Chimney Flue Insulation Process Explained

The process of insulating a chimney flue is aimed at ensuring the safety of your family and home when you use your chimney. Learn more about the process of chimney flue insulation below.

Why Insulate a Chimney Flue?

Chimney flues are subjected, especially in harsh winter weather, to a wide temperature gap. These sudden temperature changes can cause cracking and breakage of materials even as tough as brick and concrete. The other main reason to insulate a chimney flue is to reduce the moisture content of the air coming down the chimney and interacting with the heat source below. Dampness eats away at the mortar of a chimney quickly and can cause it to collapse.

The First Step to Insulating a Chimney Flue

The first step to insulating a chimney flue, done when it is first made, is called parging. Parging aids in the containment of gases from the fire, whether its source is coal or wood. This insulating layer is usually made of lime and sand, sprayed inside the chimney just as the mortar is drying. It helps force the gases up and out of the chimney instead of allowing the gases to seep out through mortar cracks. You can tell your chimney parging is failing when you see stains on the top shelf of your chimney, called the chimney breast. If you see vertical cracks in the chimney flue, this also indicates the chimney should be relined.

Relining the Chimney for Effective Insulation

After parging, one of three types of liner is applied to the chimney. The liner can be cast concrete, a solid liner applied in sections, or a metal liner.

Cast Concrete Liners

Cast linings can be applied easily and quickly to a chimney. A rubber tube inserted down the flue from the top is inflated. Every 2 meters, some concrete blocks are set into the chimney stem to make the smoke and gases move from side to side in the chimney, following a slightly winding, instead of a vertical, path. This type of liner cuts down on heat transfer but also greatly reduces air draft.

Solid Sectional Liners

These, made of terracotta clay or concrete, are most often used in large, wide, vertical chimneys that are very straight. It is easy to remove and replace these liners, as they can be built in when the chimney is restored.

Metal Liners

Rigid, stainless steel metal liners are used for the large open flues of wood-burning stoves. The liners are attached to the chimney flue in sections with clips at the overlapped joints. The space between the liner and the flue is filled with vermiculite or other granular material.

Double-skin liners, also made of stainless steel, have a smooth interior and a rough, ridged exterior. They are used on any type of chimney flue, and due to the ease of installation, can be removed, repaired and reinstalled quickly.

Gas flex liners are used for closed stoves and gas boilers only.

Therefore, when insulating your chimney flue, take note of the insulation process that was used the first time, and plan to use that type again.