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Ok to underfill wall cavity?

| Posted in General Questions on

I’m building a home in climate zone 5 and have an average temp of Dec, Jan, Feb of 20 degrees. 

the wall assembly I’m considering is 2×6, 24 inch on Center with 2inch zip R sheathing.  I know in climate zone 5, the chart says that with a cavity R value of 19 the exterior insulation should be at least 7.5.  However after doing the calculation, I found I will actually need a higher exterior insulation R value.  Currently the zip R I’m using is R9, so I can’t fill the entire cavity up to R19 without causing the interior face of the zip R foam to fall below the dew point.  My proposed solution is to fill the 5.5 inch 2×6 cavity with 1 inch closed cell spray foam against the zip R foam, effectively bringing the “exterior insulation” up to 15.  Then I plan to fill the remaining 4.5 inches with a 3.5 inch Roxul batt.  This would keep the foam definitely above the dew point, but would also leave 1 inch of vacante space between the drywall and the roxul batt.  Would the roxul batt stay secured in place against the foam or would it need to be secured?  Thanks.  I’ve added a mock up to show what I mean.

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Replies

  1. brian_wiley | | #1

    There were a couple of things that struck me about your question. The first is just the relative percentages. Just so we’re on the same page, cz5 requires 27% of the insulation to be on the exterior, at least according to this article. https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing

    That’d mean that your Zip-R panel at r9 would be about 32% of your wall, so it looks like you’d be good without any sort of additional treatment.

    The other thing I’m wondering is if we should be thinking about the zip-r in a different way? Of course it does provide continuous insulation for that thermal break, but the sheathing is on the outside. If I’m understanding things correctly, those ratios are for when the sheathing is sandwiched between layers of insulation and therefore at risk of being a condensing surface without drying potential. If that is the case (and someone please correct me if it is not) then those ratios don’t apply when using a zip-r panel.

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