secondary double glazing

[lustrum]

Hi all,
I am hoping to double glaze my sigle glazed teak windows which are in very good condition. I have read on the web of a company who supply polycarbonate (perspex like) panels which can be fixed on the existing frames using magnetic tape. I would be interested in hearing if any of you have used similar products and what were the results. My main concern is condensation betweeen the panes.
Many Thanks,
Lustrum