Basically converting the downstairs of my house , into a couple of flats , the windows are perfectly good(not rotton etc) sash windows , around 7 foot high by around 4 foot wide , firstly it would be very costly to invest in number windows for the place , due to them been curved at the tops and over a certain height , so toughen glass would be needed , so i was thinking to make it much more hopefully cost effective was to fit internal secondary glazing , i've no experince which such a concept and was wondering , if really its a bit of a waste of time etc , or if its good stuff and cost effective ? anyone got any advice or past experinces with the stuff????
TIA