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Vapor Barrier

Vapor Barrier

Vapor Barrier

We usually specify a vapor barrier for warehouse slabs on top of 4" subbase. On this particular project, the client does not want to use the vapor barrier because of cost and he heard there can be problems from their use. I have also heard this before as well.

 The geotech says that he doesn't see a reason to have a vapor barrier, but he would leave the decision to us and not mention anything about it in the report. I know there are some books on slabs on grade by ACI and PCA, but we do not have one at this time. What would you suggest, leave it out or not? Would a layer of sand between the slab and barrier help?

RE: Vapor Barrier

Vapor barriers are suggested to be used where flooring material over the slab on grade is moisture sensitive.  As for warehouses where floor may be just bare concrete, vapor barriers may be omitted.

The 2" layer of sand below the vapor barrier is to ensure that the membrane doesn't puncture (due to rough surface of subgrade).  2" sand above the vapor barrier is to avoid wet concrete to be in direct contact with the membrane.

Provide approximately 6" overlap between sheets.

That's all I know about this.  Any other thoughts?

RE: Vapor Barrier

There are situations where moisture can wick up to the underside of the slab and through a crack (there will be cracks) and into your living space.  This might damage floor coverings, rugs, drywall etc.  

Pay attention to: your soil type, the elevation of your water table and site drainage.  If any of these things cause you concern, add the vapor barrier.  If all three are okay, you probably don't need the barrier.

I recall one engineer who was worried about pouring concrete directly on the ground sheet because he thought it might cause curling.  Curling can occur where one side of a thin slab cures slowly while the other cures rapidly.  The concrete will physically curl.  I've seen it only once (in a 2" topping slab on a wood deck).

RE: Vapor Barrier

There are reams of reports on this topic, however all I can say is years ago I got a real A-- chewing because I left a vapor barrier out of a floor slab.  I did it after reading about curing etc, etc,.  After that, I ALWAYS use and recommend a vapor barrier, even in a warehouse.  Just because there is no water table problem, it doesn't mean your floor slab can't transmit moisture from the subgrade.  I've acutally seen a floor slab "sweat" in hot humid weather.

If at some later date you want to put a floor coating or sealer on the warehouse floor, it won't stick if the vapor pressure from the subgrade is too high.  Why take the chance?  A 6 mil clear poly vapor barrier costs about $.02/SF labor and $.015/SF material, so for $.035/SF I think it is cheap insurance.

That's my $.035 worth!

RE: Vapor Barrier

There is a need to provide a 'slip membrane' below all large concrete slabs. Together with preventing damp ingress into the building it will also allow the slab to shrink evenly by removing the friction between the sub grade and the concrete. Thin slabs will crack to hell if it is not in place, even with reinforcement. It must be used!  

RE: Vapor Barrier

This vapor barrier issue is a case of pay me a little now or pay me lots and lots later. As Jheidt2543 states, It is cheap insurance.  The vapor barrier will prevent the 'locked slab' syndrom that AJUK has described so well.  If your client is looking to value engineer the product, suggest that he look elsewere.

RE: Vapor Barrier

I think the positives outweigh the negatives for the vapor barrier, however I have been overridden by greater authority. Thanks for your help.

RE: Vapor Barrier


Just be sure that you document your objection to omitting the vapor barrier. "Greater Authority" generally has a very short memory.  

"Concrete Construction" magazine has had numerous articles on the vapor barrier issue.  I do believe their final determination was to use the vapor barrier.  There are ways to handle or at least greatly reduce the amount of curling. (My previous post should have read curling not curing).  You might look at back issues of "Concrete Construction" for additional research.

RE: Vapor Barrier

I have heard that a sand layer beneath the slab can help prevent curling of the slab because moisture is allowed to escape from both the top and bottom of the slab, rather than having the top of the slab cure before the bottom.  On the other hand, I've heard a lot of negatives about placing a layer of sand between the slab and the vapor barrier.  Here's a good article that expounds on some of the con's.


RE: Vapor Barrier


Thanks for the article link, I will read it shortly.  I have seen numerous articles about the pros and cons of a sand layer.  I have also had to place them in the field, during my career working for general contractors.  The addition of the sand layer between the vapor barrier and slab increases the cost of a floor slab significanly.  The concrete crew slows down quite a bit due to the difficulty in placing the sand layer without punchuring the vapor barrier.  Also, when pouring the concrete, moving around in the sand is much more difficult.

Finally, when you think about the reasoning for the sand layer, what are you really accomplishing?  The theory is that you allow the moist from the slab to travel two ways.  But, the moisture in the sand layer has to disipate eventually. The only way to go is up, so what you are doing is DELAYING the migration of the moisture, but it is the same amount of moisture!  Curling is caused by the differential in moisture content of the slab from top (the dry  side) to the bottom (the wet side).  I think the best way to avoid slab curling is to prevent moisture migration into the slab from the subgrade by using the vapor barrier AND by pouring the slab with the minimum W/C ratio to hydrate the cement without excess water.  Lastly, don't seal excess moisture into the slab by starting the finishing operation until the bleed water is out.

That's my $0.02

RE: Vapor Barrier

A vapor barrier/retarder is NOT always required.  This issue must be resolved on a project by project basis.  I use the article entitled "Where to Place the Vapor Retarder" by Bruce Suprenant and Ward Malisch -- it has a flowchart in it to help in the determination.  In a nutshell, if the slab does not have a vapor sensitive covering or a humidity controlled area, no vapor retarder is required.  When a vapor retarder is required, it should typically be placed directly below the slab, except when the slab is placed with the roof membrane in place and the project has a humidity controlled area, there should be a layer of dry granular material between the vapor retarder and the slab.


RE: Vapor Barrier


Placing the vapour barrier is done to prevent the migration
of moisture through the slab and to separate the warm air
from the cold.  The slab will crack over time, that is a
certainty.  Placing the vapour barrier now is insurance
against moisture problems later, provided the building is
heated (which I'm sure it is).  Incidentally, I recently
put in a slab-on-grade without a building above it, and
vapour barrier was not put in, primarily because:
1)The slab will crack over time
2)The slab was for outdoor use and was exposed to weather
3)When the slab cracks, the rainwater is not allowed to
  drain freely through the slab if a vapour barrier
  is present.
You don't have the same situation, but the principle behing
the vapour barrier is the same.


RE: Vapor Barrier

Just a few thoughts....
Vapor barriers often cause more problems than solutions, but that's mostly a construction/constructibility issue.

For large slabs on grade, the vapor barrier does nothing to control cracking.  This is done by appropriate thickness control and jointing.  Yes, the vapor barrier does provide some relief of friction restraint, but again, this can be overcome by appropriate thickness control and jointing.

As jheidt pointed out, "sweating" does occur and a vapor barrier can help to mitigate this.  In subsequent arguments about the sand layer/buffer between the slab and vapor barrier with respect to vapor drive, keep in mind that vapor migration in concrete slabs is transient and depends moreso on the interior temperature and the relative humidity IN THE SLAB near its surface.  Allowing moisture in the slab to dissipate in two directions is a good thing...it usually does not come back in the same quantity whether it goes up or down.

Put the vapor barrier in.  Put it 3 to 4 inches below the slab with a clean sand buffer, just make sure you don't create a "bathtub" effect by adding the sand layer, i.e., don't let it have access to an outside water source, either groundwater, surface runoff, drains or the like.

RE: Vapor Barrier

I had been waiting for your response on this one Ron.

RE: Vapor Barrier

The main issue here is that concrete is porous to moisture. Do you need the warehouse to be moisture-proof? That is, when anything is left on the floor, do you want it to remain dry. The answer by most clients will be "... of course I want the << complete the expletive >> floor to be dry".
If your client is cheap enough to demand the opposite then make darn sure you get it in writing and signed. The first time he sees a cardboard box of valuables all wet because of moisture suction through the slab he won't be pleased!
There could be some sites where moisture transmission will either not occur or will occur only slowly but they are very few and far between.
Use the membrane, and use it automatically on all such sites!

Anthony Tugwell
Project Director & Consulting Engineer - now in Australia

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