>2. I have a portable AC that has to run a duct to the window.

If you only have a single duct, then I don't think insulating it is going make much difference.

AC's have to exhaust heat on one side and exhaust cool air on the other. Both sides need an intake and exhaust. Think of a window AC. The outside portion sucks air in from the outside and blows it across the hot coils to extract the heat. The inside portion sucks air in from the room and blows it across the cool coils and back into the room.

The cheaper portable ACs have only one duct to the outside--the exhaust. Where is the intake air coming from? Yes, the room. So the AC is sucking in cool air from the room and exhausting it out the duct. As you can imagine this is incredibly inefficient.

It is so inefficient, that I don't think you are going to save much by insulating the exhaust duct. You would be better off getting a two duct AC.

However, if you still wish to insulate, then I would use a product called Reflectix:

http://www.reflectixinc.com/basepage.asp?PageName=Duct+Insulation&PageIndex=619