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Slab on grade, zone 7, subfloor/hardwood Q

user-6900238 | Posted in General Questions on

Hey, guys, gals. 

Here are the facts:

1956 slab on grade 4″ , Costal Alaska, property drains well.
Stem walls all have 3″ new XPS, 36″ deep.
Slab  has a few very small cracks. 
Old vinyl removed, a thin layer of Black mastic remains, but not enough to create gaps or high spots.
I did a simple moisture test  (6 mil plastic taped to floor for 2 weeks) this past summer in 3 different places. No moisture was trapped. 
I have height problems. Not me, my ceilings. slab to joists is already under 8′.
I have 5″ acclimated fir flooring. 16’ers so no joints. 
My slab is pretty flat. I think its good for 3/16 over 10′ 

My plan:

5/8″ CDX glued/nailed to the slab. Felt over that, nail down flooring. 

My Questions:

Vapor barrier for insurance? Perhaps Taylor adhesive under the plywood? Plastic under it,  and no glue? plastic over the ply, under the felt?

Materials are sometimes had to get up here. I may not have access to T&G ply. Thoughts? I don’t want to deal with a ton of un-level joints in the subfloor.

I had considered 1/2′ rigid foam and 5/8″ sleepers to aid in the warmth of the room but I’m not convinced it’ll make that much difference. Thoughts?

*Best I can tell, its a 3-4″ slab, then 1″ of maybe rock wool, then gravel/ag, dirt, no rebar.

*Ive been told the mastik acts as a vapor retarder too for what its worth.

It’s time to order the materials, yesterday!


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  1. plumb_bob | | #1

    I would put down directly on the slab either rigid EPS with taped ship lapped joints, or a poly sheet with taped joints. Either system should be detailed as an air barrier to control the ingress of both vapour and radon and other soil gas. Then down goes your sleepers and wood flooring.

    The insulated system will provide a more comfortable floor and would be my first choice.

    1. user-6900238 | | #5

      Thanks for that. I'm treading towards an insulated floor too but the only real way I can do it and not cause problems with doors and stairs is to apply the 1/2" EPS to the floor in-between sleepers, then nail the flooring to the sleepers. I don't have room for another layer of plywood. Do you see any major flaws in this plan? It not ideal I know.


  2. Expert Member


    This is one of the assemblies most commonly discussed here on GBA and I'll leave to others to make their cases for their preferred system. One thing that puzzles me is how you would mechanically attach wood nail-down flooring when the sub-floor is tight to the slab. Staples or nails penetrate and extend through the bottom of the plywood when it is suspended. What happens when they can't do that?

  3. user-6900238 | | #3

    1 1/2" or shorter T-nails don't go deeper(vertically) than 5/8". Does that make sense?

    I have read several articles on the topic but none seem to be a real duplicate of my questions. None I've had the time to find anyway.


    1. Expert Member
      MALCOLM TAYLOR | | #4


      I didn't mean to imply you should just look at other similar discussions, more that the advice you will get around how to deal with potential moisture against the sub-floor in direct contact with the slab will probably vary wildly. I don't have a good enough idea of how the assembly will fare to comment usefully.

      1. user-6900238 | | #6

        Ah! I see.

        Thank you.

  4. Jon_R | | #7

    > 6 mil plastic taped to floor for 2 weeks ... No moisture was trapped.

    I suspect you mean "no condensation was visible". It may have been 99% humidity under the plastic, which wouldn't be good for wood.

    The wood in your floor will be dry if there is much more drying to the interior than there is wetting from below. It isn't clear how much you have of either, but it's a good guess that a wood floor would allow much more upward drying than polyethylene would allow wetting. Fully adhere the polyethylene if you have any concerns about mold odor.

  5. plumb_bob | | #8

    The 2 things you are trying to achieve are vapour control and soil gas control, while building a good base for your wood flooring.

    I think you could manage vapour with the insulation between sleepers idea, but maintaining an air barrier would be difficult.

    I am sure somebody will suggest an epoxy coating to the slab to solve both problems, this in theory is a good idea but I have no practical experience with it.

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