Damp patch on wall on old chimney breast

[JustAnotherJane]

Ground floor internal wall with a sealed up chimney breast has a damp patch.

The previous owners removed the chimney breast from the 1st floor (making the bathroom larger).
The chimney breast is still in place on the ground floor.
We spotted a damp patch about 2/3 to 3/4 of the way up the wall a few months ago so had a look at the pipes in the bathroom.
We discovered there was a slow leak (persistent drip) from one of the pipes and thought that was it. Fixed the leak and made good the wall below.

This weekend in the same spot the wall was damp again so we checked the pipes above and those that we can get to all look dry (including the one that was previously leaking).
Advice on next step? Photo attached some of the marks are stains that I didn’t manage to shift last time but the bottom patch was damp to the touch.

 

[^woody^]

That looks like condensation or flue gas staining, and may require the plaster to be hacked off, the wall sealed and replastered.

 

[mrrusty]

IMHO this is 99% salt contamination. When it was a chimney combustion products and rain down the chimney created all sorts of chemical reactions - mainly acid generation reacting with lime based products - resulting in salt contamination of the bricks. The salts are hygroscopic. When the wall is plastered, as the water evaporates from the plaster it draws salts to the surface. These then attract moisture out the air when conditions are damp causing wet looking patches and sometimes surface crystals.

In my first hand experience you can sort this by isolating the room air from the salts. A fix that may work is to paint the wall with a damp seal paint before decoration (I've had some success with this). A better fix is to strip the wall back to brick, seal it and then plasterboard with foam adhesive which isolates the PB (wet dab adhesive can still draw salts, as I've found)

The fact that it is a bathroom with lots of water vapour is contributing to the problem.

 

[JustAnotherJane]

Thank you both for your feedback. It sounds like I’ve presumed wrongly that if it was sealed it wouldn’t be anything to do with the chimney . We do plan to remove the chimney breast but can’t do so yet (need money to do what we planning) so it seems a shame to pay to sort this out now.

 

[^woody^]

Paint the surface with a stain block - see products from Zinser. You'll need to do the whole breast so that the stain does not creep around.

These paints tend to be gloss, so then I'd if you want to emulsion, you'll need to use lining paper first.