Permeability nonissues & buildability issues in a very dry, warm climate (3B?) – GreenBuildingAdvisor

Hi.

Longtime poster, occasional return visitor. Recipient of a few Q&A Spotlights. 

Availability of materials where I am, in the CA high desert, is a mixed bag. Even Rockwool is tough to procure (yes, I’ve taken this question up with Roxul directly, too). Nevertheless, exterior Comfortboard80 is still my plan for the walls.

Speaking of my walls, here’s my question: in this extremely dry firestorm world we’re living in, why should I care about permeability of my vapor barrier? I think perhaps I shouldn’t. We’re desperate for a little humidity around here; let the occasional moisture of occupants dry to the inside where our throats and our skin can experience its fleeting joy. 

Getting ZIP around here is a nonstarter. How about I peel and stick outside all my plywood sheathing with a Class A impermeable fire barrier along the lines of Grace Vycor, then a layer of rockwool (I insist I’m gonna get it hell or high water) and my cladding.

Title 24 might make me waste money filling my stud bays, TBD, but let’s first hope they’ll get to be empty.

Edited to add: per Chris Corson’s old approach in a cold and wet climate, I used buried poly sheeting on an exterior polyiso build, 2010, in Minnesota with zero issues. And this desert climate is bone dry.

Thank you in advance to any professionals still hanging around here from 15 years ago. All right. Let me have it!