Chimney Breast Damp Issue

5 Feb 2013
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United Kingdom
Hi All,

Have an issue with damp on a chimney breast wall. Property is a ground floor flat in a converted Victorian 2 storey property. Walls are 9 inch solid walls. Chimney breast is on the gable wall. I have removed the air vent and can see that the chimney had a void area behind the breast.

At first i believed the damp might had been down to salt contamination within the plaster and this was drawing the moisture out of the air? Damp patches on the plaster at the bottom of the wall could be from water penetration from the chimney not being capped? Property has since been empty for approx 6 weeks and the patch has not dried out. Is it worth running a dehumidifier?

Has anyone else got any views on the damp patches?


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A wall that thickness can take a long time to dry out, once you have installed a vented cap to the chimney a dehumidifier will speed things up.
hi, Rossi90.

If you can see if there is any dirt and debris build up below and around the air brick on the inside of the Chimney Breast, if so a small narrow shovel and bruised arms will get this debris out.

If possible, a long shot I know but see if the entire height of the Flue is clear? there may be a birds nest blocking the chimney pot or the nests sometimes collapse into the flue.

One idea would be to run a fan in the room with the air flow onto the damp walls, you do not need Heat just a large volume of air flowing across the damp patches will assist in drying the wall.

one other Idea is to force a draft up the old flue? if possible aim a fan at the opening that you have, the old vent in the plaster, the idea is that a large volume of air will force the Flue to dry but from the inside? but warn your neighbour above that there may be a sooty smell in that room above, if the Flue is not entirely sealed as it passes through the upstairs neighbours property?

As for a De-hum? it will be cheaper to at least try the fan, if the fan does not work then resort to the De-Hum

Thanks for your replies. There are no debris built at the base of the chimney.

As for the cause of the damp could this have been related to condensation as the property has been left empty and unoccupied with no heat or ventilation for around 3-4 months?

We are going to install a dehumidifier to dry out the walls and see where this lead us.
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Being empty, unheated or vented would certainly exasperate the problem!

The introduction of heat to the room will be drawing out any moisture which has gathered in the flue over winter.

Unless there is any other underlying problem it will dry out in time now the room is heated and vented.
Is it capped and vented or not capped and with an open pot?

ie can rain get down the flue?
1. A humidifier will work - it certainly wont exacerbate the condition no matter
the source of the staining. But only operate it while someone is on the premises.

2. A continuous low temp from a rad, and good ventilation work best with condensation.
But there are no signs of condensation on the (colder) wall to the right hand of the c/breast?

3. The staining on the c/breast is probably a result of chemical reactions to the unswept soot in the flue. The flue chemicals liquidise & penetrate to the finished surface of the c/breast.
Does the first floor c/breast show similar signs?

4. There are stains very low on the c/breast which might be rising damp from the earth filling below the fire opening back hearth.

5. Your c/breast should be opened up & smoke tested & swept. It pays to have all house flues tested & swept at the same time.
The chimney stack should be inspected (& pics taken) when a terminal is installed.
Is the gable brickwork & pointing sound? Can you see any signs of soot penetration grinning thro?
Do you have a basement?

6. If the staining is as a result of chemical penetration of the c/breast then its simpler to remove all the c/breast plaster & render back up with a sand & lime mix.

7. You would do well to dig out the front hearth and cover the area with a patch of suspended floor. Check for fungal damage before covering.

8. Is the wall on the left a partition - whats behind it?

9. FWIW: one of the double outlets seems to have been badly installed.
Where do the floor stains below the outlets come from?
1. A humidifier will work - it certainly wont exacerbate the condition

How will a dehumidifier work?

The dampness is from the flue, and you are talking about drying it off from the room side, so the plaster stays constantly damp.
There are no other signs of dampness in the property only on this chimney breast.

Would this chemical reaction have been built up over a number of years? Similar to salt contaminated plaster which can draw moisture out of the air?

If we was to hack off the plaster back to brick and re-plaster would you expect the damp marks to come back? if so how quickly?

There is an old unused basement below which I currently don't have access to. Will try and gain access to see if there are any further obvious signs.

Wall on the left is to the bathroom. We have checked and there are no signs of any leaks within the bathroom etc.
The damp is from the flue. You can see how it is following the flue line.

It's either coming down the flue as rain, or its flue condensation. That is what you need to solve.

Then you need to hack the plaster off, treat the bare breast with a stabilising and blocking solution and replaster.

ignore item7.

The stains are salt (chemical) contaminated plaster.

It most probably has "built up over a number of years".

If you was to hack off, render up and remedial skim finish, plus the flue sweeping and terminal vent installation - and any stack work, or external re-pointing then you could more or less live in peace with the flue/chimney breast.
At worst, render will give you 20 years of blemish free decorations on the c/breast.

The basement might have a fire opening and a flue that runs past your flue. In other words, your chimney breast contains two flues.
The basement flue should also be smoke tested and swept, and terminated correctly.

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