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VB under the footer? Radon.

John Klingel | Posted in General Questions on

I’ve been wondering about radon, and just looked at the “Vapor Barriers, Radon, Basement Slabs…” blog. There, I noticed that the sub-slab vapor barrier was continuous under the footer, apparently to help keep radon out. A vapor barrier under the footer concerns me; if you ever did get water in there, it would not dry out until it, perhaps, wicked up the foundation wall, into the slab, and out. Maybe water can sit there forever and not bother anything. ?? Is a VB under the footer even necessary for radon control, if you have radon pipes under your slab and the compacted gravel around the foundation walls is continuous with the gravel under the slab? It seems that any radon under the footer would migrate through the gravel and out your vent pipe. No?

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  1. Riversong | | #1

    Radon/vapor barriers are not typically installed under the footing, but should be taped to the foundation walls and at every seam and around every penetration. Radon is a very small molecule that can pass through an opening too small to see.

    Subslab stone or mixed gravel with a full perimeter radon vent that is connected to a stack through the roof should be sufficient to evacuate soil gasses in 95% of the cases. In the remaining 5%, an inline fan is necessary to increase the subslab negative pressure. And it's always best to minimize in house stack effect leakage which will draw basement air upstairs and compete with the radon vent. A leaky fireplace is a classic competitor to a radon vent.

    However, it's not a bad idea to put a capillary break under the footing, or between the footing and both the foundation walls and the slab. Otherwise, there may be wicking of water from the ground. If the exterior perimeter drain is above the bottom of the footings, as is normally the case, then the capillary break should be above the footing.

  2. John Klingel | | #2

    Robert: Roger that, but why is there a vb under the footing on that blog? Perhaps excessively wet soil? j

  3. Riversong | | #3

    why is there a vb under the footing on that blog?

    Because that's a monolithic thickened-edge slab, so there's no choice but extending the vapor barrier under the entire structure. That's one of many reasons I use separate perimeter grade beams and interior slab. That allows me to isolate the slab both thermally and hygrically from the footing and the ground. It also makes it easier to locate sublsab plumbing and radiant heating loops when the perimeter and height is already defined.

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