Vapor Barrier over DMX+OSB?

Discussion in 'Floor Preparation' started by ec10, May 15, 2018.

  1. ec10

    ec10 New Member


    I am re-doing my basement floor and will be installing cheap laminate flooring from home depot. I installed DMX one-step (DMX 1-Step Unique Air Gap Underlayment Prevents Mold and Mildew) on the concrete floor and above this I laid 3/4 in tongue and groove OSB.

    My question is: should I
    1. Include another vapor barrier between the OSB and the laminate planks or
    2. Include (non-vapor-barrier) padding foam/felt between the OSB and the laminate planks?

  2. Mike Antonetti

    Mike Antonetti I Support TFP Senior Member

    I think the OSB method will fail. Usually it would be double layered OSB and I think one layer would be at 45 degree to the other and fastened 6” in field, 2” around perimeter of each sheet.
    • Like Like x 1
  3. ec10

    ec10 New Member

    Hi Mike,
    Thanks for your reply, although it really doesn't address my question. In any case, what is the nature of the failure you're worried about? I wouldn't think I would need to double layer the OSB over a concrete foundation.

  4. Daris Mulkin

    Daris Mulkin The One and Only Charter Member I Support TFP Senior Member

    Mike gave good advice with the double layer and the boards are not perfectlt flat. Anchoring one to the other will flatten it out. You could also do a sleeper system. Either way you do it I would think if any moisture at all it will soak up into the OSB[beaver barf] and swell.


    • Like Like x 1
  5. Graeme

    Graeme Pro Member

    Option 2, in my opinion. You already have a vapor barrier that will protect your floor from moisture coming up through the slab. If you were to put a second layer of vapor barrier on top of the OSB, you run the risk of trapping moisture, potentially compromising the OSB substrate.
    • Agree Agree x 2
    • Like Like x 1
  6. ec10

    ec10 New Member

    Thanks Graeme,

    That makes a lot of sense. I did find one product that is a vapor barrier but claims to have a channel on the bottom side for air/moisture to flow. Have you had any experience with this?

    Appreciate your help,
  7. Mark Brown

    Mark Brown On The Surface Flooring I Support TFP

    that traffic master sure looks a LOT like the garbage old Roberts 3/1... I wouldn't buy that hype.

    poly shims... what does that even mean, because I bet it means "little Styrofoam balls"

    I do agree with graeme on this one. If you have deemed you osb to be sound and secure and protected with a moisture barrier then by all rights I would have to assume a quality 2 in 1 would serve you right.

    As for how sound that osb is well that's up to you. If it were me, I would be shooting for Mikes suggestion... or build a sleeper but we are past that.
  8. Chris 45

    Chris 45 Director of P.R. on some deserted island. I Support TFP

    I’d be more concerned with ‘cheap laminate’ in your situation. And I’m guessing you’re doing all this because of possible or existing moisture issues, otherwise you would just lay your floating floor directly over the concrete. With DMX, and whatever you put on top of it, is all supposed to be gapped from the walls. Then your base boards are supposed to be installed with a gap between the floor and the base to allow everything to breathe. That cheap laminate will suck up all that moisture like a sponge.
    • Agree Agree x 1
  9. Graeme

    Graeme Pro Member

    I think Mark has that one spotted... it looks like the Roberts 3-in-1 from days gone by. As it says it is a vapor barrier, I would not use it. You would be able to get a "non-vapor barrier" underlayment for less money, or spend the same and get one with better acoustical properties instead.
  10. ec10

    ec10 New Member

    Thanks all,

    So I'm concluding that I want to avoid a vapor barrier. So I need something like this? Roberts Black Jack 100 sq. ft. 28 ft. x 43 in. x 2.5 mm Premium 2-in-1 Underlayment for Laminate and Engineered Wood Floors-70-026 - The Home Depot

    I'ts really hard to tell since all products will at least say they "protect against moisture" even if they are not an actual vapor barrier

  11. Mark Brown

    Mark Brown On The Surface Flooring I Support TFP

    You are correct that most if not all of the foams used are vapor barrier. This is just the nature of closed cell foam. I would just not seal the edges where they join and staple it to the osb. If you are really concerned (I probably wouldn't be) you could even go so far as to perforate it to let any moisture breathe better... problem then is it is breathing right into the laminate.
    This is getting complicated lol
  12. Graeme

    Graeme Pro Member

  13. Mike Antonetti

    Mike Antonetti I Support TFP Senior Member

    Good call Mark on the UL, put that stuff in my friends parents house years ago, his dad was in a wheelchair, sure would like to know whether it’s still down. What a scam that stuff is.

    I’d say OSB would curl upward, the joints are the weak area, offsetting another layer gives it more rigidity. So my theory is top and bottom moisture levels should be equal, any imbalance causes it either to cup(dryer on top) or crown( dryer on bottom) I saw a wood picnic table curled up last week, I assume sun dries the top.

    I assumed you put the OSB down for insulation.

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