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Installing Solid Wood Floors Over Concrete Slab

user-6900238 | Posted in General Questions on

Here we go again.

I’ve been reading and thinking and reading and thinking. But now it’s time to decide and move on. Here are the details:

House Location: Costal Alaska, Seward. It a moderate climate for Alaska. It’s wet, wet, wet and gets cold, but not very.

Slab on grade, 1954 build, no known cracks. It’s flat-nuff.

Until I purchased it it had vinyl tiles glued to the floor with black mastik, the kind your mother warned you about. Currently, the tiles are gone and some of the mastik too. All of it may be removed if if if…

I want solid wood floors VG Fir 4″. I have a decent about of experience installing but never under these conditions.  I have relatively short ceiling height, a bit under 8′. I can mitigate the stairs and thresholds as the floor rises.

We considered  finished concrete floors but fear they would be too cold, even though I recently insulated the exterior side of the stem walls to R15.

I am not sure if there is a vapor barrier under the slab and will be investigating this today to add more data. I’m thinking there is. The builder built many houses in my neighborhood and was reasonably well known/respected from what I can tell. He did a good job, followed the rules, thought long term.  But I still don’t have proof. Maybe it’s frowned upon here?

So. Is a liquid applied vapor barrier or epoxy the best option? Then a subfloor or sleepers and insulation?

* For the record, I rather dislike engineered flooring or click or anything else for that matter that tries to be something it isn’t. I can’t ever get used to the way those types of flooring feel, sound or look.  They’re just not for me.

*As mentioned, I have experimented with removing the mastik in the event I need to finish the job so the concrete will accept a product to reduce (stop ?) the flow of vapor, give me peace of mind, hopefully.

I may have left out some details. Please advise if so. Otherwise, thank you for your responses.


P.S. The slab has about 3/4″ of ?rockwool?(not sure what its called or its type)  insulation under it. The slab is 4″ thick. What ever the under slab insulation is, its surface looks and feels like a roofing felt, tar based something-or-another. I’m not familiar, but feel it is a piece of the puzzle.  

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  1. qofmiwok | | #1

    If there are sub-slab moisture issues, it is common to have mold under vinyl. If you didn't find any, plus the fact that the slab is so old, means to me that you probably wouldn't have an issue with wood. Have you measured the moisture content of your slab?
    Another option would be to put some radiant heat tubes down then some additional stained concrete. Or what about tile that looks like wood? You get the warm feeling of wood without any moisture concerns.

  2. user-6900238 | | #2

    I haven't measured the moisture content, yet. Perhaps I should. As for sub-slab moisture issues, I'm not sure, nor am I sure how to detect said issues. But sub-slab moisture issues and mold under vinyl sound like different things. No? Can you explain further? They vinyl is gone and things seem ok. We're living in the house too, FYI.

    I'll update the original question a bit reflecting this: The slab has about 3/4" of ?rockwool? insulation under it. Its 4" thick. What ever the under slab insulation is, its surface looks and feels like a roofing felt, tar based something-or-another. I'm not familiar, but feel it is a piece of the puzzle.

  3. BCinVT | | #3

    Is the slab on a gravel base (called an "Alaskan slab" down here in VT?) That is how you will know how wet the slab might get. Since it hasn't cracked it is either on a well drained pad or soil or you just never have frost problems.
    A simple test is to tape a piece of clear plastic on to the slab. If you see condensation under it after a week or so it has a moisture issue If moisture is not an issue you can do whatever finish or extra insulation you want, but in any case a vapor barrier over the slab is not bad insurance.
    Also, it would help the slab stay warm if you continue the insulation on the stem wall on the exterior below the the slab. Good illustrations on that detail can be found on this site.

  4. user-6900238 | | #4

    Brian Carter. Are you still out there?


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