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How would you insulate this?

Jon_Lawrence | Posted in General Questions on

I need to decide how to insulate the underside of my unvented, conditioned attic.  The sheathing is 5/8″ Zip with 4″ of polyiso on the exterior.  Originally, I was planning on dense packing, but that is problematic in the area above the collar ties because: 1) it is difficult to attach netting to the underside of the rafters in that space, 2) I can’t just dense pack the entire triangle from the collar tie up to the sheathing because my ratio of exterior to total R-value would be too low at the ridge which is where the greatest risk is.

I am in Climate Zone 4 so I need at least 31% of the R-value to come from the polyiso.

I am considering using Roxul in the rafters to work around this issue.  It would be a bit shy of the rafter depth, using one layer of 3.5″ and one layer of 5.5″, but I would fit it tight against the sheathing.  I would then run Majrex from the floor, up the knee wall and along the bottom of the collar ties.

The kneewall on the other side is not the exterior wall, but a partition wall, so the Majrex on that side would be attached to the underside of the rafters.  The entire area will also be drywalled.  The light fixtures will be in the partition walls, so no penetrations through the Majrex.

I have 2 questions:
1) Will the plan using the Roxul work?
2) Am I overthinking this and should I just DP the entire area including the area from the sheathing to the bottom of the collar ties?

Thanks,
Jonathan

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jonathan,
    Q. "Will the plan using the Roxul work?"

    A. Yes, as long as the Majrex is strong enough, and well-stapled enough, to keep the mineral wool in place. (I have never installed Majrex, so I don't know how fragile it is). Another caveat: your local building inspector has to agree that it is OK to leave Majrex exposed in an attic -- the inspector needs to agree that it isn't a fire hazard.

    Q. "Am I overthinking this and should I just DP the entire area including the area from the sheathing to the bottom of the collar ties?"

    A. No, you aren't overthinking this. The foam-to-fluffy ratio matters.

    1. Jon_Lawrence | | #2

      Martin,

      Thanks for the response. The Majrex is very strong. I have gone through a few rolls already with no tears. I am using the double-sided tape which I find easier than stapling. The Majrex will be covered with drywall so no inspection issues.

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