Interior Parge on Nexcem Stemwall? (Conditioned Crawlspace)
Hi everyone, I am building a house for my sister-in-law in Southeastern Manitoba (Climate zone 7a, Canadian Shield terrain). The foundation wall is formed with Nexcem (formerly Durisol) forms, which are kind of like large CMUs made of cement-bonded woodchips with a rigid mineral wool insert. They are free-draining, vapour permeable, and not air-tight.
The site is on granite bedrock, and the forms are scribed directly to the bedrock. We will be applying 3″ of closed-cell sprayfoam directly to the bedrock as insulation, vapour retarder, and radon barrier. Depressions and crevices in the bedrock are filled with crushed stone and weeping tile and drained to daylight. There are also a radon vent rough-in pipes buried in the crushed rock. I have heard that granite is a particularly bad substrate for radon, and our inspectors are pretty on-it with enforcing radon mitigation requirements.
I had planned to parge the outside face of the forms to create an air-barrier, and give a less porous substrate for foundation dampproofing in the below-grade portions of the wall, BUT, this means that the spray-foam on the crawlspace floor terminates at the porous forms, and I’m concerned that we will be negating the value of our radon control measures by doing that. So, I’m wondering if we should rather be parging the interior of the forms to make our air-barrier contiguous from the sprayfoam at the ground up the wall to the mudsill, and using a little extra dampproofing on the rough-textured forms outside. Or, we could parge inside and out (extra work and material). What do you think?
Lastly, I’m wondering if we should be putting a vapour-retarder on the interior of the forms. There is not a capillary break between the stemwall and the bedrock, and some portions of the wall sit in bedrock depressions that will hold water. The exterior damproofing will only extend 18″-30″ up the wall, and we will transition to a double-layer of tar paper w/ stucco and lath above grade.
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