Ideas for air barrier for stepped foundation wall
Jerry Chwang| Posted inGreen Building Techniqueson
我的房子有一些不寻常的条件:
1. Extra deep footings in south wall because of poor soil conditions. 8′ below grade.
2. Using an ICF foundation, but ICF is not made out of foam board, but is composed of blocks like CMU made out of a wood chip/cement mix that is air permeable (Durisol).
I’m worried that air can go from interior through inner wall of the ICF down and around the slab into the soil underneath the slab.
Questions:
a) Is this an issue?
b) How to address? I was planning on parging interior basement walls sometime in the future with clay. For now, would parging the wall with cement below the slab to the footing work?
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Replies
Jerry,
Air leakage through soil is possible. But the direction of the air leakage is the opposite from the direction you hypothesize. A basement (or the lowest floor of a building) is usually at negative pressure with respect to the outdoors, especially in winter, because of the stack effect. That means that if you have a crack in your foundation or your slab, air can enter your house through that crack. (That is infiltration.) It is unlikely that air will leave your house by this route. (In other words, exfiltration is unlikely.)
You have correctly guessed that the best way to ensure a good air barrier with a Durisol wall is by the use of plaster. Concerning your question about parging the Durisol wall under the slab: I don't think that cracks in the wall that is under your slab matter very much. The crack that you have to worry about is the crack at the perimeter of your slab, where your slab meets the wall. (Of course, if the manufacturer of your Durisol blocks insists that below-grade walls have to be plastered, that's another matter entirely. You should follow the manufacturer's recommendations.)
An unplastered Durisol wall is unlikely to be very leaky, but a good plastering job will certainly take care of any small air leaks.
.
Thanks. You guys are great.
The manufacturer did recommend parging to footing on exterior of wall (and waterproofing), but hadn't heard of any issues with infiltration on interior side bypassing the slab, though they did acknowledge this was possible.
In the long run, I was going to finish basement interior walls with clay or something airtight, but still vapour permeable for drying. For now, I think it might be best to parge from slab down on inside, down to footing as well...and maybe waterproof as well?