Solving chimney breast damp
We often take calls from property owners suffering damp issues on chimney breasts, where they've employed a roofer to repair flashings etc. on chimneys expecting the damp issues internally to go away / dry down.
Damp patches on chimney breasts that won't dry down? Leaking chimney fixed? We explain these persistent damp patches
There is more to it than that. Where there has been long term burning of fossil fuels in a fireplace, a chimney will be lined with soot.
When water runs down the chimney and through the soot, salts present within the soot are dissolved, meaning that they go into solution and then can travel within that water.
If and when that water migrates out into direct applied plaster on a chimney breast, the water can evaporate from the face of the wall, i.e. water changes from a liquid into a gas (water vapour), whereas the salts cannot change into a gas and so revert to a solid, being deposited within the wall plaster. If you ever dissolved salts into water in a school science lesson, then evaporated off the water with a bunsen burner, it is the same.
The salts that are deposited are hygroscopic, which means that they are capable of absorbing moisture from the air (vapour).
This is why damp problems can persist after a leaking chimney has been repaired. There may be no further ingress but salt contamination within plaster is absorbing environmental vapour, causing damp patches.
These can be more visible in warm humid conditions, and a basic test can be to repeatedly boil a kettle within a room to increase humidity. If the damp patches appear/worsen, it is indicative hygroscopic salt contamination in the plaster.
The remedy is then to hack off the affected plaster, apply a meshed cavity drainage membrane and then re-plaster. The new finishes are then isolated from any salts still present within the masonry.
Salts can also deliquesce, which is when they absorb enough environmental moisture to dissolve within this, once again becoming mobile.