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Insulation and Drainage for Recently Poured Foundation

casabian | Posted in General Questions on

Hi everyone,

Brand new GBA member, excited to try out the service.

I purchased a home on Cape Cod and there were severe water issues in the basement because it was built on clay and lacked proper drainage. We decided to lift the house to solve the water problem and capture living space (bedroom, bathroom, game room, etc).

The foundation has been poured and the plan is to now waterproof with Tuff N DRI (instead of tar) and backfill with septic sand and a small amount of clay on top of the septic sand.

My first question is whether or not I should do a french drain around the perimeter. Because it’s sand, my contractor and the foundation guys think I’ll be okay without it.

My second question is the most cost effective way to insulate the slab. Foundation team said we should do plastic, then foam, then plastic and then pour the 4 inch slab.

Thanks for any thoughts and resources. I’m brand new to this and have been watching Matt Risinger and Home Renovision on Youtube.


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  1. Expert Member

    Hi Ed,

    You get one crack at providing good perimeter drainage. Maybe it isn't necessary, maybe it will become so in the future, when retrofitting it will be immeasurably harder. For the time and expense it represents it's a really good idea to include it now - especially if you are planning to use the basement as finished living spaces.

    For the slab: Rather than plastic under the foam, I would substitute a 4" to 6" layer of clear-crushed rock to act as a capillary break. Then the foam can dry to below. So rock, foam, p0ly, slab.

  2. andy_ | | #2

    In my experience foundation guys tend to be fairly cavalier about drainage.
    I wouldn't take any chances if you are going to be using the basement as finished space and have already had water issues on that site.

  3. maine_tyler | | #3

    I'm a bit confused by the meaning of: 'lift the house to solve water problems.' The house has been removed from the original foundation, the old foundation destroyed, and a new one poured in it's place? Or the foundation wall has been extended?

    Either way, it sounds like some trouble has been gone through in order to fix a water problem. It seems if such trouble is being endured, skimping on fairly standard drainage details could be considered pound foolish.

    Does the sand backfill drain to daylight? A French drain system doesn't merely provide a low resistance, isolated moat around the foundation—it provides a path of escape. Pipes and wrapped stone does this effectively, but I suppose in theory, well draining material could itself be daylighted to accomplish this. Consider, though, that sand holds a fair bit more water than clean crush stone (slower rate of percolation). Such a detail (omitting the footing drain system) would only make sense to me on a sloping lot (with nearby grade fall, such as with a daylight basement), and with well draining native soil. Even then, I'm not sure I could personally get myself to omit it. Call it insurance.

    Malcom's suggestion for: crush stone>Insulation>poly>concrete is the standard slab stack-up. One could argue that without an interior drain tile—and with a rising water table— the benefits of a crush layer is not fully realized. The 'most robust' detail would include an interior drain pipe system as well as the (more crucial) exterior system, especially if high water table is of concern.

    This is a good article here:

    Consider your insulation strategy at this point in the game as well.

    1. casabian | | #4

      Thank you all for the insight.

      @Tyler, yes the old foundation has been replaced with a new poured foundation. The lot is not sloping so it sounds like I need the exterior drain and a place (wick?) for that and gutters to drain to.

      I haven’t been able to fully understand how the water table works from my googling - this site is far more there a test or way to learn more about this from the town?

      For insulation strategy I was planning to do spray foam on walls in basement but would welcome a less expensive option.

  4. JayMart | | #5

    Hi Casabian - I'm building my own home in RI and I'm in the same exact boat. In fact, I'm having Tuff-n-Dri sprayed tomorrow on my new foundation. Would love to connect...

    1. casabian | | #6

      Nice! Shoot me an email with your number, could chat tomorrow. im edward dot casabian at gmail

  5. creativedestruction | | #7

    Don't skimp on the drainage-- this is the 'ounce of prevention' option. Install drain tile with filter fabric that stays below the slab level and run it to a sump or daylight. And positive slope at grade away from the foundation is your #1 defense.

    The company that makes Tuff N Dri also makes Warm N Dri, an exterior drainable fiberglass insulation for foundation walls. Depending on your climate zone it may meet energy code or it may not be enough.

    Malcolm's slab approach is best: rock, foam, poly, then pour. If you insulate the interior of the foundation, run the foam up the slab edge as well for a continuous thermal break.

    1. Expert Member
      NICK KEENAN | | #9

      This. In particular, "run it to a sump or daylight."

      You don't want to be relying on the ground to absorb the water. You want it leaving on the surface, at a spot where it will flow away from the house. Ideally you want the drains to exit at a spot lower than the foundation and drain by gravity. If there is no spot on the property that is lower than the drains then you want them to drain into a sump, which you pump up and out with a sump pump, to a spot at least ten feet away and downhill from the house.

  6. casabian | | #8

    Picking this one back up as they are finally getting ready to pour the slab.

    Builder is saying poly, foam, poly, slab. It is all clean sand in the basement.

    Should I demand crushed stone as the bottom layer?


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