foam/membrane over field stone foundation

Discussion in 'Remodel Forum & Blog' started by Markus, Oct 6, 2013.

  1. Markus

    Markus New Member

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    Hi,

    I recently bought a house that had a remodeled basement, but had water issues, so I had to rip out everything. I now want to do it right, and so I have a few questions:

    - My current plan is to put delta FL membrane over the field stone foundation and spray it 2 inch with closed cell sprayfoam. This would avoid that the foam directly touches the foundation wall. I also want to install a french drain and have the FL membrane go into the drain. Is this a good plan? I tried it on a small section behind the new modcon boiler I installed, and it seems to work great. My biggest concern is whether it is ok for the field stone foundation to be moist in the long run - do I need to have the field stone foundation dry to the inside, or will it be ok in the long run to seal it off?

    - I can not figure out what flooring would be ok. With the french drain there should not be too much water, but dew point etc. is of course always a problem. One constraint is that the floor hight is not great, so it should as little height as possible. I was excited about porcelain tile, but that seems very expensive since the floor is not particularily flat and a mud job would be required. I was hoping to just slightly level it, maybe put on garage epoxy or so. In the kid play space I was considering foam carpet tiles. Does that create moisture problems?

    Thanks in advance,
    Markus
  2. Dana

    Dana In the trades

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    Stone, concrete, or masonry foundations don't need to dry, not at all, in any direction, for any reason. There are examples of bridge foundations made of stone & concrete that have survived millenia of submersion and are still functioning fine. It's the wood in contact with foundations that needs to stay dry.

    Stone foundations also have lower capillary draw than concrete, and even if the footing is in water, if there's at least foot or so of above grade exposure (and reasonable roof overhangs to limit splash-back wetting), the foundation sill is pretty safe. But if there's any question at all, jacking up the house a hair and slipping the membrane between the foundation and foundation sill isn't as scary a process as it might seem.

    To put down a finish floor, even with French drains you would need both a ground vapor barrier (10mil poly, or membrane) as well as rigid EPS or XPS insulation between the cool concrete and the sub-floor. The amount of insulation required varies with local subsoil temps and climate, and whether the space will be mechanically dehumidfied. Got a zip code?
  3. Markus

    Markus New Member

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    Location:
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    Dana, thanks for your great advise. I am in Belmont MA, not too far :)

    Great to hear, so I will use delta FL with two inch closed cell foam from inside. The foam will seal the membrane on top. Good point regarding capillary draw, I was wondering about that after reading on Bulidingscience - It should be about a foot at least, so I hope I am fine. Anderson insulation recommended to spray the relatively small gap between wall and ceiling where the sill plate is with open cell foam, because it would be hard to fill it with closed cell they say. Do you think thats problematic (capillary draw) or advantageous (maybe sill plate dries a bit to inside)?

    What would you do for floor? The current floor is a pretty bumpy thin concrete floor. You say I would need a vapor barrier and insulation even for porcelain tile? I am worried insulation adds quite a bit of height. Is there a way to insulate without adding too much height? (xps - is 1/2 inch enough? - can I put some fairly thin flooring down directly onto it, e.g. plastic laminate flooring (snap lock), or do I need a concrete layer on top of XPS (ok for fire retardant?) ? I guess air pocket (delta fl dimple membrane) is not needed then, xps would be better? I guess foam backed carpet tiles don't count as insulation ;) ?
    The goal is something like tile or vinyl for laundry area and workshop, and plastic laminate or even wood laminate or vinyl or carpet for playspace.

    What do you think are the options without finishing - someone recommends to just paint the floor with garage epoxy ... ?

    One more thing: Do you happen to know how low a french drain pipe should be? It seems you don't want it too high to actually keep the water table well below the floor, but also not too low to not pump too much water. I will also have challenges since at a few places there might be shale / slate directly underneath the thin concrete. (any good company for that?)

    Best,
    Markus
  4. Markus

    Markus New Member

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    Location:
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    Let me simplify my flooring question:
    - What is a good low cost floor with no insulation - some leveling required (epoxy? stained cement layer? porcelain not good?)
    - what would be an insulated option that adds the least amount of height (e.g. self leveling cement - 10 mil poly - 1" XPS - laminate directly on top)?

    Thanks, Markus
  5. Dana

    Dana In the trades

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    Often times a bumpy concrete floor in a fieldstone basement is just a 1" rat-slab poured on dirt. You'll figure that out when you dig out for the French Drain.

    If it's a flimsy slab on dirt it's easy to demolish, then dig down 8-9", add at least 3" of clean 3/4" gravel (mechanically compacted), with 2-3" of EPS (R8-12) on top of that, with 10 mil poly atop EPS, and a 3" steel reinforced slab (wire mesh type is good enough- no need for re-bar), on which you can apply the finish floor. The gravel is a pretty good capillary break and allows any seepage to find it's way to your French drains UNDER the slab. EPS is both cheaper than XPS and greener, with a more stable R-value over time. (In 50-80 years as XPS loses it's blowing agent it's R value falls, eventually reaching the same R/inch as EPS was on day-1.)

    At the slab edges put a back-stop of 2" EPS extending vertically to meet the wall-foam as an expansion joint, mechanically and thermally de-coupling the slab from the foundation.

    If you try to cheap-out, pouring concrete over the bumpy stuff it usually ends up cracking, but with some digging you can end up at the same height, or even a bit lower if you like, as long as you don't undercut the foundation.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    If you find that you have a good, structural slab, and want to level the floor, there are products that would allow you to do that. One being self-leveling cement (slc). It's not cheap, and can create more problems than you already have, but if done well, will give you a really nice, flat floor to deal with. Both tile and any laminate or other snap-together floor want it REALLY flat (the laminate actually needs to be flatter than it would be for tile). Once you have it flat, the only company that makes a tile underlayment sheets approved for use on a slab (other than say cork) is Wedi, which is available in various thicknesses with the varying R-factor. There are a couple of things like EasyMat that is available in various thicknesses, but I'm not sure what R-factor you really need. The only tile underlayment that is approved for use over green concrete (less than 28-days old is considered green) is Ditra. This has an advantage for vapor management in that it has a grid of channels that run the entire length and width of the mat.

    Unless you have some insulation under the floor, or add some heat (not as efficient unless there's insulation), the tiled floor will be cold. Tile, though, once installed and the thinset and grout cure, is pretty imperious to any water damage, so has a leg up over laminate.
  7. Markus

    Markus New Member

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    Location:
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    Dana, I wish I could do that (and I'll investigate), but there are a few challenges: I suspect that there is quite a bit of slate directly under the rat-slab, and that is part of the reason for the little bumps. I know that at some places there is a foot of dirt/gravel first, but at at least two places there is slate right underneath. So it will be difficult enough to get a french drain in - I hope they can jackhammer a bit into the slate without damaging the foundation. I also suspect that I would quickly undercut the foundation. Reading some of your old posts I now lean towards: self leveling cement - 1" or 3/4" of XPS - 3/8 of plywood bolted in - laminate or whatever permeates.
    You think cracking is a problem? wouldn't the stack still hold together? I was dreaming that I could skip the plywood, but it seems its needed for bolting down and firecode.
  8. Dana

    Dana In the trades

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    Without site inspection and some test drill holes to figure out what's under the slab & where, and how the foundation meets the natural slate formation it's hard to make this call. If you need to put relief cuts into slate hardpan to protect the foundation while removing some slate-depth it's not too tough do do (a diamond blade & circular saw works if you only need a couple of inches.) If there's a LOT of slate that would have to come out it's obviously a bigger job.

    Whatever else you do, you WILL need a ground vapor barrier between any sub-floor and the soil. If you're pouring concrete over foam the vapor barrier needs to be on top of the foam, but if it's going to be a wood subfloor it can go either under or over the foam.
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