Vapor Barrier
Vapor Barrier
(OP)
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?
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
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
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
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
RE: Vapor Barrier
RE: Vapor Barrier
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
http://www.buildingscience.com/resources/foundations/sand_layer_under_slab.htm
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
DaveAtkins
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.
Milldude
RE: Vapor Barrier
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
RE: Vapor Barrier
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