How do I block a draught from an unused chimney?

A disused open fireplace is usually the single largest hole in a room. It works exactly as designed, drawing warm air up the flue and pulling cold air in through every gap to replace it, whether or not there is a fire.

Safety

Step by step

1. Confirm the chimney is genuinely disused.No fire, no stove, and no gas appliance venting into it — including a back boiler or an old gas fire that has been disconnected but not capped. If there is any doubt, have it checked before blocking anything.
2. Sweep it first if it has not been done.Soot and debris will drop onto whatever you fit and make a mess when you remove it. A sweep also confirms nothing is nesting up there, which is common in a chimney left open.
3. Measure across the flue, not the fireplace opening.The balloon or excluder sits up inside the flue where it narrows, not across the front of the hearth. Reach up and measure the throat — it is usually much smaller than the opening below it.
4. Fit a chimney balloon for the simplest option.An inflatable cushion pushed up into the flue and inflated until it seals against the sides. Inexpensive, removable in seconds, and it deflates automatically if a fire is lit under it — which is a genuine safety feature.
5. Consider a fabric draught excluder instead.A wool or fabric plug that wedges into the flue. Slightly more expensive, easier to remove and refit repeatedly, and it does not perish the way an inflated balloon eventually does.
6. Leave some ventilation.Most purpose-made products are deliberately not airtight, and that is correct. A completely sealed flue traps moisture, which condenses inside the stack and shows up as damp patches on the chimney breast.
7. Label it clearly.A tag hanging into the fireplace, or a note on the mantelpiece. A visitor lighting a fire under a chimney balloon is exactly the scenario the label prevents.
8. Deal with the hearth as well.Once the flue is blocked, the remaining draught comes around the edges of the fire surround and through gaps in the hearth. A bead of flexible sealant around the surround finishes the job.
9. Check the cowl or pot at the top.For a chimney permanently out of use, a capping cowl that keeps rain and birds out while still ventilating is the long-term answer. That is roof work and worth paying someone for.
10. Check for damp after a few months.Look at the chimney breast in the room and, if accessible, in the loft. Staining or a musty smell means the flue is not ventilating enough and the block needs loosening.
11. Remove it before any fire is lit.If the fireplace is ever brought back into use, take the block out and have the chimney swept and checked before lighting anything.

Tips

• Do this in September rather than November. It is the cheapest heating improvement available and it takes one evening.
• A chimney balloon left inflated for years perishes. Check it each autumn rather than assuming it is still doing its job.
• If the room still feels draughty afterwards, the remaining air is coming from the gap between the surround and the wall, not the flue.

Common mistakes

• Sealing the flue completely airtight — Moisture in the stack has nowhere to go and condenses inside, producing damp staining on the chimney breast — a much bigger problem than the draught.
• Blocking a flue that still serves an appliance — A gas fire, back boiler or stove venting into that flue will put combustion gases into the room. Carbon monoxide has no smell and this is how people are killed by it.

If it doesn't work

Questions people ask