if condensation is building up on the glass ( inside the house) then youll want to remove the trim and check that the RSO is insulated ...if the condensation is between the panes of glass then time to change the door ...579$ for a 5 footer 639$ for a 6 footer....RONA
Joko your idea is a good one ...R7.5 ISO board is 29$ for a 4x8 sheet...if you use tuck tape ( red stuff) to seal it ...pick up a Xylene based cleaner to remove the red film of glue it transfers to your trim/jams when its warm enough to take it apart
Good Luck