How to Internally Insulate a Chimney? – Can I Just Ask? | Ep.23

Chimneys are one of the most common yet overlooked sources of heat loss in older homes, and in a retrofit context almost every property will have at least one to deal with. Despite their cosy associations, an unaddressed chimney creates a significant gap in the building’s thermal and airtightness envelope — drawing cold air in and allowing warm air to escape. The approach covered in this episode focuses on creating a continuous insulation line and a proper airtight seal at the chimney, rather than simply ignoring it or blocking it off crudely.

For chimneys with an existing flue still in use, the recommended starting point is to line it with a flexible liner and backfill around it with vermiculite or perlite. This keeps the liner warm so combustion gases continue to travel up without cooling and condensing — and critically, that backfill needs to reach the same level as all other insulation layers in the building, whether that’s loft or roof insulation, to maintain a continuous envelope.

At the opening itself, a blanking plate creates the primary airtight seal, and where a wood burner or fire is still in use, Multipor mineral insulation board — an A1 non-combustible rated material — is used to insulate directly into the opening. Wood fibre can then be used freely around the outside of the chimney breast, since the breast itself doesn’t get hot; the only practical consideration is that chimney breast brickwork is often only around 100mm thick, so hammer-action fixings should be avoided when securing boards in that area. Beyond that, it’s a straightforward process that integrates neatly into a standard IWI installation.