First, there’s some relevant discussion to my question in these threads:
https://www.greenbuildingadvisor.com/community/forum/general-questions/25062/rainscreen-performance-during-wildfires
https://www.greenbuildingadvisor.com/community/forum/general-questions/19325/rainscreens-wildfire-hazard-and-other-unintended-consequence
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/wrapping-older-house-rock-wool-insulation

However, I am contemplating a detail I’ve not seen discussed, that may address the cladding problems that come along with continuous rockwool insulation in a way that would not be considered for construction in most situations.

I am new construction in SoCal 50 miles from the coast, zone 3 (CA building zone 9), 1500ft, 22″ rainfall average, on a steep slope above riparian/chaparral in a very high fire severity zone.

My baseline R20 (cheaper, better than required by title 24 but not by the 4-5-10-30-60 rule) wall detail is:
– latex
– type C or X drywall
– 2×6 with rockwool batts
– 1/2″ plywood structural sheathing (seams taped with Wigluv 60 or a cheaper substitute as the primary AB)
– 5/8″ Densglass Fireguard screwed to studs
– housewrap (primary WB/VB, secondary AB)
– [possibly horizontal hat channel screwed to studs]
– vertical seam concealed fastener 24 gauge steel cladding sealed at seams, screwed to plywood [or hat channel]

That’s a 1-hr wall. The sealed metal cladding is more of a “drainscreen” but could see some ventilation depending on our detailing at top and bottom. The “chimney” surface behind the cladding is non-combustible.

An alternate wall detail I am considering would get close to the recommended R-30:
– latex
– type C or X drywall
– 2×6 with rockwool batts
– 1/2″ plywood structural sheathing (seams taped with Wigluv 60 or a cheaper substitute as the primary AB)
– 2-2.5″ ComfortBoard IS/CIS, tacked to plywood with washered screw
– 5/8″ Densglass Fireguard (screwed through the insulation to the studs without Heco Topix or squash blocks or Z-girts)
– housewrap (primary WB/VB, secondary AB)
– [possibly horizontal hat channel screwed to studs]
– vertical seam concealed fastener 24 gauge steel cladding sealed at seams, screwed to plywood [or hat channel]

This is getting closer to a 2-hr wall (and I appreciate there are likely to be weaker links and intend to pay close attention to windows and doors, most of which are not situated in the areas of greatest concern. I also have an unvented roof without an overhang). My claim is that the stiffness of the Densglass obviates any of the concerns with squashing the Rockwool insulation, as the screw’s load is distributed over a competent area of the insulation. Hat channel vs no depends on the engineering for attaching the relatively lightweight cladding to the plywood (I may be ignorant and this just isn’t done). Vertical seam is less elegant in this regard as the cladding cannot be screwed directly into studs. However, it appeals aesthetically and is more consistent with existing development.

I intend to build a trial assembly to discuss with our contractor, but before I did I wanted to ask the Pros (especially those familiar with Rockwool exterior insulation): do you see any problem with the second assembly?

Cheers,

S