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Basement moisture vapor issues – would like to finish basement

ScottSS | Posted in General Questions on

I’ll try to make a long story short – I would like to finish the majority of my basement, it’s around 2700 sq. feet, will leave some unfinished for storage.  Flooring would be either laminate or vinyl planks.
Newer house, less than 5 years old. 
Climate zone 5
No vapor barrier installed under basement floor.  
Dehumidifier has been running non-stop since the floor was poured – maintains about 45% RH in the winter, up around 50-54% in the summer.

Testing has been done (except for pH):
Average RH value of slab at 40% depth is 94% (tested in 5 areas)
Average MVER result of slab is 6.1 lbs/1000 ft (tested in 5 areas)
High sodium: 1700-1200 ppm, 1-3 mm below surface
High potassium: 2600-2200 ppm, 1-3 mm below surface
High chloride: 1091-1189 ppm, 1-3 mm below surface
Total 5512-4688 ppm, max level for epoxy is 1600 ppm… urethane is 4800 ppm.

I tried a sodium silicate sealer, it seems to have stopped some transmission as seen by objects placed on the floor no longer showing marks when moved or cardboard isn’t molding.  This doesn’t help much in the expansion joints, and it seems like it may have caused a larger issue in possibly increasing those levels tested above.  It did not impact the RH or MVER ratings at all!

I was looking into epoxy coatings, but the above sodium/potassium/chloride tests were all too high, risking delamination.  The contractor states that a urethane product would be able to handle those numbers, but at 2x the cost of epoxy.

I’m looking at alternative solutions, I need to eliminate the vapor infiltration from the slab, but i’m not sure if that is completely necessary once the space is finished/conditioned.  I’ve looked at dimpled membrane products like Delta-FL, DMX 1-Step, but am worried that if I don’t seal off the edges, they will not support an equilibrium within the air gap they provide, leaving the moisture to leak into finished areas with drywall or some other material.

I’m at a loss – am I forever doomed to have an unfinished basement, or is there some way to get this done besides tearing up the floor and installing the under-slab barrier?

I’ve read through this article, but it’s still not quite clear as to what I can do with the results above:

I’d love to get 30 minutes with the author – I believe contributes here!

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  1. Expert Member
    Peter Yost | | #1

    Hi Scott -

    Finding a finish that is water and vapor tight for concrete slabs is difficult, as you know from personal experience and as you have discovered in Joe Lstiburek's BSC piece you cite.

    In our basement, the concrete slab has no moisture barrier underneath. We slowed the vapor transmission with a negative side waterproofing (see this blog:

    The rest of the vapor load we manage in the summer by either opening up our 7 basement windows when it is dry outside or with a dehumidifier when it is humid out.


    1. ScottSS | | #3

      Thanks Peter. Looks like you and I are from the same state (although I don't live there anymore).
      I originally looked into a company that did the negative side waterproofing, I assume you went with the drainage pipe around the interior walls with sump pump?

      My hesitation is that I don't have a full blown water problem, just a water vapor problem. I would probably have to use the dimpled membrane on all concrete surfaces in my basement, or use the Schluter Ditre product and do tile, just to be sure I don't have water condensing anywhere. I'd have to get a price on that, and consider that cost vs. the headache and price of ripping out the floor and installing those drain pipes (just in case, since it's open), and a proper vapor barrier under the slab.

      I believe my problem stems from a spring of some sort, or water table on ledge, but I've never had water infiltration, just humidity. What I do know is that my footing drain runs year round, an at a good rate.

  2. ohioandy | | #2

    Consider a finished flooring that is more resilient than plank: tile. Just this week I'm helping a friend with a massive insurance claim on a basement flood. Seven years ago I put in 2,000 square feet of the click flooring, and after a minor plumbing leak went undiscovered for a couple days, ALL of it has to come out. How about tile installed over Schluter Ditra or similar, then add area rugs for comfort? The isolation membrane gives you the vapor retarder you're missing, and any water issues won't hurt anything after you roll up the area rugs.

    1. ScottSS | | #4

      Tile is on the list, but with no water infiltration, I wasn't worried about flooding - although if (when) my footing drain fails, I'll be in trouble!

      I do understand and agree with the logic, but tile floor, unheated, in the basement just didn't seem like the way we wanted to go, but it remains an option, depending on how we deal with the humidity.

  3. ScottSS | | #5

    Sorry for resurrecting this thread, but it has all of the info, and searches are not providing the exact answer I'm looking for.

    I'm moving forward with finishing my basement. I plan on doing the walls with 2" XPS, 2x4 framing with bat insulation, sill seal under the pressure treated lumber.

    I want to reiterate - I have never had water infiltration, only high humidity. No vapor barrier under the slab. I found a product called Kovara, which is marketed as an alternative to expoxy (since I cannot reliably do epoxy on my floor). The product is not cheap, but claims to provide the vapor barrier i'm looking for. My thinking is to install the Kovara right up to the framing, sealing off the floor. Flooring choice is LVT.

    Looking through other questions and reading articles, I'm seeing a lot of mention about dimpled membranes, but that allows the moisture to escape through the edges (hitting my drywall?) and doesn't seem like a reduced humidity/mold situation - it only protects the finished floor.

    I'm wondering if Kovara is really any different than laying down and sealing poly, or other products like Pergo Gold (which claims to be a vapor barrier), which are much less expensive. What happens under these products? I assume there will be oxygen coming through the slab with the moisture, but with a product like Kovara or even Pergo gold directly in contact with the floor, will that simply prevent the vapor from coming through, causing it to stay contained to the slab and any small air gaps that exist, basically coming to a stasis point (per Joe's article)? I have some radon that I'd like to mitigate as well, so the sealed option is ideal (it's low, but still there) - there is a radon pipe already installed, just no fan yet, so I always have that option going forward.

    1. GBA Editor
      Brian Pontolilo | | #6

      Hi Scott,

      I don't much about the products that you are considering. I do know that many people in your situation install poly sheeting over the slab, lapped on to the bottom of the walls when there is no vapor retarder beneath the slab and they have concerns about vapor. Seems an even better solution is a layer of rigid foam that provides some vapor control and some thermal value (yes, some builders do both). Both also can be detailed as air barriers to keep moist basement air from reaching the cool slab where it can condense. Are you opposed to insulating the floor? If you don't have water issues, these are pretty common approaches. Read more here: Installing Rigid Foam Above a Concrete Slab

      1. ScottSS | | #7

        I did read that up to the cutoff (i'm not a prime member) - I was about to sign-up for the free trial so I could finish the article since it cutoff right where the 6 mil poly came up (great editing!).

        The issue with rigid foam on the floor is losing head space and then having to redo the basement stairs, as well as the floor being too high for the walkout doors. Would placing poly and then 1/2" foam be enough? I don't know that I have the space for 1".
        I wasn't so concerned with condensation as I was with vapor transmission - once the space is conditioned, I suppose that could be an issue I haven't considered.

        Kovara has 2 products I'm considering:

        1. GBA Editor
          Brian Pontolilo | | #8

          Hi Scott.

          I understand the challenges of headroom and flooring heights. Putting down a 1/2 inch layer of rigid foam over poly shouldn't cause any problems, but it won't provide too much thermal value, and you'll need some type of subfloor which raises the total height of the floor another 1/2 inch, at least.

          Hopefully a GBA user with experience with the kovara products will see this thread and be able to offer some feedback. If you don't get any opinions on it, maybe try starting another thread with the product name in the title.

          1. ScottSS | | #9

            Will do, thanks! I haven't searched for Kovara in the forums yet, i'll try that as well.

    2. Expert Member
      Dana Dorsett | | #10

      >"I plan on doing the walls with 2" XPS, 2x4 framing with bat insulation, sill seal under the pressure treated lumber."

      Can I convince you to change the spec on the foam? XPS has no place in a discussion on "green" building.

      At 2" XPS is labeled R10, but if you read the fine print it's only warranteed to R9, and it's highly likely that it will drop to ~R8.4 (the same as 2" EPS of similar density) before the house is demolished/replaced. The higher initial R is due to the slow diffusion of the HFC blowing agents used (a mixture of HFCs, the predomint component being HFC134A). These HFCs are EXTREMELY powerful greenhouse gases (HFC134A runs about 1400x CO2 @ 100 years), and make XPS far and away the LEAST green insulation in common use today:

      By contrast, EPS and polyisocyanurate are blown with much lower impact hydrocarbons. EPS is blown with a variant of pentane that has a global warming potential of 7x CO2 @ 100 years, but most of that blowing agent leaves the foam at the factory, where it is recaptured, not vented to the atmosphere. The performance of EPS does not depend in any way on the blowing agent (which is 99%+ gone in the first few months post manufacture), and remains stable over many decades.

      If you need R10, use 2.5" of Type VIII (1.25lbs per cubic foot) or Type II (1.5lbs) EPS, or 2" of graphite-loaded EPS. If the basement has no history of flooding, 1.5" of foil faced polyiso or 2" of fiber faced roofing polyiso can be used, as long as the cut bottom edge of the foam has a 1/4" of air between it and the slab.

      BTW: An inch of EPS under the bottom plate of the studwall provides an adequate capillary & thermal break to keep the wood from wicking excessive ground moisture or accumulating excessive moisture from humid summertime air. If using polyiso for the wall foam, extending the EPS an inch beyond the stud framing, but leaving a ~1" nominal gap to the foundation wall allows the polyiso to rest on the EPS. The 1" channel between the edge of the EPS and foundation wall redistributes incidental bulk water incursions (but not total innundations, of course) sufficiently to keep the polyiso from wicking that water up.

  4. 730d | | #11

    I have found that a radon mitigation system designed to put a slight vacuum under your slab can lower the humidity in the basement in a measurable way. If there is tight soil under your slab or continuous footings dividing your basement slab into separate sections than maybe not so easy.
    Maybe worth considering ?

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