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Condensation on concrete floor

| Posted in General Questions on

Hello everyone,
I am in the first winter in our small newly built cabin. The cabin has a polished concrete floor that has 3″ of insulation under the slab but on the front elevation has no insulation on the vertical portion of the slab as there is a ledger board that a deck hangs off (I was concerned about compressing the insulation).

In retrospect I regret this detail, as I am now getting a significant enough thermal bridge to develop condensation on the baseboard and concrete floor. At least I think that is what is happening. 

This is only in a location that a curtain normally covers, all other areas are fine. So I am thinking that the curtain is trapping the moist air and it isn’t warming sufficiently to dry out resulting in the beginnings of some mold growth on the baseboard and edge of concrete. In addition, this is adjacent to the aluminium sliding patio doors that no doubt leak some cold air.

I cannot move the curtains as there is a wood stove that the curtains should not be too close to. However, this area is never seen because of the curtains therefore I am wondering if it would make sense to adhere a 1″ or so thick layer of rigid foam insulation say 6-8″ into the room directly on the slab (Blue outline on photo)? 

Would that prevent this cold surfaces from forming condensation at least right in the corner? 

If so, I have some foiled faced polyiso which I could use. Or should I use a more vapour open option (EPS for example) to facilitate drying of the baseboard? 

Thank you in advance for your thoughts.

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  1. quinnowen | | #1

    images attached to comment

  2. Expert Member
    Akos | | #2

    A bit too late for you, but there is standard hardware to hang a ledger through foam.

    I don't think insulation on the interior will work in this case, you really need the insulation on the outside. Getting the ledger spray foamed, carrying this foam all the way either bellow the soil or to tie in to the existing foam would work. Just make sure you deal with flashing above your ledger, any water that makes it behind it will have no way of getting out and rot the ledger.

    The best option in this case to add some heat to that area. The curtain is not trapping moist air (all air in your house is moist compared to concrete exposed to exterior) but it is limiting air circulation thus heat. If you can't move the curtains, add some heaters there.

  3. quinnowen | | #3

    Thanks Ekos. I will endeavour to get some spray foam onto the face of the slab. In addition I have found some very low watt 12" long tube heaters that I intend to mount in the location and operate a couple hours per day in the winter.
    Another useful lesson learnt on this never ending journey of building things right.

  4. user-5946022 | | #4

    That looks like alot of water in the photos. Are you sure there is not a flashing leak with water getting inside the wall?

  5. Expert Member
    Michael Maines | | #5

    If it is cold outside, with no vertical slab insulation I can imagine getting condensation in the wall assembly or even on the interior surface, especially at an outside corner and next to an aluminum sliding glass door which probably conducts a lot of heat directly outside. But even then, that's a lot of water--are you sure you don't have a bulk water leak of some sort? Doors are notorious for leaking at the sills, head flashing can be done wrong, ice dams on the roof...

  6. Mark_Nagel | | #6

    I'd wager that this is direct water penetration more so than condensation. Water is likely running from somewhere around that door and then back inside: I could guess that denser, cold air is helping push that water inside the building; that and a slight gap and or angled sill plate (or foundation) assisting in the travel/projection.

  7. quinnowen | | #7


    I think the photo is potentially a bit disingenuous as the uneven polished concrete has naturally dark areas. There is also some dark sealant in one photo that is an additional air seal along my baseplate. I think a good indication is that I don't get increased water during heavy day time rain events, it is largely in the morning after a cold night and rather than a pool of water its a serious of droplets on the concrete and face of the baseboard.

    All that being said, I have managed to get access to some of the exposed concrete slab on the exterior that I will spray foam then cover again with EPDM membrane. What thickness foam should one try to achieve here? I can probably only get about 1" as its going to be awkward to get in there, will that be sufficient in this climate (night time temps rarely drop below 28F) Thanks again.

  8. Expert Member
    Akos | | #8

    An 1" of foam will probably work in milder climate, you only need to bring the concrete temperature up a bit. If the patio floor is open, I would try to get as much in there as possible, more never hurts.

    The SPF kits don't work well in cold, and work even worse over a moist surface. Only do this if the conditions are right and the concrete and ledger is dry.

  9. Expert Member
    NICK KEENAN | | #9

    A way to check if it's leakage or condensation is to cover the surface in aluminum foil. If it's condensation you'll find moisture on the interior side of the foil. If it's leakage you'll find it on the exterior.

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