Advice on Chimney Liners
Correctly installed, they improve both safety and performance
Here are two documents created for the U.S. hearth industry association on the selection and installation of chimney liners. One is advice for homeowners and the other covers things that dealers and installers should consider.
If you are thinking your chimney might need a retrofit liner, the information in these documents will really help you to understand what the issues are before you make the decision.
Note that the rules for liners are slightly different in Canada, with the result that insulation around liners is not used as often as in the US.