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Damp on chimney breast wall 1st floor bedroom

I live in a 1958 detached 3 bedroom centrally heated brick house with radiators. The chimney is on the side of the house which is about 10' away from the house adjacent & doesn't get any direct rain on it. The chimney is built on the outside of the original house wall and therefore the walls behind it inside the house are flat (flush). I suffered with problems of damp in the chimney with a leaking stack and so when I got new roof tiles I had the chimney taken off at loft height. The chimney had been a real open fire and I had it swept by a professional chimney sweep. The top of the chimney is open in the loft and sealed from the weather. The bottom of the chimney in my lounge has a decorative only gas fire which has a permanent open vent. I then installed a further 2 airbricks on the outside chinmey wall (one at the top & one at the bottom) which has the original DPC at the bottom.
Despite all of this and experimenting covering (capping) and uncovering the top of the chimney stack inside the loft to increase/reduce airflow I am still getting damp readings (over 44% on damp meter) on my plaster in the 1st floor bedroom on the wall immediately behind where the chimney stack goes up on the outside just below ceiling height and about 1' from floor height. When I have inspected inside the chimney (by looking down from inside the loft) I get these same readings on my house chimney wall nearest to me and the mortar, but not on the side walls or outer wall of the chimney.
The 1st floor bedroom is well ventilated (windows always open).
When I leave the window closed with the room radiator on with the dehumidifier switched on the levels start to fall.
Any ideas guys how I solve this issue it's really puzzling me ?

Ok so since asking the question I have removed the plaster off the wall back to the brick. The bricks behind and the mortar show high levels on the damp meter as mentioned and yet to the left and right of the plaster either side of the chimney this is fine ?
Would it be worth getting the outside chimney stack removed ?

Discounting the removal of the exterior chimney breast as an unnecessary task, thank you. Would I need to place a tanking membrane on the brick wall after application of salt neutraliser before installing an insulated plasterboard ?

Many thanks for your help.

Would it best practise to batten the wall with treated battens then put a foil backed insulation or polystyrene insulation between the batons & then put a foil backed plasterboard on or do i need to leave an air space void between the battens and the new plasterboard ?

3 Answers from MyBuilder Chimney & Fireplace Specialists

Best Answer


Although it is difficult to assess without seeing it, from the description you have given, and assuming the flues are adequately vented, it sounds like you may have some sulphur contamination from the bring of old fossil fuels. Sulphur is hygroscopic and will absorb moisture from the air giving high moisture readings.

The only solution for this is to remove the contaminated plaster, neutralise the salts and replaster.

Again I emphasise I have made some assumptions from your description but I hope this helps.


*** added after question edited ***

So the stack has been removed to below roof line.

You also mention you had a leak in this area (hence removing the stack) so this can not be still occurring.

Taking all this into account the evidence suggests that you have limited sulphur contamination in your brickwork and plasterwork exacerbated by the old leak. As mentioned earlier sulphur will absorb moisture form the atmosphere so will be high one day and low the next.

Your only solution is to remove the plasterwork, apply a salt neutraliser and then replaster as mentioned earlier.

Second edit *****

When you say "left and right of the chimney" I assume you're referring to the walls either side and not the chimney cheeks.

As the evidence suggests contamination from salts from the burning of fossil fuel then the results your getting are to be expected.

*** third edit ****

By stack I assume you mean remaining breast against the external wall as you've stated that the stack has already bee removed.

In that case I think that's financial overkill for a bit of salt contamination that can be cured just by neutralising and replastering correctly as you would still need to replacer anyway.

Ultimately it's your decision of course.

*** fourth edit ****

You could do or you could coat it with a slurry coat like triton t55 (Google it) which is like a cementitious membrane. However if you're battening out for the boards you'll need to be aware of compromising any membrane you use with the fixing screws.


Either would suffice. Just be aware that condensation can occur in the void so leave a small gap top/bottom to allow air to flow through. This depends on atmospheric conditions within the property of course.



Answered 18th Oct 2018

It’s very difficult to make an accurate deduction without seeing it in person, but maybe consider two possibilities 1) an accurate damp tester normally requires holes drilled into the wall/brick to allow ‘testing probes’ to measure amount of damp inside the brick. 2) you may be measuring condensation on the wall surface. Condensation gathers on cold surfaces, you chimney wall will be cold due to no insulation and not being used.
Open a window for air circulation and see if the moisture level goes down, if so either open window more frequently, install vents, or fix insulation backed plaster boards over chimney breast to provide warmth.
I hope this helps.


Answered 9th Sep 2018

There are only two ways Damp affects a property the roof and of course the rising Damp by using a Damp meter you can access where the Damp is coming from just because you see a Damp area it does not mean this is where the problem starts covering problems up is not a cure this is why people end up with bigger problems the age of of the house does help


Answered 15th Jun 2019

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