Full length mirror on damp out-side wall

[yottie]

My wife wants a full length mirror fitting in the bedroom on an outside wall
which I'm presently stripping prior to patching and applying a damp-proof(?) paint. The wall was previously papered but I intend to "let it breathe"
without any paper on, the outside has a good coat of Weathershield.
Assuming that this damp problem will re-occur what is the best way to fit this mirror, wooden backing, battens, tin foil....? would welcome any help.
NB this is a 100 yr old cottage.
Thanks.

 

[^woody^]

Glue some 3-5mm polystyrene to the back of the glass (like the stuff on rolls used to line walls) and then fix the mirror on battens or similar, to give a 15-20mm air circulation gap behind it -