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Has anyone ever run a vapor retarder up the inside of foundation walls?

AGoldstein | Posted in Energy Efficiency and Durability on

I’ve been reading Pretty Good House, which I think is a great book and I saw on page 98 they show a basement wall detail with the vapor retarder under the basement slab turned up the concrete wall and taped to another yellow vapor retarder directly on concrete walls that presumably goes up and over the foundation under mudsill, is taped to sheathing and lapped over with WRB. (The picture doesn’t show this part but it is on the next page in a different wall detail.) This seems to me like a good way to keep moisture out of the basement but my only thought is if you apply some rubber or elastomeric damp proofing product on the outside of the basement walls before backfilling, unless you use a vapor variable membrane on the inside like intello instead of vapor retarder, it seems like this would trap moisture in the concrete. I was curious if anyone has done this and what they did on the outside of walls? I was thinking a drain board on the outside as an alternative to elastomeric or rubber could allow concrete walls to dry towards the outside (underground) but that might be impossible as the ground always has some moisture in it. I guess the bigger question is can a basement ever be completely dry considering it is moist underground at all times?

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  1. Expert Member
    DCcontrarian | | #1

    There's absolutely nothing wrong with trapping moisture in concrete. It makes the concrete stronger.

      1. Expert Member
        Michael Maines | | #10

        Those are some interesting and unusual claims. The first is sales copy from a moisture meter company; some of their claims may be true but if warm, humid air has a significant effect on concrete, somebody should alert the engineers who build bridges or builders in tropical climates where concrete is the main construction material. The second link focuses on microbial growth; if that really causes significant structural breakdown, then why are most existing, damp concrete foundations still in place? I have a BS in structural engineering and have been designing and building homes for 30 years. Concrete continues curing and getting stronger forever, though it slows significantly after the first 7-28 days. Moisture is required for the chemical reactions to occur.

        1. Expert Member
          DCcontrarian | | #12

          I guess there are places with dry soil but every place I've ever lived the soil below a basement is damp all the time. If moisture were a problem for concrete I think we'd know about it.

          I mean, we try to keep the moisture out, because concrete will wick moisture and it's bad for the rest of the house.

        2. Expert Member
          DCcontrarian | | #13

          The basement waterproofing business is chock full of charlatans and quacks. Whenever I hear someone talk about concrete needing to dry, or a "high water table," I immediately picture the AFLAC duck going "quack, quack, quack."

        3. AGoldstein | | #15

          True, the sources I provided are biased as they're selling products. I believe concrete used in underwater applications has admixtures that prevent washout when it's poured, so I think it's chemically different than concrete used on a home foundation. Isn't that right? But it's a good point. My concern is more about keeping the basement dry. If there's condensation it has to go somewhere, and it seems as the drier side will always be the inside of the foundation wall, vapor drive is towards the interior, condensation is likely to occur on the inside.

          If a vapor retarder on concrete walls was taped to vapor retarder under slab it seems plausible condensation could reach the perimeter drain or at least the insulation below the slab and stay out of the building envelope. I wouldn't want to run a dehumidifier constantly to keep my finishes in my future basement from rotting or growing mildew. I've never asked a client after building a house if the humidity levels of their basements are under 50% without resorting to a dehumidifier, but I have seen lots of musty basements, which is something I want to avoid.

          A couple people have mentioned avoiding putting poly directly behind sheetrock, which I know is a terrible idea. I only see that when pulling off sheetrock from a wall that was built decades ago when builders thought it was a good idea, which is why vapor variable membranes are used now. But I was simply talking about putting poly against concrete with an air space between poly and insulated 2x4 wall built in front of it.

          1. Expert Member
            DCcontrarian | | #16

            Everyplace I've ever lived the inside is the dryer and warmer side. Vapor drive can go either way in those conditions. But yes, you need something on the interior side of the concrete to block vapor from entering the living space. There can't be anything to the exterior side of it that can't stand getting wet.

      2. Expert Member
        MALCOLM TAYLOR | | #11


        Concrete dams and weirs, concrete pavers, buildings on piers, concrete lamp posts, retaining walls, exposed concrete cladding. As Joe Lsitburek said: "It’s OK for the concrete to be wet. The concrete doesn’t have to dry to the inside.”

  2. Expert Member

    A Goldstein,

    DC is right. However this is what Martin wrote in his blog:

    "Avoid the use of polyethylene. Basement wall systems should never include polyethylene sheeting—neither between the concrete and the foam insulation, nor between the gypsum drywall and the insulation. In these locations, polyethylene can trap moisture, leading to mold or rot."

    So no vapour-barrier, but the foam itself is a fairly effective vapour-retarder.

    Can a basement ever be completely dry? No, but I'm not sure that matters much. As long as it is kept at a reasonable RH by ventilation or de-humidification, the spaces should be comparable to those above ground.

    1. Expert Member
      DCcontrarian | | #3

      OK, the whole issue of whether basement walls can dry at all is pretty contentious, but I'll bite: if you have a layer of polyethylene between the concrete and foam insulation, what is there to rot?

      1. Expert Member
        MALCOLM TAYLOR | | #5


        Nothing. I think the rot is a problem in the second location Martin mentioned. Not to put words in his mouth, but maybe it should have read: Poly against the concrete can develop mold. Poly behind the gypsum can cause the framing to rot.

      2. Expert Member
        BILL WICHERS | | #6

        If you have a studwall, poly on the interior side of the studwall will trap moisture between the concrete and the poly, which can be an issue for the framing.

        I see no issue with poly directly against the concrete, and I have done that myself in my own home behind insulation, since I have a few walls that can weep, and I used a dimple mat and poly sheeting taped together behind EPS foam board so that any water that gets through gets carried down to a perimeter drain behind the foam. I see no issues here, since, as you said, there is nothing to rot there. Even if some mold grows back there, the entire perimeter of the dimple mat is sealed with polyurethane sealant to the block wall for that very reason (and to help with humidity control in the basement).

        Concrete doesn't care if it stays wet forever, it will just get stronger with time. Remember that concrete is the material of choise for hydroelectric dams, and they stay wet forever. There is no need for concrete to "dry", regardless of the direction it faces.


        1. Expert Member
          DCcontrarian | | #7

          Not only is there no need for concrete to dry, it's almost always a bad idea to try to let it dry. I say almost because there's no harm in letting an above-ground concrete wall dry to the exterior. But below ground it's not going to dry to the exterior. And you never want it drying to the interior, there's no circumstance where it's better to have that moisture inside your house than in the concrete.

          1. Expert Member
            MALCOLM TAYLOR | | #8


            Looking back at earlier blogs on the same topic I think the recommendation not to include poly against the concrete is a holdover from advice about walls with fluffy insulation:

            "Should I include a polyethylene vapor barrier?

            No. Basement wall systems should never include any polyethylene. You don’t want poly between the concrete and the insulation; nor do you want poly between gypsum drywall and the insulation. You don’t want poly anywhere.
            Paul Ellringer, an energy and mold consultant in Saint Paul, Minnesota, has a collection of slides showing moldy basement insulation. In most cases, these basement walls were insulated with fiberglass batts, and included two layers of polyethylene — one on each side of the studs. Ellringer calls this a “diaper wall,” and reports that most of them are a mess.

          2. Expert Member
            DCcontrarian | | #14

            I've seen my share of soggy fiberglass. But I think he's putting blame in the wrong place saying it's because the insulation is sandwiched by polyethylene.

            The conventional wisdom when building an assembly is that one side is going to be warmer and moister, the other side is going to be cooler and dryer, what you want to do is block moisture from entering on the warm side and allow it to leave on the cold side. Vapor drive goes from warm to cold and that should keep your assembly dry.

            If you can't build your assembly that way, because you can't dry to the exterior, you have a "wrong side vapor barrier" or WSVB. These are most commonly seen in unvented roofs. What you have to do then is put enough impervious insulation against the cold side so that condensation can't happen within the insulation. Even in the coldest climates there will be times when it's hotter outside than inside, in those times the vapor drive is toward the interior and you can get some drying of the assembly.

            So what happens in a basement wall? It's the ultimate WSVB. The outside is against the soil, which pretty much everywhere in the US stays below indoor temperature year round. So to the extent there is vapor drive, it is from the interior out. Interior air at 70F and 50% RH -- basic interior conditions -- has a dew point of 50F, which is right where the basement walls will be in much of the US. So any permeable insulation against the concrete wall is going to be susceptible to condensation.

            The basic solution is like any other WSVB, impermeable insulation against the cold side. But there's a twist: does a basement wall ever have a drying vapor drive the way an above-ground assembly does?

            It would have to be to the interior. Vapor flows from warm to cold and from moist to dry. In a conditioned basement, the concrete is always cooler than the interior air. It is possible for the interior air to be dryer than the inside of the insulation, so you could get a situation where the heat is pushing the moisture one way and the diffusion is pushing it the other. You might get some drying in that situation but it would be limited.

            I'm not a big fan of fiberglass insulation in basements, but if you're going to use it you want to have an impermeable layer of foam behind it, and you want it to be vapor-open to the interior.

            That's a long way of saying the problem isn't the plastic between the insulation and the concrete. That does no harm and can be helpful in keeping moisture from wicking out of the concrete. The issue is having a vapor barrier on the warm side of the assembly, because that's where you're going to need drying, coupled with not having something impermeable between the concrete and the fiberglass.

  3. Expert Member
    Michael Maines | | #9

    That foundation detail in the PGH book is not a blanket, "do it like this" detail, just one way that one of the authors has done it. On that project, a requirement was no foam inside the building envelope, so they used a little foam on the exterior and a vapor retarder on the interior so they could use fluffy insulation without worrying about moisture-related problems. That author uses WUFI as a hygro-thermal model so they are confident that it's a safe assembly. In that specific assembly, having a vapor retarder between the cellulose and the concrete is important.

    I typically use what I think is a simpler detail, though it does require foam on the interior. Concrete is a nearly ideal exterior material so I like to leave it exposed. I waterproof it below grade with either damp-proofing and a dimple mat or a rubberized elastomeric membrane. On the interior I use foil-faced polyiso, sometimes alone and sometimes with a stud wall and fluffy insulation. It's just as safe, or safer than the assembly in the book, but it does require foam insulation on the interior.

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