Insulating range hood exhaust duct in vented attic with leftover rockwool

I’m in Seattle, climate zone 4C. I have a vented attic (with fun fun half story and kneewalls). Through it, I am running an 8in round duct for the range hood exhaust that goes from the kitchen ceiling out the roof. It’s not carrying conditioned air I’m looking to blow into the house, but it still seems like a good idea to insulate the duct to reduce thermal bridging and cut down on noise. The rest of the attic will have about R60 of blown cellulose insulation.

So the simple thing to do is to just get some of the foil-faced fiberglass duct sleeves. Those only give R6 though, which seems kind of low. Also, I have some leftover rockwool ComfortBatt (and Safe ‘n’ Sound). I’m wondering if there’s a way to reuse that and achieve a much higher R-value. My understanding is that it’s generally recommended to have a vapor barrier on duct insulation to avoid condensation. Just to confirm, that applies to range hood exhaust ducts too, right? (I assume it would, but double checking)

Assuming a vapor barrier is required, would it be reasonable to wrap the duct in rockwool batts (probably holding it on with twine or something) and then put something else around that as a vapor barrier? If so, any recommendations on what to use? I’ve gathered that the bubble-wrap reflectix insulation stuff is next to useless on its own, but my understanding is that has to do with it needing an air gap from the duct. Would it be effective as a radiant barrier and vapor barrier when wrapped around some “real” insulation? Is the radiant barrier worth with it? Could I just wrap the whole thing in 6mil plastic sheeting (which I also happen to have extra of)?

Bonus question: inside the thermal envelope would it be OK to wrap the duct in rockwool where it sits in the kitchen behind the decorative chimney? I’m guessing in that case no vapor barrier is needed as it’s unlikely to get sufficiently cold or humid, but would it still be a good idea to wrap the rockwool in something just to keep it encapsulated?