Glaze

(When insulating a liner for a wood flue or a fireplace flue)
Creosote is just smoke that condenses on cool surfaces. So the colder the smoke inside the flue, the more creosote buildup you’ll have and the harder it will be.

The creosote can turn so rock hard that it cannot be brushed off using conventional methods of cleaning. This is called “Glaze” (which is very flammable and can cause chimney fires!) So insulating a liner for a wood flue (or fireplace flue) helps to keep the smoke hot to prevent this glaze from building up. Insulating also does away with the “clearances to combustibles” needed around chimneys (per code.)