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Interior sealing for damp basement — good idea?

[email protected] | Posted in General Questions on

I have a 1912 house in Michigan (zone 5) with an unfinished basement.  The basement walls are poured concrete and are often slightly damp, some places worse than others.  There is occasional water on the floor in heavy rains, maybe a few times a year.  The walls are painted and seem to have some sort of thicker, rougher coating under the paint, either an old Drylok-style coating or a thin skimcoat of something.  As I was prepping a corner of the basement for repainting recently, I noticed that some of the old wall paint/coating was coming off and in those places there is often efflorescence and spalling underneath — it’s like little blisters under the paint filled with dust.  Some are maybe nickel-sized, others a few square feet or so.  At most, the spalling has been maybe 1/2″ – 1″ deep, but most places are much less and there is plenty of the wall paint that remains intact with no evidence of efflorescence or spalling.  There’s no cracking in the walls and I’m not concerned about the structural situation currently, although it seems that another 50 or 100 years of this will probably lead to problems.  My current project is to remove the existing deteriorated coating where it’s come loose, and then scrub away the efflorescence and any loose material.  This will leave me with maybe 80% of the existing wall coating intact.

My question is how to best move forward from there without making things worse, and ideally improving things.  I have no plans to finish the basement (!), but a bit drier and cleaner would be nice.

The gutters are intact; the downspouts let out at least a few feet from the house and the grade slopes away from the house.  There’s simply a high water table in my neighborhood and many basements are damp.  Digging down to the footing from the exterior isn’t feasible due to concrete driveway/porch on two sides and a crawlspace on a third.  My understanding is that an interior perimeter drain isn’t feasible because the house likely has no separate footing (unconfirmed, just based on local practices of the era) and the slab is likely at the bottom of the wall, so digging down for a footing drain would undermine the wall.

This leaves me with interior coatings (or omission thereof).  Here is what I’m considering: skim coat of lime mortar as a sacrificial parging, followed by Thoroseal to manage water and then a couple coats of paint to manage water vapor and for aesthetics.  I’ll only be able to do the parging and Thoroseal on the little islands of bare concrete.  Are the Thoroseal and paint a bad idea?  I know some would say to leave the wall uncoated so it can dry to the interior, but to me it seems like slowing that evaporative flow of water from exterior to interior would be a good thing and slow down the efflorescence.  Is the sacrificial parging necessary if I also Thoroseal and paint?

If the paint starts to bubble off over the years, that’s not a big concern — I just want to avoid additional harm to the concrete.

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  1. GBA Editor
    Martin Holladay | | #1

    Have you read this article yet?
    "Fixing a Wet Basement."

  2. [email protected] | | #2

    Yes, but I'm not sure where to go from there. The easy exterior fixes have been done; exterior excavation isn't an option. I don't think we can do an interior drain either, unless someone has come up with a solution for old homes with no footings.

  3. Expert Member
    Dana Dorsett | | #3

    With a foundation that wet, have you measured the moisture content of the foundation sill?

    Without a capillary break between the sill and damp concrete sealing the interior is likely to increase the moisture content of the sill.

  4. canada_deck | | #4

    I am following this with a lot of interest.

    A few uninformed thoughts:
    1) If you try to seal the concrete from the inside, you may prevent the inside wall from getting wet but you are also eliminating the ability for the concrete to dry to the inside so the mass of concrete may be wetter for longer which is obviously not great and may even lead to moisture going higher up the block or spreading further across the block. Along that line of thinking, maybe you need to embrace the fact that your foundation will let some moisture in and concentrate on ensuring that it can dry properly and quickly when that happens.

    2) Perhaps you are underestimating the potential of water to flow underground and the ability to influence the local water table through water management measures across your entire lot. How big is the lot? You say the gutters drain a few feet from the house. An easy starting point would be for it to drain 10 feet away. Have you observed the gutters during a storm. Are they all working?

    3) Does anyone ever drill holes through the foundation to drain water into a sump pump inside of the basement? Is that a thing?

    1. Expert Member
      BILL WICHERS | | #7

      #1, concrete can stay wet forever and it doesn’t care. Remember that they build concrete dams to hold back rivers. Concrete likes to be wet. Capillary draw does allow the water to rise higher in a sealed wall though, which is why a capillary break becomes more important when you seal the wall against moisture with waterproofing, insulation, etc.

      #3, yes, “weep holes” are sometimes used to drain the hollow spaces in block walls. There are interior drainage systems designed to work with these holes to carry the water away. I suppose weep holes could drain water from the exterior of a poured wall to an interior drainage system too, but I’ve never tried that or seen anyone else try it.


  5. Expert Member
    BILL WICHERS | | #5

    Drylock makes a chemical cleaner that dissolves the efflorescence. You will probably want to use that, or something like it, while doing your cleaning.

    I had a similar issue to yours, although with a block wall. I extended the downspouts much farther out than usually, about 8-10 feet. That made a big difference. I still get too much water on that side of the house, so a French drain is in the works. I know in my case the surface water is going down into the ground and getting in, so drying out the surface will help.

    Do you have a slab floor in your basement, or just dirt? There are cut-in perimeter drain systems for slabs. You cut a slit around the perimeter, install the drain, concrete over the top and a little lip sticks up against the wall to catch any water.

    There are better water sealing coatings out there than drylock, but they’re all band aids if your wall is leaking — none will last forever. If you recoat, you need to completely remove the old water block coating so that the new one can penetrate the surface to bond. Surface prep is really critical.


  6. canada_deck | | #6

    Another uninformed thought here. You say:
    "Digging down to the footing from the exterior isn’t feasible due to concrete driveway/porch on two sides and a crawlspace on a third. "

    Practically speaking, there is very little water that should be entering the ground through the driveway or through the area covered by roof/crawlspace. Water can get through the porch but even that can be managed if needed (for example, you could go to a fiberglass porch deck and drain it away from the porch.)

    So you can now draw a big perimeter ring around which there is very little water coming in from above. You could dig that down as deep as you wanted and install a perimeter drain, potentially even with a sump pump.

  7. [email protected] | | #8

    Thanks to all for your responses.

    Dana, thank you for your point about the moisture content of the sill. The sill is actually spray foamed, so not really accessible to check. Hopefully the foam is providing a barrier from interior moisture, and it's drying to the exterior. Not much I can do about it anyway...

    Zephyr7, the floor is concrete slab, but I'm not sure (and the local reputable foundation contractor isn't either) how to do an interior perimeter drain when the bottom of slab is very likely to be approximately equal to bottom of wall, with no footing below. Where does the drain go without digging out and undermining the wall bearing?

    As for extending the downspouts, there's one that lets out just a couple feet from the house, but at an outside corner and onto a concrete driveway that slopes well away from the house. I could try to extend this one and see if it helps at all, but it's across the house from the dampest areas. The others let out at least 10' away from the basement. Gutters work well and we have them cleaned regularly.

    Canada_Deck, the porch is concrete slab as well and extends pretty much the full length of one wall, so there's definitely no water getting into the ground for 8' in front of that wall, and yet that's one of the two dampest walls. The other dampest wall has a concrete driveway along the entire length, which slopes away from the house. These two dampest walls are on the uphill side of the neighborhood -- the topography generally slopes toward our house from that direction for blocks and blocks, even though the grade within 10'-20' of our house slopes away from the house.

    The concrete driveway does have some cracks (enough for weeds to grow in), which I've never bothered to seal because it seems like the amount of water infiltration through those would have to be minimal to negligible. The driveway extends right to the house, and that joint is caulked (and also sheltered by the eave), so it's not coming in there.

  8. Jon_R | | #9

    You can install dimple mat on the floor with a sump pump to remove the water.

    Don't neglect making sure that slope away from the house is a water barrier (it doesn't help if water flows right through it).

  9. canada_deck | | #10

    Re: " These two dampest walls are on the uphill side of the neighborhood -- the topography generally slopes toward our house from that direction for blocks and blocks, even though the grade within 10'-20' of our house slopes away from the house."

    Again, I'm not an expert on this particular topic... but this looks like a key piece of information.

    It sounds like there is a swale (basically a low spot) 10-20 feet away from your house. If water is collecting there (from your driveway/porch/gutters in the one direction and from the prevailing grade in the other direction) and then absorbing into the ground, it's not a stretch to assume it may be working its way back to your basement underground - especially if the overall topography slopes in that direction.

    A simple measure could be to ensure that any surface water at that point gets drained to the downhill side of your yard. To get fancier you could drain a deeper cross section.

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