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XPS for furring attic kneewall?

CrisPA | Posted in General Questions on

Hello all,

I need to improve an attic kneewall. It’s 2×4 with badly-installed fiberglass batts. While bemoaning that it’s not 2×6, I realized I could furr out 2 more inches and be much better off for the cavity insulation. But as I’ve learned here, the wood acts as a thermal bridge.

So… could I use 2″ of XPS (e.g.) on the studs to make the bays 2×6, put in 5.5″ Rockwool (R23) between, then sheath behind with 1 or 2″ XPS? I’d need long nails or screws to get to the wood for the sheathing and the furring part (anti-cobbling?) sounds kind of tedious.  But is it a dumb idea?

Thanks,
Cris (4A)

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    Don’t bother furring it out to fit thicker batts. Put in R15 mineral wool, which will fit in a 2x4 wall cavity, then sheath the entire rear (exterior) side of the wall with 2” polyiso. The polyiso is more R per inch than XPS, and XPS is not a popular material on this site because of the blowing agents used.

    The furring would just add needless work. The polyiso, or the XPS for that matter, both have greater R per inch than the mineral wool batts, so you’re better off using the rigid insulation as insulation (as sheathing) and not as furring strips to make room for lower performing batts. If you want a higher R assembly , just use a thicker layer of rigid foam.

    Bill

    1. Expert Member
      Dana Dorsett | | #2

      I'ts a lot easier to install 1.5" x 2" thick strips of foam board in the micro-attic behind a kneewall space than it is to install full-width sheathing.

      Rather than XPS (which is the least green insulating material in common use, and loses performance over time as it releases it's climate-damaging HFC blowing agents), cut the edge strips out of foil faced polyiso, which is dead easy to do with a steel wallboard taping knife that has been sharpened on it's edges:

      https://www.finehomebuilding.com/2009/01/29/theres-a-better-way-cutting-rigid-insulation

      To have something to fasten any air-barrier material to it's better to use half-inch CDX or 1x furring on the foam board, creating "Bonfiglioli strips".

      https://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf

      So for the 2" depth adder, half-inch CDX + 1.5" polyiso through-screwed to the studs would give enough room for R23 rock wool, and would more than double the performance of the framing with an R9 thermal break.

      For an attic-side air barrier on the batts it's safe to use perforated aluminized fabric type radiant barrier stapled to the Bonfiglioli strips. While not a true air-barrier, the perforated RB is good enough to prevent wind-washing loss of performance even on low density batts, and with perforations won't create a moisture trap. For the room side air barrier use the drywall, detailing it as an air barrier.

  2. Expert Member
    Peter Yost | | #3

    Bonfiglioli strips!

    Nice one Dana!

    Peter

    1. Expert Member
      Dana Dorsett | | #4

      I didn't coin the term- I think Martin may own that one.

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