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Damp Proofing

Damp on chimney breast above first floor air vents

In our house the fireplace has been converted into a cupboard in the kitchen, it’s opened a few times a day so not permanently sealed but not constantly air flow. It’s not sealed to above as we occasionally have a little bit of soot come down.
The chimney breast is in the centre of the house so no external walls involved.
On the first floor there are ventilation bricks about 6 inches off the floor on two sides of the chimney breast, they are open as you can see through them, and about a foot above the level of the ventilation bricks are damp patches on all four chimney walls, about 1-1.5 foot in diameter. There have never been upstairs fireplaces.
I’m thinking condensation as doesn’t seem to be coming down from ceiling?
Any tips very gratefully received

6 Answers from MyBuilder Damp Proofing Specialists

This condition is called ammonium sulphate contamination . It comes from previously burning coal . Ammonium sulphate is a salt , therefore hygroscopic which means it will absorb moisture from the air . Ventilation does help but eventually a waterproof render to those random patches is the only answer .

2019-09-27T09:45:01+01:00

Answered 27th Sep 2019

It’s possible rain water is coming down the chimney from the top - if there is an offset in your stack, the rain water may be causing a problem at that point- does your chimney have a rain cowl at the top? If not, that would be the first option I would try.

2019-09-27T09:45:01+01:00

Answered 27th Sep 2019

It’s more than likely efflorescence caused by fossil fuels. The efflorescence attracts the moisture that you produce which give the appearance of the chimney looking damp.
Option 1 cut out the infected plaster treat brickwork with an ant salt solution and replaster with the appropriate materials. (Not gypsum)
Option 2 wrap the chimney in a dry base fleece and replaster

2019-09-27T09:45:02+01:00

Answered 27th Sep 2019

It sounds like the plaster has become salt contaminated, making it hygroscopic, it is a common problem on chimneys, the only real solution is remove plaster and re render with salt retardant additives.also make sure the pots are ok on the roof

2019-09-27T09:45:02+01:00

Answered 27th Sep 2019

If the chimney stack is still there and hasn't been capped off it could be rain coming down inside of chimney if it has been taken down then you need to look at condensation which is a easy fix

2019-09-27T09:45:02+01:00

Answered 27th Sep 2019

It's very difficult to diagnose the problem without see the damp marks but it is most likely water ingress from the chimney pot.
Rainwater could wet any debris in the old flue and the brick work inside the flue would absorb the water.
I would get the chimney swept and get a vented cap fitted to the pot, this will stop rainwater getting in and still allow air movement.
You may need to re-plaster where the damp patches are or at least apply Stain Block before re-decorating.
You will find someone trustworthy off mybuilder to advise you, all advice & estimates should normally be free.

2019-09-27T17:50:02+01:00

Answered 27th Sep 2019

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