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Install Rockwool in the crawlspace with exterior Rockwool in carpenter ant zone?

ToddMartel | Posted in General Questions on

I am a homeowner in the process of having 3 inches (R12) of Rockwool Comfortboard 80 exterior insulation installed as a retrofit  to the exterior of my home in climate zone 6B in a forest in Western Montana.  We will likely install it from the soffits to the ground–covering part of the concrete foundation.  My conditioned crawlspace is not done well, and I want to do the best to make amends now.  Current studbays in my crawlspace are lined with R19 (dirty) fiberglass batts and fiberglass is currently also crudely extended down from the stud bays to cover the concrete foundation and touching floor. On one side of my crawl space, unfortunately, the original builder did not use a sill seal (though it appears to be elsewhere in the home) between the concrete foundation and sill plate,  and pressure treated lumber sits directly on the foundation.  I have checked the moisture content of the wood, and it has not been elevated excessively at any time of year, though it makes me  a bit anxious.  There is some very minor mold beneath the batts in the bays, but it’s not bad, likely in part due to colder winters here and our warm summers with low humidity, which dries things out.

My plan was to 1) apply Xypex to the concrete foundation where it meets the sillplate where there is no sill seal, minimizing the flow of water through the concrete as it crystalizes, and thus acting as a virtual capillary break as opposed to trying to jack up the house and putting in an actual capillary break.  Does this sound reasonable?

2) Pull the dirty fiberglass batts, clean the mold in the bays, spray the studbays with BoraCare, and after installing exterior Rockwool Comfortboard, consider Rockwool batts in the crawl space studbays.  I am not sure about putting rigid foam/sprayfoam in studbays by the ground, as I am literally surrounded by endless acres of trees and thus carpenter ants (even if I spray on the surface of foam with Boracare?).  I am not sure if I should worry about this, but with exterior Comfortboard, the Rockwool batts shouldn’t be at risk for condensation, as the sheathing should stay warm enough where this isn’t an issue, right?  I had planned on just leaving the concrete foundation exposed to the interior and covered with Comfortboard on the exterior.  Rockwool batts would also allow for more drying of the sill plate where there is no sill seal.  Thanks!


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  1. KeithH | | #1

    DIY here:

    Any chance of a picture? I can't quite visualize your project.

    On a previous residence, I installed R-12 comfortboard inside the crawlspace (using plastic masonry fasteners to attach it) without spraying a silicate as a vapor barrier (I would probably use RadonSeal or similar if I did it again) and comfortbatt with canned foam in the truss ends. The crawlspace has an active radon system (fan plus membrane) and there is no sign of moisture or mold before or after the roxul installation. It seems like it worked great, protecting the house an excessively cold crawlspace in the winter while preserving the summer heat mitigating mitigating effects of the earth crawlspace. Also lowered the radon from action levels to near zero.

    I didn't put any studs in the crawlspace and there weren't any before hand. I can't see what advantage they would offer over PMFs.

    In my current home, I did end up spraying the concrete walls with RadonSeal Plus, adding stud bays on the flat on top of the exposed footer, filling the bays with comfort batt (studs on the flat allowed 2" of continuous mineral wool between the concrete and the studs), and covering the whole thing with intello plus. But it's a concrete crawl space used for storage so I didn't want exposed insulation. I filled the truss ends with canned foam air sealing, comfort batt insulation, 1/4" hardware cloth for rodent resistance, and intello plus for vapor control. Again, it has an active radon system.

    I hope that helps.

    I'm guessing the pros here will refer you to articles suggesting that you shouldn't do what I do and I'd guess that they are right. But in the dry rockies, you can get away vapor management approaches that simply bad ideas in the NE.

    1. ToddMartel | | #4

      I did add a picture, though my camera on my phone is pretty much shot. The small hump at the bottom of the wall with the yellow-ish fiberglass is my concrete foundation that is covered in fiberglass. The bays are R19 fiberglass batts on OSB. There is a touch of mold on the OSB behind those batts, as there is no exterior insulation currently, though were we were going to install the 3 inches of Comfortboard 80.

  2. Robert Opaluch | | #2

    Exterior rockwool "install it from the soffits to the ground–covering part of the concrete foundation"? If you intend to install exterior rockwool and make ground contact on the exterior, what would stop carpenter ants from getting up between the rockwool (or whatever you install over it) and the concrete foundation wall, then into your framing, without you noticing? No termite/insect barrier anywhere?

    1. ToddMartel | | #3

      I had planned on installing a Z-girt flashing at the foundation and probably using Cobra Vent attached to the wall and spanning the Rockwool, rainscreen cavity, and attached to the furring strips. The foundation was going to be airsealed with Prosoco Fast Flash, and the lower sheating and studs treated with Boracare. Would that not be sufficient? Would it be better to install the Rockwool a few inches into the soil?

      Excuse my ignorance, as I am a home owner. I was initially primarily concerned with putting foam products in my crawlspace with carpenter ants in the area and with part of my foundation not having a sill seal between the foundation and my sill plate.

      1. Expert Member
        MALCOLM TAYLOR | | #5

        User ...181,

        I'm with you on limiting the use of rigid foam in areas prone to carpenter ants. Your plan sounds good to me. I'm not sure how well the Xypex will work as a capillary-break, but it's better than the alternative you describe.

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