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Securing insulation batts perpendicular to wall studs?

hertzr | Posted in General Questions on

I recently re-did my Cap Code’s attic insulation. Air sealed as best I could and installed R-30 rock wool batts in the floor joists and R-15 in the knee wall cavities. I then nailed 1″ XPS foam board on top of the floor joists and on the backside of the knee wall, covering/air sealing the newly installed batts.

It was cheaper to do a bulk purchase of the R-30 batts, so I have enough leftover to place a second layer perpendicular to the floor joists on top of the foam board. I could also do the same on the knee wall if I can figure out how to secure the batts.

My question is – anyone have ideas on how to best secure batt insulation perpendicular to wall studs? Am hoping to not re-frame behind the existing wall studs (the R-30 batts are made for 2x8s).

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  1. user-2310254 | | #1


    Can you tell us your location? Insulation advice is often climate-specific.

  2. GBA Editor
    Martin Holladay | | #2

    There are a couple of issues here. In theory, your decision to install a cold-side vapor barrier (the 1-inch-thick XPS) could come back to haunt you, because you've created a condensing surface where moisture can accumulate. The colder your climate (and the colder the attic), the bigger the risk. Moreover, the leakier the ceiling, the bigger the risk.

    In general, you never want to install a horizontal vapor barrier on the exterior side of attic insulation.

    The vertical vapor barrier on your kneewalls is less risky, because the stack effect won't affect your kneewalls as strongly as it will the horizontal foam on your attic floor.

  3. hertzr | | #3

    Thanks for the reply. Am in climate zone 4A (central MD).

  4. GBA Editor
    Martin Holladay | | #4

    If I were you, I would remove the horizontal XPS that you installed above the attic insulation. You can leave the vertical XPS on the kneewall.

    Now that you have so much leftover XPS (because you can't install it horizontally), you can add another layer or two of XPS on the exterior of the kneewall -- solving your problem of attaching the mineral wool batts.

  5. hertzr | | #5

    Interesting, thanks! I thought 1" XPS is a class III vapor retarder (~1.5 perm rating at 1") vs. a vapor barrier - that was the main reason why I went with it. That and to provide a better air seal on the floor and knee wall areas.

  6. GBA Editor
    Martin Holladay | | #6

    Most sources list a lower permeance for 1 inch of XPS. My charts usually say 0.4 perm to 1.2 perm, depending on the brand. That will allow a very limited amount of drying to the exterior.

    If you did a good job of air sealing the ceiling layer (under the bottom layer of mineral wool batts), you might get away with the XPS. It would make me nervous, though -- especially on cold nights.

    Adding another layer of R-30 mineral wool over the XPS would warm up the XPS layer and reduce the risk. As I said, you might get away with it. But if this were my house, I'd get rid of the XPS and just blow cellulose over the existing R-30 mineral wool.

  7. hertzr | | #7

    Martin, that's great information, thank you!! Was wondering about XPS's real-world perm performance (am using Owens Corning Foamular 150, which they rate as having a "max perm" of 1.5).

    As for moving the XPS on the floor to the kneewall - in your view, how thick is too thick for a vertical surface? As in, do you see danger in 2" thick XPS? What if it were 4"?

    Am trying to maximize the kneewall's R-value in the short term. My attic, which is directly behind the main bedroom, is currently very poorly vented and gets upwards of 140F on hot, sunny days. I plan on addressing the ventilation issue at some point this fall by expanding the few (3) soffit vents currently present and possibly installing a ridge vent (I'd then plug the existing gable vents).

  8. Expert Member
    Dana Dorsett | | #8

    A maximum spec is maximum value, not the nominal value. For design purposes assuming 1.5 perm max foam more than 1 perm would be a mistake.

    There is no increased moisture risk for thickening the foam on a knee wall, as long as it's all on one side.

    A ridge vent needs corresponding soffit vents, ideally with more than half of the total free vent area being at the soffits (soffit vent area greater than ridge vent area) to reduce the amount stack effect infiltration drive from the floors below. Attic ventilation is about purging moisture, not cooling, and the cooling benefit of attic ventilation is modest at best.

  9. GBA Editor
    Martin Holladay | | #9


    Q. "Do you see danger in 2-inch-thick XPS? What if it were 4 inches?"

    A. The thicker the rigid foam layer, the safer the assembly. For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation.

  10. hertzr | | #10

    This is all great info. Thank you all for the guidance!!

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