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Community and Q&A

Bentonite on basement walls

user-6512277 | Posted in Energy Efficiency and Durability on

I recently bought a house in Marine 4C climate zone that has an east facing daylight basement. The previous owner installed an interior french drain that drains into a sump pump. The drainlines for the downspouts were rerouted from the front(west, which slopes slightly into the house) to the north and south sides. The problems they were trying to resolve have apparently been fixed because the interior concrete walls of the basement are not damp and the sump basin is bone dry after above normal fall and winter rains.

I intend to finish the basement with a combination of foam insulation boards attached to the concrete walls and blown-in cellulose in stand off 2×4 wall cavities. I had always intended to use geotextile bentonite fabric or panels or trowel it on as a mastic on the surface of the concrete walls to prevent the possibility of moisture penetration. My question is this: Is bentonite a choice for front line defense against water penetration and do I need a vapor barrier between the bentonite and the foam board? Most applications of bentonite are on the exterior of the foundation walls. Does bentonite work on the interior surface or would water pressure make it peel off the walls? The stuff is not very sticky and getting it to stay on the walls in the first place is difficult. It must require wall prep and additives. Will bentonite sheets or mastic work on a concrete slab and will any floor finishes adhere to it.?

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  1. user-2310254 | | #1

    I would install r-15 of rigid foam and skip the air permeable insulation. If cost is an issue, consider using reclaimed foam, which is much more affordable than virgin material. The foam (properly detailed) will serve as a vapor barrier as well. Of course, it won't help you if you have bulk water issues. Have you read Martin's article on insulating a basement wall?

  2. user-6512277 | | #2

    I did just read the article. Thanks for the link. I guess I should leave the stud wall empty.

    What I am more concerned about is the trouble the previous owner went to in installing an interior french drain and sump. There was obviously a water intrusion problem at some point in the recent past(the drain work and sump pump look relatively new). Unfortunately the previous owner is deceased and the realtors were not interested in finding answers so I'm mostly on my own.

    My question relates more to waterproofing on the negative side. The concrete walls have been painted and the paint has adhered. I want to apply waterproofing and can only do it on the negative side. I don't know if hydrostatic pressure would cause a troweled bentonite on the negative side to fail. Or if a bentonite panel would work better. Or if bentonite is even the right product. I want to be sure to stop water intrusion before I go to all the trouble of finishing the stud walls. I surely don't want to rip everything out later.

  3. GBA Editor
    Martin Holladay | | #3

    Like you, I have never heard of the use of bentonite on the interior side of a basement wall. That is reason enough (at least for me) not to try it.

    There are standard measures available. Available "waterproof" coatings for use on the interior of a basement wall include Thoroseal, UGL Drylok, and Xypex. You could try one of those products.

    For more information on these products, and on the topic under discussion, see Fixing a Wet Basement.

  4. Dana1 | | #4

    Bentonite is clay, and will not block water or water vapor. When saturated-wet it's extremely slick- commonly used as a well drilling lubricant.

  5. user-6512277 | | #5

    Martin, thanks for the input.

    I noticed you put "waterproof" in parenthesis, as in "so-called". I have used both Thoroseal and Drylok on the positive side of concrete in the past but would not rely on either for a long term negative side application due to hydro-static pressure. My take is that bentonite is an impermeable layer on the outside of a concrete wall so it would be impermeable on the inside. I am trying to find 1/2" sheets and if I can locate them close by I may go that route. Will let you know how it works out.

  6. brendanalbano | | #6

    Wikipedia tells me that bentonite works as a groundwater barrier by swelling as it absorbs water. It seems like the method wouldn't work on the negative side:

    Buildipedia says that bentonite needs to be under 30-60 PSF of pressure to be waterproof, which further seems to indicate that your application is not advisable:

    It sounds to me like the pressure of the groundwater squeezing the betonite against the concrete wall is what makes bentonite impermiable on the outside of a wall. On the inside, you don't have that squeezing action.

    I'm just speculating here, as I have no experience with bentonite on either side of a wall, but it doesn't sound like a good idea, unless the manufacturer of the product you are buying specifically recommends using it like you plan to.

  7. user-6512277 | | #7

    Found this source:

    What about a TPO or PVC membrane that connects to an interior french drain?

  8. GreatLakesWaterproofing | | #8

    Old question but it popped up in one of my searches. Bentonite is a great product for exterior applications but a complete dud on the inside. Do not use it on any inside projects, either on the wall or under the flooring unless it's getting pumped through the walls or floor due to access restrictions on the outside.

    Bentonite works by swelling when in contact with moisture, if applied as a membrane during construction, the membrane might only be 1/8" thick but can swell (when in contact with water) to close to an inch thick with a consistancy of Silly Putty. Water pressure pushes this into holes and cracks, self-healing the wall. No other product that I know of will act like this living membrane.

    Interior coatings are fine but the waterproofing qualities are debatable and only with minimal water present. Stopping water has to happen on the exterior, drain-tile systems are only water management and prone to failure once enough dirt, sand or mineral build-up happens in the blocks or pipe system.

    Bentonite will block water and vapor, it's used for man-made ponds, dump linings, nuclear containment and of course, waterproofing. For existing homes like yours it's possible to pump a bentonite slurry around the foundation to seal up cracks and holes. The slurry is pumped through pipes pushed to the footing, some walls might require a hundred or more gallons of bentonite to fill the voids, now filling with water and dumping in the basement.

    If you can avoid using any interior membranes, you should, products like dimple board or solid plastic, either rigid or flexible, leading to your drain-tile will act as "mold-factories" they love the dark, wet area you've made on walls that used to be dry.

    Here's some more information on bentonite products for waterproofing foundations.

    1. Expert Member
      MALCOLM TAYLOR | | #9


      "Water pressure pushes this into holes and cracks, self-healing the wall. No other product that I know of will act like this living membrane."

      Isn't that much the same mechanism Xypex uses?

  9. GreatLakesWaterproofing | | #10

    I don't know enough about Xypex but looking over their website it looks like they only bridge small hairline cracks and it drys rigid. They have some repair coatings containing concrete blended with polymers that give them some flexibility but even this product is recommended for 1/16" or smaller cracks.

    De Neef is foam product that expands when in contact with water and along with Xypex, is a popular product for industrial applications. Both of these products are very expensive and definately follow the safe handling procedures if you use either, I've never seen them used in residential applications.

    I'm pretty sure both of these will dry after set-up and you can't smear them after that. Bentonite sets up like cake frosting, you could scrape it, move it and reapply it and it would still be effective, there's where I get "living membrane."

    One drawback is that it needs to be encapsulated and below-grade dirt next to foundations is a great place for it.

    1. Expert Member
      MALCOLM TAYLOR | | #11


      I'm surprised you haven't heard of or used Xypex. It's is not a niche product. It is routinely used for remediation in residential construction, and as an admixture at concrete batch plants for foundation walls.

      1. GreatLakesWaterproofing | | #14

        I've heard of Xypex, we don't use it in our projects. We have used Thoroseal before with good luck but those are usually side-jobs.

  10. dan_saa | | #12

    For commercial basement construction in property line condition, per recommendation of GC we specified Bentonite just as GreatLakesWaterproofing says. We also added Xypex to the concrete mix as "belt & suspenders" but assumed we could not fully depend on the Xypex due to cracking. Basically cheap just in case extra measure.

  11. Expert Member
    BILL WICHERS | | #13

    I really can't see bentonite alone acting as a waterproof barrier here. We use that stuff with directional drilling rigs as a sort of grout to help stabilize the bore holes, and is most certainly does NOT block water when used this way (water is used as the drilling fluid, and saturates everything regardless of the presence of bentonite). I like the analogy of it setting up "like cake frosting", but I would add that the pressure of a foot making a footprint in it will cause it to crack and break up -- it is certainly not very durable.


    1. GreatLakesWaterproofing | | #15

      Granular and powdered bentonite come in many different blends, I think the well-drillers use the most basic version that helps lube the drills and stabilize the holes. A high quality waterproofing bentonite will have added fillers to get into foundation cracks and holes, the bentonite granules swell when in contact with water plugging the gaps. Bentonite is also limited by the size of the crack, usually around 1/4" since the material will flow through. If the bentonite is allowed to set up properly (all the mixtures have different times) and there was a long enough column of it, it would stop your water flow.

      I was told that when the Deepwater Horizon Oil Well blew up in the gulf 10 or so years ago, the "mud" they packed in the casing was bentonite before they sealed it at the top. Bentonite is also used for sealing non-used well casings but these rarely have pressure.

      Sometimes I'll see factory flat roofs with a pile of grey stuff in a corner, that's bentonite they use to temporarily seal roof leaks, they shovel it in the wet area and it seals the holes.

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