Hello all,

I am nearing the end of a large scale reno of my forever home. Climate zone 6

The home went from having a vented attic to a non vented attic in the process (opened up for a cathedral ceiling)

we sprayed 5.5 inches of closed cell insulation directly to the underside of the roof sheathing and finished out the interior build. Fast forward 2 years later, I noticed the roof bowing in some spots in between rafters. Not super noticeable, but I notice it.

upon further inspection the roof sheathing is only 3/8 ply (not something I noticed before)

the roofs asphalt shingles are at the end of their lifespan, so I am planning for a new roof anyways.

I would like to do a stamped metal shingle roof.

my plan is to strip the old roof down to plywood, and add a new layer of 5/8’s plywood to strengthen and straighten out the roof, lay underlayment and install new metal shingles.

I am getting a lot of conflicting information about whether the underlayment needs to be permeable or not.
roofer wants to put a high quality synthetic non permeable underlayment down.

my concern is I would be just trapping potential moisture or condensation between roofing layers.

Does the underlayment need to be permeable or non permeable?
 
if permeable, how many perms?

any recommendations on underlayment?

and lastly do you see any flaws in my plan or how we are going about it all.

any help would be greatly appreciated.

Thank you