Thus when it is cold it will get very cold and that will mean that it will attract condensation – which is a by-product of the burning of your fires (hence the term 'non condensing furnace'). This will attach onto your chimney liner and this is a serious problem as condensation is acidic and highly corrosive – meaning it would eat away at the liner and eventually completely deteriorate it.

Worse, if you get condensation on your chimney liner, then you will likely also get creosote on your liner and this is very danger as creosote is a dangerously flammable substance which can cause chimney fires. The colder your chimney is, the more creosote will stick to it, and the more creosote is on your chimney, the colder it will become.

However bear in mind that this insulation is more crucial for wood burning chimney liner or coal chimney liner. For gas or oil burning fireplaces you will usually be fine with the flue liner on its own.