I have deep windows frames like this:
Rather that adding a whole secondary glazing frame on the inside. Could I add a spacer/battening to the inside of the existing frame and second pane of glass? I could hold the glass in place with either more battening or glaziers putty. So if the window has 3 seperate panes, I would do this for each.
Sounds like it would work to me. What are your concerns? Poor insulation, condensation? The spacer will be visible from the outside and make the window slightly smaller but I think I can live with that.