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Community and Q&A

Evaluating the Bonfiglioli Wall for Climate Zone 6

johngfc | Posted in Energy Efficiency and Durability on

Zone 6b, Colorado

Elev: 7300’,  ~ 22”/yr precipitation (~ 50% as snow)

~7500 HDD now but declining ~ 37 Days/yr (1980-2019 data)

The site faces S, on a S-facing hill, full sun, and warm for the area.

 Would/could a Bonfiglioli wall (B-wall) as described in FHB: be suitable for this CZ6 location? The wall is (inside to outside):  drywall, vapor retarder (e.g. MemBrain), 1×3 aligned with studs, 1” foam strip on stud, 2×6, sheathing, WRB/rainscreen, cladding. Reportedly R-30 with high-density fiberglass in the stud bays (no foam), and requires careful air sealing.

I’ve seen but I didn’t note any evaluation of the suitability of the B-wall for CZ 6. It’s clearly advocated (by some) for CZ5, but appears non-compliant with IRC and other GBA discussions – e.g., which advises exterior foam at least R-7.5 for 2×6 walls in CZ5 and R-11.25 for CZ6. Lstiburek (repeatedly) advises against a wall where the sheathing is cold (but has a double-stud with sheathing in the middle:

 Related: – long and detailed discussion for _thick_ walls. Martin (comment #27) says external insulation was (at that time) allowed throughout the US. In our area, IRC 2018 applies.


1.     Is a B-wall suitable for my CZ6 location? I’ve been leaning to 2×6 with 2″ of external form or rock wool, but the B-wall looked interesting, if it’s safe.

 2.     Can a B-wall actually achieve R-30 with fiberglass, as claimed?

3.     With the B-wall, would there be a noticeably lower risk using dense-pack cellulose rather than fiberglass?

4.     The B-wall isn’t consistent with IRC CZ6 guidelines for external insulation (20+5, 13+10). Is external insulation always required by code in CZ6? B-walls, Larsen and double-stud walls seem to be promoted for cold climates without external insulation.  (a) Is moisture likely to be a problem with the B-wall described above, and if not, why?  (b) Is there an exception to IRC and other guidance (E.g. Moisture Control Guidelines 2020 – ABTG Research Report No. 1701-01) for these walls?

5.     With declining costs of PV, increasing ASHP efficiencies, and warming trends, is there a comfort or financial case for ~R-40 over ~R-30 (U 0.025 vs U 0.033) walls in a “warm” zone 6 location? Would improved glazing likely be a better investment?

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  1. Jon_R | | #1

    Make sure that the wall complies with the recommendations in Table 2 A or B here.

    No, code doesn't require exterior insulation. But it's a good idea. So is cellulose (vs fiberglass).

  2. jwolfe1 | | #2

    You asked some great questions. I'm sorry none of them were answered.

  3. Jon_R | | #3

    Lots of additional options/answers here:

    > is there a comfort or financial case for ~R-40 over ~R-30 ... improved glazing

    No and use BEopt.

  4. mdb_az | | #4

    Bumping this question as I’m curious to see it answered!

  5. Expert Member


    Bonfiglioli walls only make sense compared to the alternatives (deeper studs, Mooney Walls, exterior insulation) if you are doing the work yourself. Otherwise the labour component is much too high.

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