Hi. I have a large attic which has staircase access. There is also an air handler in the attic, and the ductwork is mostly in the rafter bays of the roof. I have lots of air seal-penetration types of issues in the attic floor that need to be addressed as well.

I would like to finish the attic. So, the question is… what’s the best way to insulate?

I am thinking that iso above the roof deck, or sip panels, or something along those lines, with a shingle layer (or approriate material for a normal looking residential roof) would be the best because: no thermal bridging, air seal issues in the attic floor become less of a prioirity, the handler and ductwork are now in the envelope, and I do not have to move ductwork or fur anything out to make room for insulation on the inside.

Is redo-ing my roof this way the best approach from an insulation and space-saving standpoint? I am thinking total cost would be more, but not too much more than a closed-cell, fur-out move duct approach.

I guess the other option is to keep the attic vented, and insulate around the finished area?

Any advice would be appreciated.

(NJ shore near the water, climate zone 4).