Why Damp in Chimneys Is a Serious Problem (and What to Do About It)
When people think about damp issues in a home, their minds usually jump to leaky roofs or condensation on windows. But one of the most overlooked – and potentially damaging – sources of damp is the chimney.
Even if your chimney looks perfectly solid from the outside, hidden moisture problems could be causing significant damage inside your walls.
Here’s why you should take chimney damp seriously — and what you can do to prevent it.
How Much Water Can a Chimney Hold?
You might be surprised to learn just how absorbent old bricks can be. Some types of bricks can hold up to 1 litre of water per brick. Multiply that by hundreds of bricks stacked up through your home, and it’s easy to see how even a relatively small chimney can become a massive water sponge.
When bricks become saturated, several problems can follow:
Internal Damp Stains: Water can seep through walls, causing ugly damp patches inside your home.
Masonry Damage: Repeated freeze/thaw cycles (especially in colder months) can cause bricks to crack or spall.
Mould Growth: Damp environments are ideal breeding grounds for mould and mildew, which can damage your home and affect your health.
Disused Chimneys Are Especially Problematic
If a chimney is no longer in use, it can become even more troublesome.
Without a constant flow of warm air from fires or stoves, the inside of a chimney can become cold and damp — an ideal environment for moisture to settle and accumulate.
Additionally, disused chimneys are often sealed incorrectly at the top and/or bottom. A completely sealed chimney traps moist air inside, preventing any natural evaporation and leading to worsening damp conditions over time.
The Importance of Proper Ventilation
Good ventilation is critical to prevent damp problems in chimneys — especially unused ones.
If you have a disused chimney, it should be capped at the top to prevent rain ingress, but still vented at both the top and the bottom to allow airflow.
Some practical options include:
Installing an airbrick or ventilator grille in the chimney breast at ground level.
Fitting a capped ventilator or specialist cowl on the chimney pot to keep rain out while letting air circulate.
Without ventilation, any residual moisture trapped inside the flue has no chance to evaporate — meaning dampness will only worsen over time.
Other Causes of Chimney Damp
Apart from trapped moisture, several other factors can contribute to chimney damp problems:
Failed flashings where the chimney meets the roof, allowing water to seep in.
Cracked chimney pots or mortar joints.
Porous or degraded brickwork, particularly on older properties.
Lack of a chimney cowl, allowing rain to pour directly down the flue.
Condensation from modern heating systems venting into old, cold chimneys.
Often, it’s a combination of several issues rather than just one.
What to Do If You Suspect Chimney Damp
If you’re seeing signs of damp near your chimney — staining, mould, crumbling plaster — it’s important to act quickly.
Here’s a basic action plan:
Inspect the chimney from outside and inside if possible (or arrange a professional survey).
Check flashings, pots, and brickwork for damage.
Ensure correct ventilation is in place for disused chimneys.
Consider applying a breathable water-repellent treatment to external brickwork to reduce rainwater penetration without sealing in moisture.
Repair or rebuild damaged sections if necessary.
A professional surveyor or experienced roofer can help you identify the exact cause and recommend the best solution for your home.
Final Tip:
Never fully seal a disused chimney without proper ventilation — it's one of the biggest mistakes that causes damp problems years later.
