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Why? (I'm on here to learn, this is the kind of discussion I'm looking for. )
Because of mold issues... use XPS foam directly on the concrete floor, follow manufacture instructions how to seal properly and that will provide you with a continuous semi permeable vapor barrier... not to mention it will enable water vapor to diffuse harmlessly while providing moisture barrier.

Good luck :thumbsup:
 

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In my experience Greg is right, you don't want anything completely impermeable or it will completely trap moisture and not be able to evaporate into the air, thus creating a habitat for mold. You want to slow the moisture to a manageable level for the amount of air underneath the structure so the water vapor doesn't metastasize into liquid which will over time exponentially increase the moisture underneath.

If it can transfer into the air at a slow enough pace it will go bye bye, if not than it just recycles itself along with the added moisture from the concrete. Its the reason tyvek is not impermeable nor is tar paper.
 

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In my experience Greg is right, you don't want anything completely impermeable or it will completely trap moisture and not be able to evaporate into the air, thus creating a habitat for mold. You want to slow the moisture to a manageable level for the amount of air underneath the structure so the water vapor doesn't metastasize into liquid which will over time exponentially increase the moisture underneath.

If it can transfer into the air at a slow enough pace it will go bye bye, if not than it just recycles itself along with the added moisture from the concrete. Its the reason tyvek is not impermeable nor is tar paper.
Deck...Thanks.... I sorta undersytand where you are coming from.... but there is a material difference....

when you are roofing with permeable felt or siding with permeable/breathable Tyvek, you are protecting moisture entrapement from your sheathing or decking that can rot...

that is not the case with a concrete slab....

I guess you think that a delayed or slow migration of water vapor from a damp SOG, is somehow better for flooring material than a more rapid migration.

Justaposed to this view is that most (or at least a majority) of underlayments for SOG by manufacturers are a vapor barrier, not a permeable vapor retarder. Although, I admit that some are a retarder.

Not saying you're wrong.... just a discussion.

Best
 

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If the finish is going directly on top then I would agree that impermeable is necessary, however this is speaking of an air gap in which case naturally water vapor will be in the space no matter what. Plastic will trap the moisture coming up from the concrete and mold or mildew will develop.

Here in Florida everything is concrete and impermeable is not good with an air space with all the humidity we have. It was explained/proven to me by PE/Archy who is a genius and an a-hole and forced me to eat crow.
 

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If the finish is going directly on top then I would agree that impermeable is necessary, however this is speaking of an air gap in which case naturally water vapor will be in the space no matter what. Plastic will trap the moisture coming up from the concrete and mold or mildew will develop.

Here in Florida everything is concrete and impermeable is not good with an air space with all the humidity we have. It was explained/proven to me by PE/Archy who is a genius and an a-hole and forced me to eat crow.
Good point and consideration:thumbsup:

In fact, there is an underlayment for direct application of floating floor on SOG that has tiny bubbles/protrusions/ribs on the underside so you get a minor air flow/ventilation between the flooring and the slab...... similar to as you are pointing out.... just to a smaller/lesser degree.
 

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Because of mold issues... use XPS foam directly on the concrete floor, follow manufacture instructions how to seal properly and that will provide you with a continuous semi permeable vapor barrier... not to mention it will enable water vapor to diffuse harmlessly while providing moisture barrier.

Good luck :thumbsup:
Are there any other semi-permeable vapor barriers you would recommend besides XPS foam?

I believe someone mentioned tar paper, which I believe is inherently semi-permeable.

The takeaway being, on slab or poured concrete on top of slab, it's best if you use a semi-permeable barrier that allows for vapor to pass-through in order to prevent any potential moisture trapping issues ?
 

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Pouring concrete over a slab you would want plastic unless its suspended over top (which I honestly don't see a reason for)
 

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The takeaway being, on slab or poured concrete on top of slab, it's best if you use a semi-permeable barrier that allows for vapor to pass-through in order to prevent any potential moisture trapping issues ?
Yes, allow the moisture to enter the space and deal with it mechanically. Much better than trapping it and allowing problems to develop unseen.
 

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How come no one has mentioned sealing the concrete. I did it for years.
Tony.... What have you used/recommend for sealing concrete relative to moisture migration from below.....

I've used sealers to prevent surface staining etc.... are those very effective in blocking moisture migration.

Would UG Water Bloc be effective on SOG?

TIA
 

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Tony.... What have you used/recommend for sealing concrete relative to moisture migration from below.....

I've used sealers to prevent surface staining etc.... are those very effective in blocking moisture migration.

Would UG Water Bloc be effective on SOG?

TIA
Every time I have to seal concrete and go to a supply yard they never carry the same brand sealer.I have used it for sub grade and slab.I have layed Vinyl and carpet on it. Never had a problem.What ever you get make sure you read there label. Everyone has there own way of using it.Once sealed do not penatrate. If you are going to put a sub floor over it Lay your joist and PL your scabs down and wait until it sets then nail. The PL adhesive holds better than nails anyway.
 
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