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: https://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf) 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 https://www.greenbuildingadvisor.com/article/efficient-walls-budget 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.  https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing, 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: https://www.greenbuildingadvisor.com/article/lstibureks-ideal-double-stud-wall-design).

 Related: https://www.greenbuildingadvisor.com/article/is-cold-sheathing-in-double-wall-construction-at-risk – 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.

Questions:

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?