Jonathan wrote:

On Wednesday, 1 November 2017 16:48:35 UTC, Fevric J. Glandules wrote:
Helping someone out with this... their back door has a panel in
the lower half which must be all of 2mm thick. I am thinking
to insulate this in some way but cannot decide on the best
material to use. As "not being ugly" is apparently a consideration.

Any thoughts?

On our timber doors I removed the moulding and used bubble wrap followed
by another piece of ply to hold it in place,

Thanks for the suggestion!