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Dry-stone stacked foundation, water and radon

user-7381202 | Posted in General Questions on

Gosh, do I need help w this one. It’s a 1820s house in Columbia County NY with a largely stone foundation and backed against a small mountain. The “root cellar” with mechanicals and a dirt floor is under the living room, the rest is crawl space with dirt floor or slab. The 10×14′ cellar has heavy springtime water ingress where the dry stacked stone meets bedrock. The previous homeowner reported ~12″ of standing water during a tropical storm Irene and the sump losing power. The exterior water ingress can only be managed so much by gutters/french drains, given grade against mountain. There’s no appetite for the cost of a full excavation to “footings” – if that’s even possible w this type of foundation. It’s literally dry stone stacked w/o mortar! There’s also a 3 month reading of radon at 4.0. So… I’m thinking a layer of washed gravel in the cellar is necessary to allow the water to flow to the sump. Then a radon-rated vapor barrier on ground/walls, taping best able around multiple mechanicals and piers (it’ll be a beast) – and adhered to where there actually is mortar, directly under the sill plate. Questions: 1) do you think an interior french drain to the sump is necessary? 2) spray foam insulation on walls necessary? had a few subs recommend, but worry about freezing/drying 3) would you pursue a radon system drawing under the vapor barrier or an HRV w radostat? I don’t want to unnecessarily over-engineer this… Oh, and there’s mice galor! So thinking of caulking sill/rim joists and filling w rock insulation btwn. Heard they hate that stuff. Many thanks in advance for any advice and/or validation.

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  1. user-7381202 | | #1

    Here's some pics.

  2. Expert Member
    Peter Engle | | #2

    That's going to be a challenging foundation and you have to be gentle with it because there is a significant risk of collapse if you change the loads in any serious way. Is it really just dry stacked? Sometimes these walls look that way, but there's actual mortar in the heart of the wall. Here's a few thoughts:
    -If stability is an issue (the wall is bowing inwards anywhere), excavation and stabilization are required. Spraying the wall inside and out with gunite would be one solution. Building forms and filling the space between the stone and the form with concrete vibrated into place would be another. Sometimes there's no choice. If you do this, at least you can finish with a smoother surface to work with.
    -If you can't do any better with the grading, you can intercept surface water with a curtain drain. Basically, dig a trench uphill from the house, line it with fabric and washed gravel, all the way up to the surface. Extend at least one end of the trench to daylight around the house and downhill. Install perforated drainage pipe in the trench and solid pipe to daylight.
    -excavating the exterior, applying gunite, a waterproofing barrier, and "footing" drains on top of the ledge would be my first choice for the exterior.
    -Don't count out spray foam. If you plaster (or gunite) the interior to bring it closer to flat, then add spray foam, that would control water pretty well. The water would be forced to run down through the loose stones to the base of the wall. You would still need a drain there to carry the water to the sump. Either dig the drain into the ground, or set it on top and add enough washed stone to come up to level. DON'T undercut the wall. Then add a plastic air/vapor/radon barrier and a slab.
    -The subslab drain can be your radon collection system for a vacuum-style radon system.
    -If you still need mitigation after all of this, you can consider a HRV/ERV. I would run it continuously, not on any type of controller.

  3. user-7381202 | | #3

    Thanks Peter. Very much appreciated... "gentle" is the operative word here I think. And appreciate the detailed guidance, esp emphasis on avoiding undercutting or destabilizing. I'll take a better look to see if there's mortar deeper in besides. Already have gutters, regrading, a curtain drain and swale for surface run off going in. May suggest pausing until their impact is clearer... Though I suspect with 500' of mountain above, the ground water is going to follow that bedrock. In zone 5a, do you think there's a risk of frost destablization if gunite/foam from interior only?

    1. Expert Member
      Peter Engle | | #6

      I don't think applying gunite/foam on the inside only will cause any problems. In fact, if you added wire mesh to the wall before gunite, you can probably improve the stability of the wall and its resistance to soil pressure. Frost heave acts in the direction of heat loss, so as long as your basement is warmer than the surrounding soil, you shouldn't have problems, even though the total heat loss decreases with the insulation.

      You're probably right about the ledge. If the water is perching on top of the ledge, it's going to follow the contour of the ledge into your basement. The only way to stop that above the house would be to cut actual channels in the ledge for the bottom of the curtain drain. An interior subslab drain might be your only option, placed as low and close to the ledge as possible.

  4. 1869farmhouse | | #4

    I had a similar foundation wall. I used high density spray foam on the interior side. So far, so good.

    I will note that there were no signs of structural issues, only water and cold coming in.

    1. user-7381202 | | #5

      Thanks! How far north are you?

  5. Deleted | | #7


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