Which vapor barrier for polished concrete finished floor?

[sixwoofys]

Which is your preference for polished concrete finished floor? I've heard that a (poly) vapor barrier directly in contact with the 4" slab is preferred (for use with some flooring types) as opposed to a 2" layer of sand between the (poly) barrier and the concrete slab.

My finished floor will actually be the concrete itself.

Which moisture barrier would work better?

Poly barrier, then 2" sand, then (mesh) reinforced concrete or..
2" sand, then poly barrier, then (mesh) reinforced concrete?

Thanks in advance for the help!

 

[msucog]

if moisture is a concern, always use vaper barrier. the heavier the barrier, the more protection against moisture transmitting through. by the way, moisture can still transmit through most vapor barriers to some degree.
personally i prefer the sand (or GAB-both compacted) to be under the vapor barrier but i've heard it argued both ways. i picture it as the sand helping to act as the moisture break and then the vapor barrier acting to stop/limit the moist-vapor. plus, it seems more practical to put the vapor barrier down last to reduce puncture. i will usually not recommend only 2" (my thoughts are that if it's that important to be there, it should be at least 4" thick). most of the time when ground water is not an issue, we just recommend the moisture barrier (as the minimum).

 

[DarthSoilsGuy]

i've seen both designs used. I don't like the poly-sand-slab, because:

1) if water/rain can get in your slab prep area it makes a mess. how do you drain it when your slab is already supposedly flat and such a small thickness? the slab prep has a sand mush on it. since it's so thin, i don't imagine the settlement to be an issue, but i don't like it. Contractors have an easy fix for this....a probe rod (assuming there is some drainage gravel under the VB)

2) how is the concrete getting placed? will the concrete disturb the sand layer and will the sand mix in with your slab concrete or will you have a thin slab in some spots b/c of this sand displacement without mixing? i've seen pumped concrete plow, and plow is the best word, the sand course.

 

[sixwoofys]

Thanks for the great information.

I am planning to place the concrete by truck directly onto the prepared surface (sand).

I am leaning towards placing the barrier first, then 3 - 4 inches of sand, welded wire on chairs and then concrete. I am hoping that wetting the sand will prevent the plow factor (?).

The job will also be a monolithic pour (both footings and 4" concrete slab will be poured at the same time). So obviously the corners and edges will be quite thick and gradually becoming the 4" slab as we move more towards the center of the rooms.

One reason I am wanting to place the vapor barrier first is to allow the moisture located at bottom of the slab to cure with, and be in direct contact with the sand underneath. In order to give the moisture somewhere to go other that through the top surface. As the finished floor will be the actual concrete itself I have no real concern with moisture affecting wood flooring, adhesives etc.

Thanks again for the help. Any other ideas or information, experience will be greatly appreciated.

 

[TDAA]

We advocate omitting the VB for a sandy site, with no ground water issues. We are in a dry climate though.

 

[theCorkster]

ACI 302.2R-06 presents a discussion and flow chart to address this question; the approach depends on whether the base material will be placed with water tight roofing in place or not. They also discuss the pros and cons of using granular material over / under the vapor retarder (barrier implies an impregnable boundary, which is a myth).