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Community and Q&A

Managing Moisture at Interior Wall Sill Plates

texaskyle | Posted in General Questions on

I am renovating a slab-on-grade home that was built in the 1970’s and we have a problem with moisture accumulating at sill plates of interior walls.

There have been multiple flooring types used over the decades and most recently was a tongue and groove wood floor floating on a ~3mm thin padding which was on top of a impermeable plastic sheet. The plastic was taped at the seams creating a totally impermeable barrier between the slab and the floor except at the walls. (~30% of the floor was tile that has been floated over)

We would like to go back to wood floors but I (the homeowner) don’t want to find an alternative to allow the slab to breathe and the moisture to dissipate. I have been given the options to “glue the wood down” after sealing the concrete or going to a tile that looks like wood.

What other options do we have?
Will sealing and gluing the floor (except under the sill plates) still allow moisture to accumulate at the sill plates in an unusual amount?

Any insight would be greatly appreciated.

Expanded Info:
The home is being renovated because of mold and piss poor construction techniques used in a “facelift” that was done 5 years ago. The home has been totally gutted, all sheathing replaced with zip, and closed cell spray foam going into the attic (conditioned attic) and in the 2×4 walls for improved performance.

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  1. nynick | | #1
    1. texaskyle | | #2

      oh thats interesting! thank you for sharing

    2. Expert Member
      DCcontrarian | | #4

      I really don't like the corrugated plastic basement subfloor products that are sold under a variety of names. I guess what I don't like is the way their marketing shows a complete absence of understanding of the building science involved. Here's what that website says: "This equalizes pressure so the concrete has proper ventilation to dry out. It also separates the floor so the cold concrete won't bring down the temperature of the room."

      The absolute last thing you want to be doing is allowing your concrete to dry to the interior, that just brings an essentially limitless supply of moisture into your conditioned space. What you should be doing is keeping that moisture out of your space in the first place. And while insulation under the floor is good, 3/8" of air space that is free to circulate isn't really the best option.

      There are four main issues that you have to deal with in a finished basement. The first is liquid water penetrating the basement from the exterior. If that is happening, you have to deal with it before doing anything else. The way to deal with it is redirecting the water and giving it a route out, either a sump pump or a drain to daylight.

      The second issue is moisture wicking through the basement walls. This is blocked with a vapor barrier on the interior.

      The third issue is moisture that is in the interior air condensing against the cool concrete. The nature of wood creates a particular problem. When air is cooled its moisture capacity decreases. When wood is cooled its moisture capacity increases, at the same atmospheric humidity it will absorb more moisture when it is cooler. A piece of wood that is permanently cooler on one side than the other will be permanently moister on the cool side, which will cause the wood to warp and eventually rot. Modern building codes require any wood in contact with concrete to be treated -- even if there is a capillary break to protect it from wicked moisture.

      The fourth issue is any moisture that gets into the finished materials through spills or leaks. There is minimal moisture drive in a basement to drive that moisture out of the finished materials. Moisture tends to move from warmer areas to cooler areas, a typical finished basement is warmer than the surrounding earth year-round, so the moisture drive is always from interior to exterior. But a properly constructed basement has vapor barriers that prevent moisture from escaping to the exterior.

      So I advise against using any sort of permeable materials in a basement. But hey, it's your house, if you really have your heart set on a wood floor, here's what you want. First, you have to make absolutely sure to keep exterior water out through grading and drainage. Then you need to have some sort of vapor barrier to prevent wicking, and an insulating layer to keep the bottom of the wood close to room temperature. If you already have sub-slab insulation you're set, otherwise a layer of closed-cell foam works as both a vapor barrier and insulation.

      Then you just have to make sure water never leaks on the inside.

      1. nynick | | #5

        What do you advise for an old basement slab with no insulation underneath? There are no water problems in this basement. I was planning to insulate the walls with CC or PolyIso, then use the corrugated plastic sub-flooring with taped seams on top of the slab. Next would be T&G particle board and then LVL. Basement would be conditioned.

        1. Expert Member
          DCcontrarian | | #7

          I don't like the corrugated plastic products at all, it's just the wrong approach for the actual problem.

          For real wood I'd like to see at least half an inch, and preferably more, of closed cell foam insulation. There are lots of ways of doing it, none of them great, all have their disadvantages.

          When putting down a floor there are basically two approaches. Either every piece of the floor has to be firmly attached to the subfloor, as with tile or hardwood planks. Or the whole floor can be firmly attach to the other pieces so that its in effect one single piece that is too heavy to move. It is either not attached at all or only attached in a few places to the subfloor. This is called a floating floor.

          In general, floating floors are most popular in basements because concrete is a pain to attach to (unless you're using adhesives). With a floating floor you can float the layer of insulation as well. There are also various ways of floating a wood subfloor over a layer of insulation and then attaching a non-floating floor to the floating sub-floor.

          If you want to do a plank floor, it needs to be nailed to something. One way is a floating subfloor. Another way is sleepers -- pieces of 2x4 lying on their sides that are attached to the concrete and insulated in-between. The wood planks are nailed to the sleepers.

          I would also look at wood laminate, which gives the wood look and allows for a floating floor.

      2. texaskyle | | #9

        I really appreciate your input. I am not sure if this changes anything but this is NOT a basement. Its the foundation of a single story home.

        1. Expert Member
          DCcontrarian | | #12

          It doesn't really matter.

          The slab is in contact with the earth. Earth has two problems: it's damp, and it's cold. The dampness you can address with a vapor barrier and capillary break.

          Earth under a house will stay at more or less the annual average temperature all the time. Around here that's about 55F. Let's say your house is normally at 72F, 50% RH on the interior. At 55F that same interior air is at 93% RH.

          There is an equilibrium moisture calculator (EMC) at:

          At 72F, 50% RH the EMC is 9.2%, which is great for wood, it will be stable and last forever. At 55F, 93% RH the EMC is 22.7%, which is well beyond the point where wood will warp, mold and rot. Wood that is consistently cold on one side and warm on the other won't last.

          I believe the channeled plastic products make the problem worse. They allow enough airflow that moisture can travel freely to the cold side. However, they provide neither meaningful insulation nor enough airflow to warm the cold side.

          1. texaskyle | | #13

            okay. Where did you get the 55F? Is that known for Texas?

          2. texaskyle | | #14

            After reading your comment a second time. If my home (which will have central dehum) maintains a <50% (for example) then even with the "sweaty" slab, the wood will remain healthy?

            Does it matter if the wood is in direct contract with the slab?
            If there is a permeable membrane/pad between the slab and the wood?
            What if the wood is glued down, what would the life expectancy of the glue be or do you know of a resource that has that info?

            TIA, you have been very helpful so far!

          3. Expert Member
            DCcontrarian | | #15

            The usual way of estimating the average annual temperature is getting your average monthly temperatures and averaging them together.

            To your second question, the only thing that helps between the wood and concrete is insulation. The moisture is coming from the interior. You're always going to have a moisture differential in the wood if you have a temperature differential. If I recall, the moisture content where wood starts to rot is around 19%. You might be able to stay below that if you keep your interior humidity low, but you're still going to have issues with the wood warping.

          4. Expert Member
            Akos | | #16

            Might require a bit of effort to get down south, but one of the insulated subfloor panels might be a good option for you. This avoids most of the pitfalls of directly laying hardwood over concrete and should be a much cheaper flooring install as it can now be nailed down.

            Making sure the moisture is managed around the site as Armando pointed out is the most important step so is sealing the slab.

      3. texaskyle | | #11

        Thank you DC! This is not a basement. Does that change your comment any?

        I am the home owner here and we do not like the feel of tile for our living room. If there was a way to get wood we would like to go that way.

  2. Expert Member
    ARMANDO COBO | | #3

    The first thing I would look before contemplating what flooring to install in your house is to tackle the source of the moisture. Check for good drainage outside your house. Typical houses in TX are built on flat lots, no positive drainage on the lot and few inches above finished grade, if any at all. Sometimes I even see brick bellow grade, perfect for moisture wicking.
    Homes should have 6"-8" clear slab above finished grade next to the house, and finish grade is best slopped at 0.5"-1" per foot. Also, I would not install grass for at least 36" away from the house, preferably with granular fill, gravel or crushed stone, and drip irrigation on low-water plants.
    Make sure you have good guttering and downspouts with a good management plan for the storm water runoff from the roof. If you don't have enough slope or if you have zero chance of positive drainage, create one to a swell and a French drain away from the house or you may need to install a sump with a pump.
    Again, the first thing on your priorities list is to stop moisture from coming in... after that, make sure you tear down at least 24" from the bottom of drywall, dry and treat the wall frame and sheathing with products like Kilz, a mold and mildew spray primer. Replace insulation and cover with new drywall and trim.
    After your wall is "healthy", there are plenty slab sealants and drainage matts you can install before you place down any kind of flooring.
    If you are going to install brick veneer, make sure you install mortar screens every 24"-36" to catch spills, otherwise you'll have a bunch of mortar at the bottom of the brick ledge providing a pathway for water to come inside.

    1. texaskyle | | #17

      Armondo, thank you! This is actually something we put off a couple years ago but we are doing it now. A circle drive way was installed several years ago and it trapped a bunch of dirt between the house and the driveway. It is a swap for serval days after a rain. I did not think that it would be contributing to the slab moisture levels though but that makes perfect sense.

  3. walta100 | | #6

    Is there a capillary break between the sill plates and the concrete?

    “I have been given the options to “glue the wood down” after sealing the concrete or going to a tile that looks like wood.”

    If the homeowner can find a tile, they can live with I say put in the tile. Wood on a slab is an iffy at best and silly after a failed attempt.

    Tile on a slab is bulletproof and will last forever.


    1. Expert Member
      DCcontrarian | | #8

      I would put snap-lock vinyl tile in the same category. It's a little easier on the feet than ceramic, but very durable. I'd say it's slightly more demanding than ceramic in the flatness of the concrete, but slightly less work overall to install.

    2. texaskyle | | #10

      I dont this there is. If there was, its not there anymore. This is not in a basement.

  4. walta100 | | #18

    If the house is gutted now maybe the time to fix it correctly and install a capillary break under the walls.

    “I have been given the options to…”
    Consider is your reputation for quality work worth destroying to keep this job? When the customer starts telling me how to do my job I begin to think I maybe at the wrong job site.

    I say do it correctly or do nothing. Easy for me to say it is not my pay check.


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