Vapor Barrier needed on Mat foundation?
Vapor Barrier needed on Mat foundation?
(OP)
I've got a small building addition - which is really just a two story set of stairs and an elevator/lobby. We set the whole thing on an 18" mat foundation over the entire footprint.
I understand about slab curling and generally have the vapor barrier issues figured out.
But with an 18" thick foundation mat, that is also to serve as the lower floor slab, the question has come up as to whether a vapor barrier under the 18" mat is required.
Normally, with a 4" to 8" slab, I would not use a VB if the slab was to serve as an exposed concrete floor as curling, cracking, etc. is helped a bit by not having a VB underneath.
And where there is carpet, tile, etc. I would use a VB for helping in long term control of moisture from below.
But this 18" mat will have vinyld tile applied to the top surface so I'm sensitive to vapor pressure coming up from below.
Would it make sense to not use a VB underneath (thinking that it would impair good curing through the mat thickness) and then notify the architect that special care, or tests, would be required prior to placing any tile on the concrete? The tests would check for vapor pressure. Might require a longer wait prior to placing tile but that's all I can think of.
I understand about slab curling and generally have the vapor barrier issues figured out.
But with an 18" thick foundation mat, that is also to serve as the lower floor slab, the question has come up as to whether a vapor barrier under the 18" mat is required.
Normally, with a 4" to 8" slab, I would not use a VB if the slab was to serve as an exposed concrete floor as curling, cracking, etc. is helped a bit by not having a VB underneath.
And where there is carpet, tile, etc. I would use a VB for helping in long term control of moisture from below.
But this 18" mat will have vinyld tile applied to the top surface so I'm sensitive to vapor pressure coming up from below.
Would it make sense to not use a VB underneath (thinking that it would impair good curing through the mat thickness) and then notify the architect that special care, or tests, would be required prior to placing any tile on the concrete? The tests would check for vapor pressure. Might require a longer wait prior to placing tile but that's all I can think of.






RE: Vapor Barrier needed on Mat foundation?
Incidently, it is not longer called "vapor barrier"! We call it "vapor retarder" since no product can claim 100% that it is a barrier to moisture.
RE: Vapor Barrier needed on Mat foundation?
The vapor barrier will probably be worthless excess for those warranty and code issues as noted.
RE: Vapor Barrier needed on Mat foundation?
Concrete Construction Magazine had two good articles related to this, June 1996, and July 1998. I can upload those if you'd like.
RE: Vapor Barrier needed on Mat foundation?
RE: Vapor Barrier needed on Mat foundation?
Thanks to everyone for the responses.
RE: Vapor Barrier needed on Mat foundation?
When you said:
"Normally, with a 4" to 8" slab, I would not use a VB if the slab was to serve as an exposed concrete floor as curling, cracking, etc. is helped a bit by not having a VB underneath."
Would you still not put the barrier under a gravel layer? We had a warehouse where the vapor barrier was left and they complained about the exposed slab being moist. I know this topic has been discussed ad nauseum but just curious when you said that.
RE: Vapor Barrier needed on Mat foundation?
Now if the warehouse was in a "low" area and the water table high, perhaps I would.
I've never put the VR below the granular base...some suggest that is a better way to go. I've always had trouble with that for two reasons - one, the granular base probably would puncture the heck out of it and two, with the VR below the granular base all in place, waiting for the concrete, it might rain. Then you have semi-perched water - a saturated granular base to deal with. In tilt construction this is even more critical as the openings in the slab at the interior columns allows water in, but with a VR you can't get it out.
RE: Vapor Barrier needed on Mat foundation?
RE: Vapor Barrier needed on Mat foundation?
RE: Vapor Barrier needed on Mat foundation?
RE: Vapor Barrier needed on Mat foundation?
According to PCA's Concrete Floors on Ground, a 4" concrete slab concrete slab is impermeable to the passage of liquid water unless the water is under considerable pressure, but concrete several times that thickness will allow the passage of water vapor.
Water vapor normally passes through the concrete and evaporates at the top surface of a warehouse floor, but a vinyl tile will seal the moisture where it may eventually cause problems.
If you cannot protect your construction from rain, maybe an additive such as KIM or XYPEX would create a moisture barrier.
RE: Vapor Barrier needed on Mat foundation?
helping to reduce interlock between the underside of the concrete and the gravel base.
helping avoid the water being absorbed by porous soil under.
As well as helping with moisture
RE: Vapor Barrier needed on Mat foundation?
RE: Vapor Barrier needed on Mat foundation?
http://tinyurl.com/caq38m
What is different wrt vapor barriers in Australia?
RE: Vapor Barrier needed on Mat foundation?
RE: Vapor Barrier needed on Mat foundation?
Most of the engineers I know place the VR under the granular layer because of the fear of curling that actually does happen in the U.S., though I tend to follow ACI's guide as much as I can. I think you pick up some additional friction that can be used to resist lateral loads when the VR is not directly under the slab also, which I know also hurts you in terms of slab shrinkage cracking.
RE: Vapor Barrier needed on Mat foundation?
hokie66...yes, we have problems with high shrinkage and curling in the US. Those are often related to an untrained workforce and failure to understand concrete technology, so the general rule is to compensate for it by keeping the joint spacing small.
I have investigated cracking in hundreds of slabs, and I can't tell you how many of them I've seen with shrinkage cracks roughly 1/2 way between widely spaced control joints and/or cracks running parallel to the sawcut control joints that were sawn a day or two after placement.
Our problems include: the specifiers often know little about concrete technology, the contractors know almost nothing about concrete technology (I realize that's a broad brush to paint all contractors, but I see many more who don't understand than those who do understand), and the finishers want to get the the pub, as you noted.