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

Insulating Chimneys Without Losing the Plot (or Santa)

At Christmas, chimneys tend to feature heavily in the imagination. A glowing fire, a bit of festive cheer, and, apparently, an overnight visit from a man in a red suit. In building physics terms, though, chimneys are far less romantic. They are one of the most stubborn sources of heat loss, air leakage, and thermal discomfort in the existing building stock.

For anyone working in retrofit, they are also unavoidable. Most traditional buildings have at least one chimney, often more, and dealing with them properly is essential if you want to maintain a continuous insulation and airtightness strategy – without creating moisture problems or fire risks along the way.

So how do you insulate a chimney properly, while keeping it safe, functional, and compliant?

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Prefer to watch Chris and Andy from our technical team go through this question. Check out the latest episode of Can I Just Ask? Watch Now!👇

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Why chimneys are such a weak point

Historically, chimneys played a critical role: evacuating combustion gases while providing ventilation and airflow to open fires. In a modern context, however, an open flue is effectively a permanent hole through the thermal envelope.

Even when the fireplace is no longer in use, chimneys can:

• Act as a major heat-loss pathway
• Introduce cold draughts deep into the building
• Undermine airtightness strategies
• Create condensation risks within the flue and surrounding masonry

The aim in retrofit is not to “get rid” of the chimney, but to bring it under control – thermally, airtightness-wise, and hygrothermally – while allowing for safe ventilation of the flue where required.

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Start with the flue: line it properly

If there is an existing flue, the first recommendation is almost always to line it. A traditional open flue performs poorly on almost every level.

A flexible flue liner improves safety and performance, but it also enables the next crucial step: backfilling around the liner with an insulating fill such as vermiculite or perlite. This serves two key purposes:

• It keeps the flue gases warm, helping them rise and reducing the risk of condensation
• It significantly reduces heat loss through the chimney structure

Critically, this insulation should be taken up to the same level as the rest of the building’s insulation. If there is loft or roof insulation, the flue insulation needs to align with it. This continuity is what maintains a coherent thermal envelope, rather than leaving the chimney as a cold bypass.

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Airtightness at the opening: blanking and lining

Once the flue is dealt with, attention turns to the fireplace opening itself. This is typically addressed with a blanking plate to form a robust airtightness layer.

Where a stove or wood burner is still in use, it’s essential that any insulation brought into the opening is non-combustible. This is where mineral insulation boards come into their own. Materials such as Multipor offer:

• A1 non-combustibility
• Vapour-open, capillary-active behaviour
• Thermal performance comparable to wood fibre boards

They allow insulation to be carried safely into the opening without compromising fire safety or moisture management.

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Insulating the chimney breast: wood fibre is fine

A common question is whether wood fibre insulation can be used around chimney breasts. The short answer is yes.

The chimney breast itself does not reach high temperatures, and there is no inherent fire risk in using wood fibre insulation on the external faces of the breast. This allows the insulation layer to continue seamlessly around the chimney and across the rest of the wall, avoiding awkward thermal bridges.

The only note of caution here is practical rather than theoretical: chimney breast masonry is often thinner than expected – sometimes only around 100 mm – so fixing methods need to be chosen carefully. This is one area where aggressive hammer action is best avoided.

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Ventilated capping: don’t forget the top

At roof level, the chimney still needs to breathe. Some form of ventilated capping is typically required to prevent moisture build-up within the flue while stopping wind-driven rain and unnecessary heat loss.

As with the rest of the strategy, the aim is balance: controlled ventilation where it’s needed, airtightness where it matters, and continuity of insulation throughout.

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A small detail with a big impact

Chimneys are one of those details that can quietly undermine an otherwise well-thought-out retrofit. Get them wrong, and you’re left chasing draughts, condensation, and cold spots. Get them right, and they simply disappear into the background – thermally efficient, safe, and unproblematic.

And if, come Christmas Eve, you still want to leave a clear route for Santa? That’s between you and your local building control officer.