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Community and Q&A

Interior Parge on Nexcem Stemwall? (Conditioned Crawlspace)

BenjaminPries | Posted in Energy Efficiency and Durability on

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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  1. GBA Editor
    Martin Holladay | | #1

    Most of your questions should be directed to a technical representative at the company that manufactures Nexcem blocks.

    It should be pointed out that the fact that your blocks are vapor-permeable is nothing to be excited about. There is no advantage to allowing water vapor to enter your house through a foundation wall. My first instinct is that you should extend the closed-cell spray foam up the interior of the crawl space walls, to provide a vapor barrier and an insulation layer.

    Whatever you do, don't forget to install a vapor-impermeable sill seal between the top of the block wall and the mudsill -- since you didn't manage to have a capillary break between the footing and the wall.

    Good exterior dampproofing details, a carefully installed footing drain system, and the placement of free-draining backfill material are essential for any crawl space foundation.

  2. BenjaminPries | | #2

    Thanks Martin. I have talked to the tech guy at Nexcem a couple of times. He didn't have any issues with what I was proposing, but wasn't sure whether or not it was necessary. I thought the building science familiarity of the GBA community might be more helpful. Regarding the vapour permeability of the blocks, I didn't mean to imply that that was an advantage, but thought it pertinent information to the question at hand. It does seem like less of a liability than it would be in some other crawlspaces as the house is perched atop a granite ridge with only enough backfill to cover the perimeter drain. I don't love the idea of sprayfoaming the whole inside of the crawlspace wall, since I am satisfied with the R28 insulation of the forms themselves, and would like to use a cheaper solution to limit air movement and water vapour if necessary.

  3. ethant | | #3

    Benjamin, what did you end up doing to solve this problem? How did you find the installation of Nexcem block to be? Are you using Nexcem to enclose a basement? Are you concerned about moisture penetration?

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