Capillary break over footing vs. vapor barrier under footing
stearel
| Posted in Green Building Techniques on
Many building science resources recommend installing a capillary break over the footing (between the footing and foundation wall). Is there a reason this is preferable to installing a vapor barrier (i.e. 6 mil polyethylene) under the footing, up the inside face of the footing, and overlapping with the under-slab vapor barrier?
I’m assuming the foundation wall waterproofing extends all the way down the exterior face of the footing. I’m in Cincinnati, OH, and my question applies to finished basements, unfinished basements, and conditioned crawlspaces.
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Replies
Eric,
When this topic came up in a Q&A forum in 2007, this is what I wrote:
"The capillary break can't sit under the footing because the bottom of the footing is often wet -- it's above the holes on the footing drainage pipe. If you install the footing drainage pipe far enough below the bottom of the footing to put the holes low enough to keep the footing dry, you've ended up undermining your footing. Ergo, the capillary break belongs between the footing and the wall."
Building scientist Pat Huelman agrees:
""It is been a tough road to get capillary breaks between the footing and foundation wall, but I see progress being made on that front. It is important to recognize that the soil will frequently be totally saturated at the bottom of the drain tile, and there can be free water as well. So it just makes sense that the bottom of the perimeter tile should be below the capillary break. Some argue that you can put the capillary break under the footing and conceivably this can work. But following the logic above, this would require the drain to be lowered below the bottom of the footing and that makes the engineers nervous."
Needless to say, some builders ignore this advice. One blogger who ignored this advice posted the photo shown below.
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