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Concrete slab vapor diffusion

Tim H

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In my area (central Alabama) 95 % + of new homes are built slab on grade (not post tensioned), with vapor barriers/retarders under the slab. My question has to do with installing vapor barriers on top of the slab under carpeting/laminate/tile flooring. How does the vapor emit and diffuse when the slab is sandwiched? Is the moisture 'trapped' in the slab, or does enough moisture emmission occur at the edges to allow the slab to properly diffuse vapor? Does it even matter?

Thanks - Tim

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When you place a vapor barrier on top of the slab you stop evaporation from taking place. Because it's unable to evaporate moisture, the slab will eventually reach equilibrium and won't accept any more moisture. Unless there are other sources of moisture moving into the slab - water from improperly oriented downspouts or sub-surface water that gets into that so-called "sandwich" - the amount left in the slab is negligible. I'd say it really doesn't matter at that point.



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I have a personal dislike for multiple vapor retarders; can't say it's backed up w/hard facts, but I just don't like them in any application.

It's hard for me to imagine a dual vapor retarder being a problem w/a slab, but since I've yet to see a vapor retarder installed properly in my entire career, it seems that installing two of them in a sandwich is asking for problems. Kind of a situation where no one could prove that it was going to be a problem, nor could they say w/certainty that it is fine.

I prefer going w/known quantities, which for me would be a single vapor retarder between the slab & the earth; if the slab had to breath, it could do so to the interior. If the vapor retarder was installed well, moisture migration would be minimal to nonexistent.

You should ask Bill Loden what he thinks; he works in Alabama & he is about as sharp as they come. He's over in Huntsville; you could get his address & phone over @ the ASHI site.

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How in the world do you get "attachment" with tile or vinyl if you're putting it down over plastic sheeting? I can't cite anything either, but it makes my "bad idea" alarm start low-level buzzing. What's the supposed problem with the one barrier system?

Brian G.

One Proven Idea Is Worth Ten New Theories (Maybe More) [:-mischie

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