In my student days I lived in former MOD married quarters that had been taken over by the University as digs. All the windows were single-glazed, and as this was Aberdeen it made for a very cold house in winter. I scavenged a roll of bubble-wrap and secondary glazed my bedroom window with that. It worked, as a cheap and ugly solution that significantly reduced light transmission.
If you don't want to go to the expense of double-glazing, go with secondary glazing. I can remember it being very popular in the 80's - particularly for sash windows as there wasn't a practical/affordable double-glazed option for those until comparitively recently.
Curently collecting recipes for The Little Book of Liqueurs..