Found this on a search:

"Cold weather installations require an additional backdraft damper
installed to minimize backward cold airflow. In addition, a non-metallic
thermal break is needed to minimize conduction of cold airflow through
the ductwork. The damper should be in the cold air side of the thermal
break, and the break should be as close as possible to where the ducting
enters the heated portion of the house."

I read this several times. Lot of double talk...OK. Thermal = heat and
break = break. So break the heat. Simple!

I built my own thermal break for my Jenn-Air and didn't know to call it
that - I cut a piece of heavy cardboard and keep it wedged in (where the
big hole is, behind the woven metal filter.

In really, really cold weather, I wrap a piece of aluminum foil over the
top vent and fit it under the edges and re-insert the top vent piece.
Pull it out when you want to grill or use the fan.

May not be pretty but it works. The cardboard is not visible so unless
it is very cold, you don't need the aluminum foil part.

I have a real love/hate with the Jenn-Air. LOVE the ability to grill
inside.

KZ wrote:
I've got a Jenn Air stove with a downdraft fan vented to the outside. In
the winter, cold air seeps in from the outside (I do have a louver mounted
on the outside wall). Sometimes when it's really cold you can freeze a
small drop of water on the stovetop. I usually need to put a dishtowel over
the vent which helps a bit.

The Jenn Air website suggests installing something called a 'thermal break'
which is described as a non-metallic (so as not to conduct heat) piece in
the air duct as close as possible to the outside. No one at my local stores
has ever heard of this device.

Does anyone out there know what this looks like and possible where else I
could find one?

Thanks in advance!
KZ