Pad vapor barrier
Construction of our modern ranch house is coming along. The pad is complete, all of the internal and perimeter trenches have been dug and the 1st stage of plumbing is roughed in. Next is termite control and 15 mil Stego wrap vapor barrier.
We had the geotechnical engineer inspect everything and he made an interesting comment which I have not heard before. He highly recommended cutting the vapor barrier down the middle of the internal grade beams and not draping the outside of the exterior grade beam in order to prevent the possibility of the concrete not filling these beams fully and leading to bridging (gaps with no concrete).
How does this affect the moisture barrier properties of the wrap then? Is this a common practice? What should I do? Some of the slab will be covered with hardwood floors – i do not want to end up with moisture and mold issues down the line. Thank you.
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Replies
Below discusses the second issue (do cover the below grade portion of exterior slab edge.):
https://www.buildingscience.com/documents/insights/bsi-037-mold-in-alligator-alley
deerefan,
With a bit of diligence cutting the poly shouldn't be necessary. Make sure to lay it with a bit of slack in the trenches, and fill them first. Concrete weighs about 150 lbs per sq ft, so unless the poly is very taught, the chances of it supporting the mix and creating voids are pretty slim. If you are really worried about it run a strip in the trenches and overlap larger lengths of poly on each side.
Remember the Geo-tech's sole job is the structural integrity of the foundation. Concern for the integrity of the vapour barrier doesn't come into it.
I’d be most concerned with any tight creases in the poly where it folds over on itself. Malcolm is right that the heavy concrete will tend to press the poly into place, but I’ve seen creased areas act like a knife and “cut” the concrete during the pour. This effectively casts a large crack into the concrete in the area of the taught crease in the poly sheet.
Make sure the poly is laid down tightly and is flat against the ground everywhere, and try to lay it into any depressions carefully. All you need is a little care and you’ll be ok.
Bill
What issues could result from "concrete bridging"?
Slabs with integral footings are designed to support loads while themselves being supported by a fairly uniform substrate of compacted material.
- They are designed to be a certain dimension based on what they are bearing on. If all of their surface isn't in contact with the substrate below, they will be undersized.
- They experience loads from the house above, and loads from the earth below. If there are voids, they have to bridge them. For that portion it becomes a suspended beam as opposed to a footing - and they aren't designed to do that.
If your question is about vapor rather than liquid water getting past the plastic moisture barrier, the answer is the cut makes little to no difference; see several discussions of how a vapor retarder works here on GBA: https://www.greenbuildingadvisor.com/article/air-barrier-or-vapor-barrier-building-science-podcast or https://www.greenbuildingadvisor.com/article/understanding-air-barriers-vapor-barriers-and-drainage-planes.
So long as there is decent overlapping at the cuts, the seams don't have a significant impact on vapor movement through the concrete.
Peter
Peter,
Thank you for answering. I guess I was under the impression that the plastic under the slab was meant to control both liquids and vapors from getting to it? I imagine water would be the biggest worry.
Deere,
Most slabs aren't designed to resist liquid water. If your slab is below the ground water level, you will have water in your basement -- unless, of course, you include crushed stone under the slab, with a perforated drain pipe in the stone leading to daylight.
It's the drainage system (the pipes and associated crushed stone) that prevents entry of liquid water -- not the slab itself, and certainly not the polyethylene.
Deerfan,
Overlapping poly without taping the seams risks concrete getting between the two layers during the pour. As Bill said, this may not affect the performance of the vapour barrier, but it can severely weaken the slab or footings.