Common Causes of Damp In Your Chimney & What To Do About Them

So you’ve been enjoying your stove for some time now, but you start to notice something odd. Staining and damp patches have started to appear on your chimney breast - never a good sign!

We all know that papering over the cracks is not a good solution to a problem. If you notice damp on your chimney breast, you need to treat it as soon as possible.

The most common causes of damp in your chimney breast are:

Rain water or other external moisture getting in
Condensation forming from inside the chimney
Hygroscopic salts drawing moisture into the walls

Take a look at our guide to find out more about these problems and how to fix them...

1. Chimney Damp Caused By Rain Water

The Problem...

One of the most common reasons why your log burner’s chimney might be wet is quite simple - rain water is finding its way inside. Whether your chimney is still being used or is sealed up, leaks are commonly a cause of moisture damage.

Water can leak into your chimney from a number of sources, including:

The top. If you haven't protected your chimney by a cap or cowl, rain can easily get in and cause issues.
Cracks inside. The older your chimney is, the more likely it is that cracks will form inside. These can let in rain or other moisture from inside the building.
Damage from other parts of the house. Don’t think you only need to inspect the chimney itself for damage. You’d be surprised how seemingly unrelated matters can be linked to a wet chimney breast! Broken guttering, missing roof tiles or damaged mortar can all lead to damp spreading into your chimney.

The Fix...

Fit a chimney cowl
Check thoroughly for leaks
Allow damp to dry out

If you don’t have a chimney cowl, or yours is damaged, this is the first thing you should correct. It should be easy to fit one - they can also help keep out other debris or animals that could cause blockages.

If you think moisture is leaking from somewhere else, make a thorough inspection to try and find the source. Blocked guttering is sometimes a problem, so give it a good clean out and make sure it is in good condition.

If your guttering isn’t an issue, check for cracks in your lead flashing - this is the joint between your roof and the base of your chimney stack. You will need to fix it if it is cracked. Get up into your loft and have a good look around for any more signs of leaking. You might need to get the help of a contractor to inspect more difficult spots.

Once the source of the leak has been stopped, it can take a few months for the damp to dry out.

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2. Condensation Causing Chimney Damp

The Problem...

If there are no signs of leaks in your property, the damp on your chimney breast might be a result of condensation.

Both used and unused chimneys require ventilation. However, when it comes to chimney damp, lack of ventilation often causes more problems for unused chimneys.

What causes condensation in an unused chimney?

If you are seeing evidence of damp in an unused chimney breast and there are no signs of leaks, it is likely due to lack of ventilation. When a chimney is sealed up, airflow becomes restricted, which can lead to condensation forming.

What causes condensation in a functional chimney?

Whether you have a wood burner or a gas stove, water vapour is an unavoidable product of burning your fire! If everything is working properly, this water vapour should be carried through the flue without any problems.

However, if your chimney isn’t working efficiently, it could be condensing on the walls too early. If your damp problem seems to be occurring higher up in the chimney, this might be why.

The Fix...

Add ventilation to unused chimneys
Make sure you chimney is lined properly

If your chimney is sealed up and is causing damp, you need to create some ventilation. If it has been sealed from the top, you might need to open it up again and protect it with a chimney cap, instead. You also will need to place a ventilation brick or panel at the base of the chimney to keep the air flowing.

If your fireplace is in use and you think you might have condensation causing damp, it may be that it needs lining. Chimney liners can help improve the efficiency of your chimney which can help reduce excessive condensation.

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However, condensation in your chimney could also be caused by a blockage. Make sure you get your chimney inspected and swept regularly to be sure debris isn’t building up - this can also cause dangerous chimney fires.

3. Hygroscopic Salt Chimney Damp

The Problem

If you can rule out both other possible causes, and your chimney damp is very persistent, it might be caused by hygroscopic salt.

Hygroscopic salt is a byproduct of burning fossil fuels, such as coal or gas. As they have been formed underground, these fuels absorb minerals. When we burn them, they are released and can build up inside your chimney in the form of salt.

The problem with hygroscopic salts is that they attract moisture. This can then lead to excessive damp forming. What’s more, once they have penetrated your brickwork or plaster, they are very difficult to remove. If your chimney is not lined and has been in use for a very long time, this is a likely cause of damp in your chimney breast.

The Fix...

Remove all affected areas
Apply a salt neutraliser and replaster

There can be some significant work involved in removing damp caused by hygroscopic salts. All plaster affected will need to be removed and the wall left to dry out. Then, you will need to apply a salt neutraliser before replastering.

For further protection in preventing condensation and damp forming in your chimney, you can also apply a damp proof membrane. These work by stopping moisture and salts from permeating into your wall.

How Do You Damp Proof a Chimney Breast?

Whether you have had damp problems in your chimney in the past, or just want to protect yourself from problems forming, here are a few things you can do to help:

Get regular chimney sweeps. This ensures any blockages are removed before they can cause problems. The chimney sweeper can also inspect your chimney for cracks or damage that could let moisture in.
Fit a chimney liner. If you haven't already, consider getting your chimney lined. It will improve the efficiency of your flue and protect you against damage. If you’ve had yours for some time, check it is still in good condition.
Use seasoned wood. If you are burning wood with high moisture content, more water vapour will be released. Using dry seasoned wood is the easiest way to keep your chimney clean and healthy.

Find out more about burning seasoned firewood here.

Looking for more advice on your log burner? Find more posts on our blog…

Why Does My Log Burner Keep Going Out? | How to Keep Your Wood Burner Healthy | How to Clean & Maintain Your Wood Burning Stove