SUMMARY:  Is cutting-and-cobbling polyiso into an unvented cathedral ceiling in CZ-2 (hot/humid) safe (re: condensation/rot under decking), if the decking has lasted 43 years with no decking rot, fiberglass batts in rafter bays, and no rigid insulation above the decking?

DETAILS:

I have a ~400sf unvented cathedral ceiling with 5.5″ rafters in CZ-2 (hot, humid), that was built in 1981.  The builder used fiberglass batts as insulation.  It’s woefully insufficient.  There ‘re also recessed can lights.

I’ve seen Martin’s excellent article, Five Cathedral Ceilings that work, which I won’t link to, because experience has taught me that my post won’t get posted because it’ll get flagged as spam.

Here’s how I’ve evaluated the options:

(1) Fiberglass-only.  It’s not sufficient R-value.

(2) Fiberglass + 2″ of polyiso under rafters.  Also insufficient, and the lights would have to be repositioned, which means running all new wiring, because I can’t just pigtail onto the existing wiring, because code/safety says that connections must be accessible (i.e., you can get to it without tools).

(3) Add 5.5″ of polyiso above the decking.  Requires removing existing roofing, building a perimeter, and using super-long screws to install, so probably cost-prohibitive.

(4) Closed-cell spray foam in rafter bays.  At $4/board feet, probably also cost-prohibitive.

(5) 4″ spray foam under decking, the rest fiberglass.  Insufficient R-value with 5.5″ rafters.

(6) Cut-and cobble polyiso in rafter bays under decking.  This won’t get me code-level R-value, but it’ll be around R-35, about double the R16.5 I estimate I have now.  I know cut-and-cobble is frowned upon, I think because of the risk of warm air reaching the underside of the decking, condensing, and causing rot.  But I think I already have that risk now with the fiberglass batts and no insulation above the decking, so if I were gonna have that problem, I should have had it sometime in the last 43 years, and installing polyiso shouldn’t make it *worse*.  In fact, it should be safer, since I would caulk the holy living mortal crap out of each layer of polyiso.

So, is cut-and-cobble viable in this situation, or am I courting trouble?