Vapor Barrier at Bearing Wall Footer
Okay one more basement prep question for this group this morning. Thanks to all who have helped so far.
Basement foundation in CZ6. The perimeter building footings have already been poured and walls are going up. In the middle of the basement footprint, there will be another footing for the bearing wall to sit on. Because the site is sloped, this footing will be excavated out once the inside of the basement has been built up and the footing will be poured at the same time as the concrete basement slab, which has been cleared as an acceptable solution.
Just thinking through the vapor barrier and I’ve read a bit about a VB going beneath footings — some say it’s fine, some say not to do it. In this instance, would it make sense to run the vapor barrier beneath the footing and have it be continuous across the entire basement slab?
Because the footing is going in at the same time as the slab, I can’t think of a way to run the VB over the footing but under the slab — unless I’m missing something.
Thanks all–
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Replies
I would run the vapor retarder under the footing. What are arguments against that approach?
I believe that I've seen the reasoning as wanting the concrete to have direct contact with the soil/gravel and ensuring there are no voids from the vapor barrier. But perhaps it doesn't matter
Sean,
You do want to make sure the concrete in the footer doesn't end up suspended by the poly. The easiest way to avoid that is to leave a fold in the poly under the footing, or use a separate piece that overlaps the poly on both sides.
Makes sense. I assume the idea is to lay the poly in the footer first so that it's a smaller piece and has less tension. And then just try and make sure it's as close to the bottom as possible?
Sean,
If you use a small piece, the weight of the concrete will push the poly down. If you use a fold in poly instead, and you fill the footing area first the same thing will happen. It's only when the movement of the poly is already restricted by the concrete over the rest of its area before you pour the footing that it can have problems conforming to the trench.