I have a huge stained glass window that was imported from Germany in the early 1900s (1910 as far as I can find). It was originally created in the mid 1800s. It's bowed and has some spots where the glass comes away from the lead (at least it looks like lead) lining.
It's about 8' by 4' and I need to insulate it for the winter. Cold air not only blows through the gaps, the glass itself is not very insulated at all. I can feel a cold patch and the foyer (where it and another old large window are located) is at least 10 degrees cooler than the adjacent rooms.
Is there anything I can do to significantly insulate this and the other window without risking any damage to either? It has all original moldings around it and beautiful plaster walls that I'd be very disappointed if I were to damage any of the reasons we purchased the house.