range hood ducting with thermal break

I am planning to install a range hood. A couple of sources recommend adding a nonmetallic thermal break between the duct that runs out to the exterior wall cap and the duct that runs back to the range hood--placing it as near to the wall as practical. This seems like a good idea for someplace like Minnesota.
http://www.ajmadison.com/ajmadison/itemdocs/8285116.pdf (whirlpool installation instructions) http://www.ventingdirect.com/pdfs/KitchenAidguides/9763388.pdf (kitchenaid installation instructions)
Except for the suggestion in one of the above references to use a "short length of nonmetallic duct", I have not seen anything that actually suggests specifically how to implement this thermal break. I have not seen 6" diameter nonmetallic duct anywhere--unless 6-inch PVC could be used.
I was hoping to learn how someone else actually did this.
thanks
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brent" <"north_sea_brent at yahoo dot kom wrote:

I don't have a suggestion other than to make sure that whatever you use is properly rated for that use to prevent a possible fire hazard
--
Joseph Meehan

Dia duit
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