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Community and Q&A

Bentonite on basement walls

Jeffrey Stark | 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. Steve Knapp CZ 3A Georgia | | #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. Jeffrey Stark | | #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. D Dorsett | | #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. Jeffrey Stark | | #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. Brendan Albano | | #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. Jeffrey Stark | | #7

    Found this source:

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

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