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Basement slab moisture

cwinters | Posted in General Questions on

Need advise. I have a 1400 sq ft finished basement in NC. Slab is drawing moisture up. Basement is 40% underground with a walkout basement door. Excellent drainage around home and yard. House was built in 1978. House was completely dug out, re-waterproofed, new French drains 3 years ago. No issues with walls drawing moisture or any leaks. Originally  installed lvp flooring in basement and noticed efflorescence coming through the joints 8 months after install in 2019. We hired a contractor to remove all flooring and baseboards, grind the slab, and seal with an expoxy. Company specializes in concrete flooring and expoxy. We than installed a raised underlayment and new lvp flooring. Now almost two years later we noticed efflorescence in three different spots so we removed the flooring only to find more water. Along the edges of the wall it’s completely dry. Moisture is wicking up through the slab. Any solutions on properly fixing the issue? Thanks

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Replies

  1. PLIERS | | #1

    What was the raised underlayment and how thick was it? If you have ceiling height I would raise the slab. What kind of ground is the house built on?

  2. Expert Member
    KYLE WINSTON BENTLEY | | #2

    In a new construction, there would be a capilary break (usually 4" or so of 3/4-1" gravel), layer of insulation (climate depending) vapor barrier, and then concrete. If you're experiencing mositure migration in the field of the slab it indicates that you have neither a capilary break, or a vapor barrier.

    The real solutions involve a bit of work, but aren't really complicated, just time consuming and disruptive. The topical approach hasn't worked, unfortunately.

    Slice up the slab, add an interior drainage system while it's easy, for good measure, gravel, compaction, insulation, vapor barrier, and concrete. You'll never have water related problems again.

    1. cwinters | | #4

      Not wanting to remove the entire basement slab and start over. Thanks for the advise though.

      1. Expert Member
        KYLE WINSTON BENTLEY | | #14

        I agree that it's not something I'd want to do either. Maybe another "hope it works first" solution is a high compression dimple mat type material used for horizontal surfaces. Henry has a few different products, like the DB200, and has higher compressive strengths on up. Youd need somewhere to divert the water to, which involves cutting a bit of the concrete as it is.

        You'd still have water, and all of the risks of water, but it could potentially mask the problem until a later date.

        I still look at it as taking X amount of work to get it right, and for the amount of work, Y, its going to take to implement a hopefully patch, you're going to be ahead with X, which is smaller than Y + X.

        I'd hate to buy more material, and then rip up another floor when you know good and well that it's going to be wet under there.

        I agree, all of the solutions still aren't fun.

  3. PLIERS | | #5

    That underlayment is very thin, if you don’t want to rip out your floor you would have to solve the water issue and/or at least raise the floor. How much water are you talking about, was it a pool? How much ceiling height do you have?

    1. cwinters | | #6

      I’m assuming I will find the same amount of water as I did before the expoxy. I’ll attach a pic. I’ve only taken up a few pieces of flooring where the efflorescence is and noticed water. Enough to soak up with a rag. Ceiling height is 7’ and smaller in the areas of plumbing where the drop ceiling is. Older house so not much room to play with vertically

      1. Expert Member
        MALCOLM TAYLOR | | #7

        cwinters,

        It's often hard to know what measures will solve these water problems. To guarantee results you need to take extensive, disruptive action - which is understandably is not very palatable. If you want to avoid that, the next step I would try is adding a sump. It may be able to drain enough water to relieve the hydro-static pressure under the slab, and keep it dry enough that the dimple backed flooring will be okay.

        1. cwinters | | #10

          Consider it but after don’t think it will cure the problem. I don’t believe this home has pressurized water underneath the slab but doesn’t have moist grounds and no vapor barrier.

          I also believe the epoxy that was installed had not properly cured. It was tacky to the touch. We let it cure for 7 days before covering up.

          1. Expert Member
            NICK KEENAN | | #13

            Moist ground and no vapor barrier will cause moisture to rise through capillary action, but not to the point of puddling. Capillary action is moisture moving from wetter material to dryer, in order to defy gravity there has to be a difference in moistness. If there is standing water at the top the top is at saturation and the bottom can't be moister.

  4. Expert Member
    NICK KEENAN | | #8

    You've got to figure out where the water is coming from to fix it. From what you wrote I don't think you can assume water is coming up from below. It very well could be running down undetected somewhere and pooling under the floor. It's much more common for water to flow down than up, it obeys the laws of gravity after all.

    Are you sure your french drains work? Do you know where they exit, and does water come out of them? If you have clay soil any water that gets into your cellar hole is going to want to come into the basement because it has a hard time getting through the clay. In a big rain it would be nothing for a few cups of water to run down the sides of the foundation and find their way into the basement through a pinhole.

    1. cwinters | | #9

      French drains are working correctly. I can see where they exit into a drain box. No there is no leaks or water coming from anywhere else. The perimeter of the basement is dry. We’ve tested the whole area with plastic and than this raised barrier and the results are the same. Moisture is coming Up from the slab. I agree no gravel or vapor barrier where installed when slab was poured.

      1. Expert Member
        NICK KEENAN | | #12

        In order for liquid water -- as opposed to moisture -- to come up it has to be under pressure. You can test that quickly by drilling a hole in the slab where the water appears, if there is pressurized water there it will bubble out. You don't need a big hole. Since there's nothing under the slab you've got nothing to lose. Until you've proven conclusively that the water is coming up I would assume it's coming down, because it's just so much more common for water to come down.

        Do you know what level your french drains are at? In retrofits it's really common not to put them below the footings because you can't get down there without undermining the foundation. French drains only provide protection to the bottom of the drains, if the floor level is below the bottom of the drains water that gets past the drain will end up in the basement.

        No matter what the source of the water, the solution is the same, you've got to give it a path to go somewhere else. There's no magic. The best way is give it a path where it flows by gravity. The other option is to give it a path to a place where it collects and you can pump it away with a sump pump.

  5. PLIERS | | #11

    Ceramic tile or painted concrete might be your cheapest option because they can get wet and be ok. If you want additional floor coverings cheap indoor outdoor rugs to dry out or toss when they get wet. You need to solve the water issue if you want anything else on your floors. Is condensation a possibility and it has nothing to do with rising water? Humidity can be controlled with dehumidifier, although I don’t see that happening at that level in the winter

  6. cwinters | | #15

    I’ve heard of acid staining concrete and using a water based sealer which will allow the slab to breathe. I’m not wanting to have concrete floors but would be better than wet lvp flooring. Would also entertain the idea of pouring a thin base of concrete with heating strips so it’s not cold to walk on. Worried it will crack and break up years down the road. This is my forever home and don’t want it to look cheap either. Footer drains where installed below footers. I watched the whole installation and helped out. Also has a 3’ tall gravel with drain and silt to help keep mud and clay out. Perimeter of the basement seems to be dry. Once flooring is up I can confirm but that’s what happened the first time this happened. Thanks for all the advice guys. Has to be other older homes with same problems. Sadly a roll of plastic before slab being poured would of solved all these headaches.

  7. Expert Member
    NICK KEENAN | | #16

    I don't think plastic under the slap would have helped. Gravel, probably.

    We have red clay here in DC. It's basically waterproof. If you dig a hole it will fill with water and form a pond when it rains. When you dig a basement you do everything you can to keep water out in the first place, but you've got to give it a way to drain naturally. The amount of water that needs to get in to cause problems is so miniscule that as a practical matter waterproofing is impossible, just a single pinhole will let in enough if water starts to build up in the hole. One of the things that they do now that they didn't used to is to put gravel under the slab, so that liquid water that gets underneath isn't trapped and can make its way to the perimeter drains. You might be able to achieve the same effect with trenches across the floor.

  8. PLIERS | | #17

    If you just want a warm floor and don’t care about a hard surface I would think you can kerdi board the entire basement and use ceramic tile over it. I wouldn’t use it because my kids play in the basement but I would think water wouldn’t bother any of those materials. I’m not sure if any of the experts would think that it would be a failed installation but I don’t see any issues with that. Especially with a half inch board plus tiled surface would raise the floor. You might still have issues with leaving anything that can mold on surface because water will find a way up. However the building scientists on this site would know better. If you can’t get water out you have to live with it and that would be mean only using materials that dont care about moisture concrete and tile.

  9. Expert Member
    NICK KEENAN | | #18

    I'm generally not a fan of dimple mat, but I think this situation might be a good application. You need to have a drainage plane to get water that gets into your basement hole to the perimeter drains. Normally this is a layer of concrete under the floor. If you don't want to break up the concrete (understandably), you have to make the upper surface of the concrete the drainage plane. Dimple mat would allow water to flow freely across the concrete, and you'd need some holes around the perimeter so that water can drain down into the perimeter drains. The dimple mat has to be ruthlessly sealed, you don't want water to have any ability to evaporate into your living space.

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