The basic question:

Is there any reason why we couldn’t use a flash-and-fill (foam+cellulose) insulation approach on a double-stud wall (probably about 12″ overall), using open-cell spray foam instead of closed-cell, as long as the foam is thick enough?

The background:

First off, I’m probably over-thinking this.  I do that.  I just hope I’m not asking a stupid question.  But it’s very possible I am.

I live in central Utah (Climate zone 5; think mixed hot-summer/cold-winter, but not extreme for either, and almost *always* low humidity).  My wife and I are designing a new house that we hope will be very energy efficient, with above-grade walls somewhere in the R40 ballpark.

From what I’ve read, one of the cheapest high-R-value walls is usually a double-stud wall with dense-pack cellulose.  My only concern with that, is that it’s traditionally built using a single cavity from inside-wall to outside-wall for ease of construction, but that means the air barrier is all the way on the outside of the assembly, right below the rainscreen and vented cladding.  I’m slightly worried about condensation in the winter.

I know that BSC advocates [1] for a double-stud wall where the air barrier is on the outside face of the inside wall, but I suspect that will be more complicated (therefore expensive) to construct.

So that leaves us with a hybrid approach, like this one [2].  As I understand, having the spray foam effectively moves the condensation plane to the inside face of the foam.  And the foam can help with a secondary air seal at the outside sheathing as well.  For the most part, that would probably be fine, but there are a few places where we’ll have vapor-impermeable surfaces (eg. a tile shower) against the inner face of an exterior wall.  So that section of the assembly at least can’t dry to either the indoors or outdoors.  The ony way for it to dry would be for the moisture to migrate within the cellulose along the length of the wall, until it can get past the vapor-impermable inner-surface, and dry toward the interior.

So that leaves us with open-cell foam in a similar approach.  From what I understand, if the open-cell foam is about 3.5″ or thicker, it’s supposed to be a pretty good air barrier, but still be vapor open.  That would mean the areas with the vapor-impermeable inner surface could still dry to the exterior, and most of the building could dry in either direction.

I’ve seen a few instances (eg. [3]) where people ask a similar question, but they’re talking about a very thin layer of foam, eg. 1-2″, whereas I was thinking about 3.5″-4.5″ (R12.25-R15.75) before doing the rest of it using dense-pack cellulose (probably 7.5-8.5″ or R26.25-R29.75).  The option with the smallest portion of the overall R-value outside the air barrier (the inner face of the foam) here is basically 3.5″/R12.25 foam and 8.5″/R29.75 cellulose, or 29.17% outside the air barrier.  Based on this blog post [4], it looks like we only need about 27% outside the barrier for Zone 5, so this would comply with that guideline.  And we could probably do thicker foam if we wanted.

So after all that long explanation (Yeah. I can’t write a short email either.), I’m back to the original question.  Can we use a flash-and-fill insulation approach on a double-stud wall, using open-cell spray foam instead of closed-cell, as long as the foam is thick enough?  Is the 3.5″ that I’ve read about the correct minimum thickness for the open-cell foam?  Am I missing some other consideration entirely?  Am I overthinking this whole thing?  Should I stick with the single-cavity double-stud cellulose wall (eg [5]), and trust that it’ll dry out, especially since we’ll have a rain-screen, and am in a fairly low-humidity climate?

The Footnotes/References:

[1] https://buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/high-r-value-double-stud-wall-construction
[2] https://buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/high-r-wall-double-stud-with-spray-foam-wall-construction
[3] https://www.greenbuildingadvisor.com/question/open-cell-spray-foam-for-flash-batt
[4] https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation
[5] https://www.finehomebuilding.com/project-guides/insulation/a-case-for-double-stud-walls