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What to do about condensation on slab along outside walls?

Bert Thomas | Posted in General Questions on

I am building a house with a monolithic slab (frost protected) in a cold environment (Alaska). The house has a walk out basement. I have 3.5″ of foam under the slab, but not under the footing portion of the slab. The outside of the slab/footing is insulated with 2″ foam and there is 2′ of 2″ foam applied horizontally around the perimeter of the slab, 4′ on the corners. I was advised not to put foam under the footing part of the slab (now I believe ill advised). In the summer when warm air enters the building condensation immediatly forms close to the walls on the slab mostly on the back wall side that is backfilled about 7′. I know what causes the condensation (concrete footing in contact with cold ground meeting warm summer air). Is there a solution for this problem? The walls are ICF, additionally, what would be the best wall covering/trim down by the floor where it may get damp from the condensation? The foam ICF does soak up moisture from the condensation.

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Replies

  1. Riversong | | #1

    Yes, you were ill-advised. Standard 25 pcf compressive strength XPS can easily support a two storey house.

    Either the full slab, including thickened edges, should have been thermally isolated from the ground or an independent footing should have been formed and the slab thermally isolated from the footing at the edge.

    Ignoring the foam at the thickened slab edge not only couples the entire slab to the ground (and makes the subslab insulation mostly ineffective), but also allows capillary moisture movement into the slab.

    ICFs are also a poor choice for a basement wall as they virtually eliminate any dynamic mass advantage of a concrete wall and leave EPS (typically) foam exposed on both the exterior, where it needs protection from insects, physical damage and UV, and on the interior where it needs protection from fire and physical damage.

    There is no remedy for this other than covering the entire slab with foam insulation and applying another floor covering, thereby losing the thermal mass advantage of the slab as well.

  2. Riversong | | #2

    Correction: that should have been 25 psf XPS compressive strength.

  3. Bert Thomas | | #3

    That's the answer I was afraid I might here. To add to what I already posted: the slab will have radiant heat, however, the tubing was held back about 12" from the walls; as of yet the building has not been heated. Do you think it will make any difference once it is heated/lived in? I don't think it will - I don't run heat in the summer and that is when the problem has occured.

  4. John Klingel | | #4

    Another correction: I think Robert meant psi, no psf. I know; picky, picky. j

  5. GBA Editor
    Martin Holladay | | #5

    Bert,
    I have written a blog on the use of rigid foam under footings; it will be published in a few weeks.

    Robert is right that the best remedy is to install a horizontal layer of rigid foam on top of your slab, and a finish floor above that. However, if you have radiant tubing in your slab, that really won't work.

    Another possible remedy is to keep your doors and windows closed during the summer and to control your indoor humidity with a dehumidifier. That isn't very attractive either.

    You can finish your walls with cement backerboard (at least for a 4-ft.-wide band up from the floor) covered with a skim coat of joint compound if you are worried about moisture wicking up your drywall.

  6. Bert Thomas | | #6

    Yes I do have tubing in the slab - I guess I would have to do the tubing over with another layer of concrete over foam - I suppose i could use thinner foam and concrete for the top coat layers? I really don't want to go that route if i can come up with a better alternative. Half of the basement is garage and half is living space. I am hoping once I apply heat to the slab, which in turn should warm the surrounding soil some, maybe the problem will go away, but I doubt it. Any ideas about that?

  7. GBA Editor
    Martin Holladay | | #7

    Bert,
    As long as you don't have any urgent need to remedy the situation immediately, it won't hurt to take a wait-and-see attitude.

    You're right that running the tubing will raise the temperature of the soil. Over time, the moisture in the concrete and other construction-related sources of moisture in the house will dissipate, lowered the indoor relative humidity. Both of these factors will probably reduce the severity of the condensation that concerns you.

  8. Bert Thomas | | #8

    I think I like your idea of the backerboard along the bottom - I even thought about removing an inch or so of the foam at the bottom of the ICF, however, that would probably prove futile - I think moisture would still find its way up the concrete to the foam on a hot day that caused a lot of condensation.
    This has only been a problem for 2-3 months out of the year, due to the long winters here. The house has been under construction now for about three years with no heat in it, so the ground really gets a chance to freeze deep through the walls and slab; still hoping once heated problem goes away without having to run slab heat during warmer months.

  9. dadaist | | #9

    This thread is rediculous.

    Take your IPA mug from the freezer in the warm summer.... no surprise folks... it gets covered with condensation! Take a warm mug from the dishwasher out in the summer.... and .... no surprise no moisture!

    Heat the slab and live in the house.... and drink an IPA right now to calm down from my rant upsetting you... or amusing you as the case may be. LOL

    Where is common sense today? Why is it called common???????????

  10. Bert Thomas | | #10

    DADAIST

    Simply trying to get useful information about a problem with my building project - if the post offends you, please do not reply with useless information. If you know something about this situation that you could expound on other than frosted mugs, which I understand the theory there - please expound.

  11. dadaist | | #11

    Bert, you just need to move in and heat that slab even a tiny bit. You don't need cement board, but if being super safe, use non paper drywall board that uses fiberglass for your bottom boards.

    Find this thread in a few years when you are living there and let us know how you make out.

    And relax tonight with a cold one!

    Peace bro, love yaa and Alaska too.

  12. Bert Thomas | | #12

    DADAIST
    Thanks for the information/encouragement - I sure hope you are right. I have been worrying about this for sometime now and hopefully needlessly. Just don't want a little problem to turn into a big problem down the road. The non-paper drywall sounds like a good option. I hope to get heat in there soon. That will definitely be reason for that "cold one".

    Appreciate the rapid response from all who responded.
    Always open to ideas/suggestions.

  13. Riversong | | #13

    Bert,

    Feel free to ignore posts from jokesters who won't use their names.

    But I agree that the problem might go away once the slab is heated for a season. Even an unheated slab in a conditioned basement will raise the ground temperature considerably over time. You might consider waiting through a heating season before finishing the basement walls, but moisture resistant MR drywall should be used below grade and keep the bottom edge a bit off the slab.

  14. John Klingel | | #14

    I just re-scanned the shallow frost protected foundation designs at the Cold Climate Housing Research Center (cchrc.org) and I don't see any foam under any of their footers. Is that a mistake? I'll have to call there Monday and inquire about this; maybe they have changed their minds. Thoughts? john

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