Type of insulation for flexible aluminum duct?
Insulated flexible plastic ducting is a common item available at any big box hardware store. Unfortunately not so much for preinsulated flexible aluminum ducting.
Sometimes code prohibits the use of non-metallic flexible ducting (for example often with range hood exhausts)
First I would check whether non-metallic ducting is allowed for your bathroom exhaust. If it is, then just purchase the insulated new ducting. This will be the easiest
If you are required to use metallic ducting, and the run isn't too long, then I have had good luck buying the insulated non-metallic ducting one size larger. You can then sleeve it over the metallic ducting. Sometimes it fits straight away, but sometimes you need to pull the inner ducting out leaving just the fiberglass lined sleeve.
Secure along the run with occasional zipties, and tape and tie at both ends.