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Community and Q&A

Measuring Moisture in Slab

thegiz | Posted in General Questions on

Hope everyone is doing well. I want to have some measure of the current moisture levels in my slab to get a better idea of exactly what I’m dealing with before installation of floor. I was thinking about using a calcium chloride test, a 3 pack on amazon is about $40. The flooring manufacturer recommends a maximum of 8lbs per 1000. What will the test results determine? If it’s above 8lbs and I add poly sheet do I retest with the vapor barrier? The flooring manufacturer recommends:

6mil vapor barrier or similar barrier with less than 1 perm rating
A optional 2nd layer of underlayment with a max thickness of 3mm (they make a recommended underlayment)
Then finished vinyl floor with or without attached underlayment

If I build a eps/plywood subfloor I would think I need poly or vapor barrier underneath the foam if the moisture content is very high. However with a dehumidifier controlling condensation and moisture in room, the vapor from concrete would have to travel up through the poly, past the 1.5mm foam underlayment, and through attached underpad of the flooring to cause mold underneath. Riskier than building the eps and wood subfloor but just sounds unlikely to me.

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  1. Expert Member
    PETER G ENGLE PE | | #1

    First, how old is your slab, and do you know if there is a VB under it? If it is well aged (more than a year in many cases) and has a VB, you probably don't need anything else. It might be worth testing just to make sure, and the CaCl2 test is the gold standard. If your slab is properly built as above and you get excess moisture, you should look for reasons. It's always best to keep excess moisture out of the slab in the first place.

    That said, if all else fails and your slab is wet, a VB on top and under the flooring can help. Clean the slab well before installing the VB, because any organic debris (sawdust) will get moldy and will probably smell. If using EPS and plywood under the flooring, then you'll never get moisture condensing under the vinyl flooring. The concrete slab will always be colder than the flooring, so moisture will go towards the slab, not the vinyl.

  2. Jon_R | | #2

    No matter what moisture tests indicate, I'd avoid putting wood in a basement floor moisture trap (vapor barriers on both sides). Or under a vapor barrier.

  3. thegiz | | #3

    It is a 100+year old home so guaranteed no vapor barrier in slab. What is the proper setup with vinyl if wood shouldn’t be underneath it? Are you saying vinyl should only be used over concrete and with eps/wood you need permeable floor?

    1. Jon_R | | #5

      That's my opinion. Either concrete only or something that isn't sensitive to moisture (foam and Hardibacker?). Or at minimum, an upwards permeable floor so wood can dry.

      1. Expert Member
        Michael Maines | | #6

        Jon you must have missed the previous ten threads on the subject.

        Hammer, my advice is to assume that at some point over the course of the year your slab is going to be damp. Water tables rise and fall over time. Often in late winter/early spring when the ground is frozen, it will thaw at the house and funnel rain and meltwater toward the basement. I would plan for the worst, even if the slab is dry today.

      2. thegiz | | #8

        I'm thinking removable pad and synthetic carpeting so it can be removed when needed. The floor will be covered so wondering if I need a vapor barrier over the slab. I could epoxy seal the floor as Bill suggested in another thread but wondering if there is a liquid barrier that would be easier to apply. 6mil poly would be easiest but as mentioned in article by Joe L "walking the plank" there is a risk of mold under the poly.

  4. exeric | | #4

    Shouldn't this subject only come up again on ground hog day?

  5. thegiz | | #7

    Ground hog day, very funny. I would tend to think worse case scenario would not include wood as Jon stated. Apparently that seems impossible to replace wood with backerboard or any other substrate under vinyl. If there was a way to put poly down on slab, then put down foam with backerboard or just backboard by itself before vinyl I would think safer than wood in worse case scenario. From what I understand in my 10 threads nobody does it so it must be impossible.

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