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Community and Q&A

Dimple Mat to Mitigate Water in Basement

vetitude | Posted in General Questions on

I have recently acquired a fixer-upper in Mid-coast Maine (climate zone 6A).  House is a 1906 balloon-framed Greek revival with a 65 yr old leaking roof, no insulation, no heating system, single-pane windows with panes falling out, some rotten sills, and a rubble/granite foundation/basement with large volumes of water running through it when it rains. I will have multiple questions for this project, but to start, I want to dry out the basement. I have read most of the posts and articles on this subject. Here is a link to the one most often referenced for those not familiar:
Also attached, a pencil drawing outlining the basis of my question, which in essence is, does anyone have a detail for the use of dimple mats for basements. In Lstiburek’s article there is a reference in photo 13 to a clever innovation.  Given the prospect of shoveling 14 tons of drainage stone through a basement window, I agreed readily that the dimple mat is a good idea, but there is no detail on how it meshes with the poly dropping down the wall, nor an illustration on which way the dimples face.
So, questions:
1. Can a dimple mat system solve my problem?
2. Or, a hybrid system-french interior perimeter drain (only shovel 2.4 tons of gravel) and integrate dimple mat drain?
3. Given the water that comes in under the “footing”, the large granite stone at the bottom of the rubble, I don’t think I want to dig my trench for the perimeter drain below the footing? Or do I?
4. I think the dimples should face the wall and the dirt–will the dimples eventually get embedded in the dirt, and lose their capacity to drain? Can I mitigate this problem with a layer of geotextile cloth?
I have emailed a couple of dimple mat producers, but thought the combined knowledge on this site might be a good place to start also.

Final tangentially related question for this phase of the remodel–chicken wire for holding closed-cell foam, or spray directly onto the poly surface, and if directly to the poly surface, does the type of poly matter? I see both systems in different articles. Not sure if I will actually pour a rat slab, but if I do, then building walls and insulating without spray foam may also be an option.

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  1. plumb_bob | | #1

    Hi Brad, why do you feel the foundation drainage needs to be inside the foundation? Excavating the exterior seems easier, and certainly easier to backfill. And then the water stays out of the house, period.
    The BSC article shows interior drainage because they could not work from the exterior for various reasons, but the first choice should be to capture the bulk water before it enters the building.

  2. Expert Member
    BILL WICHERS | | #2

    Dimple mat acts to maintain a small air space that allows for the drainage of bulk (liquid) water. Dimple mat is usually installed on the OUTSIDE of a foundation wall to help relieve hydrostatic pressure against the wall, which it does by allowing the water to freely flow down to drain tile around the footing. The idea here is to drain away water BEFORE it can make it through the foundation assembly, which is preferred.

    You can also install dimple mat against the INTERIOR side of a wall, in which case you need some kind of perimeter drain in the basement floor to carry away the water channeled down by the dimple mat. There is no issue with the dimples getting "embedded" in dirt, since they should still maintain enough of an air space to allow for free drainage. You do want to avoid filling tin the gap created by the dimple mat as much as possible though. Note that dimple mat is not likely to work very well against a rough or cut stone foundation wall -- it really needs to be installed against a flat surface like block or poured concrete.

    Since you mention that the water is getting in between the bottom of the foundation wall and the top of the footing, you might be OK with just a perimeter drain and no drainage plane against the wall itself. Waterguard is a good product for this purpose, but it's not cheap, and it will need somewhere to drain to, like a sump pump.

    You can't use chicken wire as something to spray foam against, since the spray foam will blow right through it at install time. You need a solid surface (which can be something like poly sheet) to spray against. The only problem with something like poly sheet is that the weight of the spray foam will tend to pull it away from the wall while it cures, so you end up with voids between the backing and the spray foam which you don't want. Spray foam is normally applied directly to walls for this reason.


  3. walta100 | | #3

    Seems to me if your dream is to turn this basement into living space, I think you need to refocus your efforts on the roof and windows. You have a lot on your plate don’t spin your wheels working on the basement.

    My guess is the rubble stone walls will dry out after you fix the roof, gutters and exterior drainage. Yes it may get wet from time to time but that is normal and expected when and how it was built.

    My guess is the basement ceiling is low before you add gravel and concrete. Digging out the floor is risky business with rubble stone.

    I say the walls have held up the house the way they are for a long time if you chouse to mess with them you risk breaking them and for what insulating the basement walls might save you 60$ a year and you will spend 10K $ seems silly to me.


  4. vetitude | | #4

    thanks for the responses.

    Bob, the cost of mitigating the water from the outside is much higher than from the inside, plus there are some attached structures that would need to be dealt with to make it all around the existing foundation. Then there is the risk of damaging a rubble foundation with the machine. Most operators will advise that damage is a possibility.

    Walta, totally agree with your points--this will not be living space, it just needs to be dry. Pressure tank, a water heater, eventually, the wall of solar controls and perhaps an ERV will land in this space. Maybe some mason jars with food put by. The dimple mat solution saves a ton of time. Likely just a bit more expensive than stone, but way easier to bring into the space.

    Bill, thanks for the input. The chicken wire question came from this article: If spray foam sticks to most poly surfaces, then I am not sure what the wire was for.
    I agree dimple mat on a rough surface may not work, hence my plan to use something like a Stego 15ml poly on the wall and have it direct the water to the floor drainage system that I hoped would be dimple mat on dirt. Then I would put down some blue board, a layer of poly and after that either a floating plywood floor or concrete if budget allowed, or just some old pallets if the budget indicates. Reviewing the Lstiburek article I noted he used the product over an existing concrete slab. It is using the product on dirt that seems novel. I found a product by J-drain that has a drainage system with filter fabric on both sides. I think that might work. And, I think you are correct, dimple down in reasonably firm soil is unlikely to get pushed completely into the dirt. In the end, the hybrid approach, shoveling in enough stone to do a perimeter drain, and using the dimple mat to replace the stone over the entire surface of the basement floor may have merit.

    Stay tuned, I have roofing questions, siding questions, and a few structural questions waiting their turn here at GBA.


  5. Expert Member
    BILL WICHERS | | #5

    I actually think using dimple mat directly over a dirt floor probably won't accomplish much. The dimples are likely to squish into the ground and you end up with no gap, and the entire point of dimple mat is to keep an open gap for drainage. I'd thought you were planning to use the material over a slab or wall where there is some kind of masonry and not exposed earth for the dimple mat to press against. Clean gravel is better when used against bare earth, and you are correct that a layer of geotextile fabric between the earth and the gravel will help to prevent the earth from gradually clogging up the gravel bed.


    1. vetitude | | #6

      Thanks Bill.

      With no bulkhead, and with no intention of raising the house I was really hoping to avoid shoveling a 4" bed of gravel in via a basement window. Geotextile cloth under the dimple mat seems possible, and J-Drain makes a product, J-drain 300 that still might work. That said, they say strength exercising keeps you young, so the shovel could win out.

  6. Expert Member
    Akos | | #7

    I'm in the land of two foot thick stone foundations, my home is one of them. These universally don't have any water proofing or drainage system and a lot (most?) have finished basements.

    With these, the most important thing to deal with first before looking at any interior waterproofing is site drainage. For example I have a place that had a river of water running through the basement with each rainstorm. To fix it, I had to move many trailer loads of dirt out of a backyard to get it to slope away from the house plus install sloped underground gutters as backup. The basement was finished without any interior dimple mat or any perimeter drain and has been bone dry since.

    If the issue is high ground water, unless you are ready to turn your basement into an inside out pool, no amount of water proofing will keep it reliably dry.

    After you have taken care of site drainage (slope, gutters and downspouts) you can look at adding in interior waterproofing but this should only be as backup. Typical detail has the wall dimple mat run a couple of feet up the foundation wall and drain into the perimiter drain. The spray foam gets applied directly over the dimple mat and the stone foundation above. No poly needed.

    Most slabs around me are poured directly over dirt, if you are lucky there is poly under it. With no ground water issues and a good interior perimeter drain you only need gravel under the slab if you have radon. In most cases you can pour the slab with just poly over the dirt . Adding an inch or so of rigid under the slab does make a big difference in comfort as it keeps the slab much warmer in the winter time, well worth the extra effort.

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