Vent-hood ducting need insulation – GreenBuildingAdvisor

The vent duct should be insulated if run through unconditioned space, the same as you’d do for a bathroom exhaust fan. If you can keep it in conditioned space, that part of the duct doesn’t really need insulation (although sometimes insulation will quiet it down a bit).

I would pick a path for the ductwork that keeps the run as straight as possible as a priority, then insulate it where needed. The goal should be maximizing airflow through the duct, which means keeping bends to a minimum. It’s not that big of a deal to insulate the duct where needed, so that should be a secondary concern as you plan the duct pathway.

Bill