replies

  1. Zephyr7 | | #1

    Hydrostatic pressure within the wall would likely cause the spray foam to seperate from the wall over time. Even waterproofing coatings will eventually fail. My concern with the dimplemat is that the spray foam would be adhered to the dimplemat instead of to the wall itself, so what would be supporting the weight of the foam/mat assembly? It may be best to only put the dimple mat in relatively narrow vertical strips in the immediate area of the leaking cracks so that the foam is adhered to the wall in other areas.

    What type of foundation wall do you have? If it’s a hollow block wall, you may be able to “drain” the wall with weep holes drilled into the hollow cavities of the bottom course of blocks. This would likely eliminate the need for the dimple mat. If you have a poured foundation, then I think dimple mat is probably your only option if you can’t install exterior drainage.

    Bill

  2. Lee Newton | | #2

    Hydrostatic pressure is the strongest force known to man. Consider not filling the cracks and instead let the seepage flow behind the dimple mat into your perimeter drains. I agree with the concerns articulated by Zephyr7.

  3. User avatar GBA Editor
    Peter Yost | | #3

    Hi RH (be great to have a name for the GBA Q&A community to use) -

    Can you grow mold behind the dimple mat? Mold needs three things: the right temperature, food, oxygen, and water. Very likely that at some point in time, all three of these could support mold growth. Question is: if it is sealed off from your interior space, and the materials on the other side can not be degraded by mold growth, do you care?

    Mold growth will be incidental and spotty at best, if at all.

    Great resource here for mold: https://www.greenbuildingadvisor.com/article/common-sense-mold

    Great resource here for waterproofing basements: https://www.greenbuildingadvisor.com/article/installing-basement-waterproofing-from-the-negative-side

    Peter

  4. Zephyr7 | | #4

    To add to Peter’s comments about mold, if mold is a concern, there are basement wall waterproofing paints with mold inhibitors in them. I don’t usually look for that type specifically, but I think zinsser makes one (and I’m sure there are others too). You could apply that mold-resistant coating to the wall prior to putting up the dimple mat to lessen the chance of mold growing in the newly created hidden void between the dimplemat and the wall.

    Bill

  5. User avatar
    Peter Engle | | #5

    Lee,

    With all due respect, hydrostatic pressure is not the strongest force known to man. It is a relatively small and predictable pressure. Water standing against a foundation wall applies about 1 psi for every 2' of standing water. That's equivalent to about 144 psf, so much greater than typical floor or roof loading, but still small compared to the water pressure inside plumbing at 60-100 psi, or hydraulic equipment pressures at 3000 psi or more.

    What makes it so difficult to deal with is that it can be there for the very long term, and that water slowly dissolves many of our masonry building materials, making its own pathways into our basements.

    Epoxy sealing the cracks might help reduce the flow some, or it might not. Worst case, it is a waste of time/money, but it won't hurt anything. The dimple mat is a good way to drain away the water that manages to get inside. Make sure it is tucked into the perimeter drain and that the spray foam covers this joint and the top of the dimple mat to prevent moisture from escaping at these openings.

  6. User avatar
    Peter Engle | | #6

    RH,

    You mention that you will not be finishing the basement, but that it will have studs and drywall. This raises potential fire code issues with exposed foam. I know Chicago has its own building codes, but I expect that they are similar to the national codes (or tougher) for exposed foam insulation. In short, the insulation cannot be exposed to the interior of the house. It should be covered with drywall or another material with similar fire-retardant capabilities. The studs and drywall will be fine, but you should also make sure that the top and bottom of the wall cavities are closed and that the foam is not exposed in storage areas or other spaces.

  7. RH68 | | #7

    Hey everyone, thank you all for the responses. I'm still working with some waterproofers and spray foam contractors to get their opinions, but I think we're on the right track. I've been told that the dimple mat is a belt-and-suspenders type approach, but for a few hundred dollars in material it seems like a worthwhile thing, as long as it is sealed in there to lessen the chance for mold growth. And yes, it would need to fastened securely to the wall in order to provide a sturdy surface for the spray foam. The furred and insulated walls will be covered with drywall to provide the required ignition barrier. We'll probably epoxy seal the primary cracks, but skip the lesser ones.

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