Original single glazed sash windows can tend to be draughty, Also because they single glazed you can occasionally get a build up of water on the inside of the glass.... Water can stain your sash windows making the inside look dirty, also it can drip down into the wallpaper and cause mould. This happens because of the temperature difference between the outside and inside. Having double glazed sash windows will in most cases eliminate this problem. Modern double glazed units have a thermal break between the two panes so cold from the outside doesn't travel to the inner pane. Having your sash windows draught excluded will also stop the cold air leaking into your house.
The pictures shown below are of a 5 sided bay where we replaced the windows with new high performance double glazed sash windows in Crouch End, London.
I'm busy working on my blog posts. Watch this space!