Hello all!

I’m planning to retrofit exterior insulation over my open beam (T&G) cathedral ceiling.  I live on the Northern California Coast, zone 3 marine (almost north enough to be zone 4).  Winters are kinda cold (40 degrees ave winter low, outside humidity generally 85%-100%, up to 100 inches of rain; inside humidity often over 80% at 67degrees, am working to lower this).  Summers stay cool (typically 65 degrees 65% humidity).  Wildfire is a concern, though I’m only a mile in from the coast (so its cool and humid).  I’m surrounded by oak and redwood forest.

Concerns:

Fire!

Wildland Urban Interface (WUI) recommendation are very confusing, and I haven’t found much on GBA yet (or elsewhere on line) about how this might affect current recommended practices. FEMA recommends gypsum (meeting ASTM C 1177 requirements) over polyiso (itself set over the sheathing)- https://www.fema.gov/media-library-data/20130726-1652-20490-4085/fema_p_737.pdf.  Would a couple of inches of Rockwool above the polyiso (and on the sides by the eave trim) do the same trick?

Condensation!

I’m planning unvented above-deck insulation, first by taping the current plywood sheathing (is 2.4” Siga Wigluv tape enough or should I go 3.5”); then put a vapor barrier (do you have a recommendation which?); then two layers of 2” polyiso (staggered and taped–  do you recommend a tape for this?), then 2” rockwool, then sheathing, another underlayment, and shingles.  Should I also airseal the edges of the polyiso sheets by the eaves? Any suggestions as to how to if so?

Finally, I wonder if upgrading the ceiling increase the mold concerns on our underinsulated walls?  

Thanks so much!!!!

Mark