Use code GIFT20
What is the obsession with gussetting the interior and exterior wall together ? Not to mention using double 2x2 top plates. These seem like some very strange design choices. Worse is that using the exterior wall as the structural wall requires the structural sheathing be placed in the most rot-vulnerable position and makes the spans longer for rafters and floor joists. By making the interior wall the structural wall as in Joe Lstiburek's "ideal double stud wall", they could have avoided a lot of problems for no greater cost. Not to mention that having the interior wall structurally sheathed would have made a stronger wall to hang the floor system from than a non-structural interior wall sitting on just slab. Although I do approve of the balloon framing and hung floor system as a way to maintain full R-value from sill to eaves.
Building a standard 2x4 structurally sheathed wall and then adding a non-structural exterior wall that supports no loads other than windows and cladding makes more sense.
Seems like the non-structural sheathing on the exterior is just added expense waiting to rot. Especially if using mineral wool insulation batts instead of cellulose, since bellying is not an issue.
The horror stories of rotting OSB structural sheathing on well-insulated walls, especially behind stucco, really bother me and the typical "double stud wall" still places the sheathing on the exterior wall.
I am wondering if a double stud wall is a solution ... if the internal stud wall is the structural wall with sheathing and the exterior stud wall is just to support cladding. Could the rot-prone sheathing be omitted on the exterior stud wall ? Many homes decades ago were just stucco over studs with no structural sheathing, after all, at least in SoCal.
I am thinking about 2x4 walls with a 3 1/2" gap between them -- interior 16"OC supporting 2nd floor and roof loads and exterior 24"OC supporting just cladding and window headers. The gap and each stud cavity would be filled with Roxul Comfort Batts R-15 for a nominal R-45 wall. The exterior wall would have no sheathing to rot and the interior wall's OSB or CDX would have 2/3rds of the R-value between it and the weather, keeping the dew point away from the sheathing. The exterior wall would only be covered by two layers of 30# felt with hemmed and taped laps, then furring to create a 1/2" rain screen gap, then paper-backed stucco wire and 3-coat stucco.
Interior wall would sit on floor joists and rim joist atop a perimeter crawl space foundation topped by a 2x12 sill plate but the exterior wall would be balloon frame all the way from sill plate to the eaves with a 3/4" CDX connecting the top plates together. Likewise for the window and door bucks. Otherwise the two walls would not be connected by gussets or long screws as so many retro-fit exterior insulation add-ons show. It doesn't seem necessary to tie the two walls together that way and no article ever seems to explain why it would be necessary.
The interior wall would have no penetrations and be closed off with poly taped and caulked. All electrical wiring would run through a horizontal 2x2 strapping service cavity with shallow surface mount boxes on the studs, and the drywall attached to the strapping with no attempt to air or vapor seal the drywall.
This seems like it would allow the interior humidity vapor to dry back to the inside and the weather driven moisture and vapor to dry to the outside.
All of this would be in the SoCal mountains with snowy winters and summers rarely above 85F, so I don't quite believe the "Climate Zone Chart" I've seen applies. Seems like it should be a dry zone 5 at least.
If this does not meet the definition of a "double stud wall" and it is not a "Larsen truss", is there a term for what I am considering ? Is there a reason it is a bad idea ?
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