Improving insulation and sound

Why 10mm? Do you expect to need it to be bullet proof as well?

Good thick heavy curtains will prevent more heat loss than secondary glazing when they are closed. Hard to predict how well it will perform against traffic noise since it depends on what sort of noise and how it is coupling to the room. Wider spacing and having the inner pane damped rather than rigid and taught like a drum skin will help.

You will need a UV stabilised grade with the UV protection facing outwards. Glass stops some UV but not enough to avoid degradation of water clear plastics in windows.

Normal double glazing isn't a very good sound insulator as the gap is too small. If you fit secondary glazing the thickness of the wall away from the original you'll get very good results. Glass will be better than plastic because it is denser - the thicker the better. And it needs a good air seal all round.

Also if there are drafts from the original windows, secondary glazed sliding panels can rattle annoying ly!

Brian

To work correctly, sound wise, both need to be air sealed. Which means the secondary glazing should have runners similar to that used on cars which holds the glass firmly.