Permeability nonissues & buildability issues in a very dry, warm climate (3B?)
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!
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Replies
Jon as in Jon_R?
Hi, Malcolm—afraid not!
Thanks Jon, I was just curious.
In climate zone 3 - especially the dry parts of it, I think that wall will work well. The only risk I can see is if you don't put insulation in the stud cavities, what is now a mid-wall vapour-barrier becomes a cold-side one when you air condition the interior in hot weather. That might mean some condensation on the outside of the membrane - but would that really affect much? I'll be interested to see what others say.
I think you are overcomplicating your build. Unless you specifically want the open bays for design (which is very hard to do by the way), insulate there.
If you want better fire safety you can go with something like densglass for sheathing with metal siding. By the time the fire gets through something like that, your windows and doors are long gone.
Simple readily available materials and pretty close to standard construction. Densglass is pretty easy to install with either a coil nailer or with autofeed screw gun and it is pretty easy to tape for air sealing. There are now also coated gypsum sheathing options out there that are similar to Zip, you can try local commercial building suppliers.
No issue with Title 24 with no cavity insulation.
Review Sections 150.0(a)-(d) in the code.
Thanks everybody for showing up. This is good input. Been going back and forth over best approach for the combination of price, availability, performance, and buildability for a long time, and while this isn't settled, it's still good info.