Solution for damp patches on chimney breast?

[DanielWestern]

I have recently had a room decorated in my old Victorian terrace house, where there is a chimney breast. The wall covering the chimney breast was previously covered with old lining paper so I was not aware of any damp issues. A few weeks after decorating during some cold and rainy weather, some damp patches appeared in the new paint in the chimney breast area.

After some reading around I am pretty sure that this is related to hygroscopic salts in the brick/plaster attracting moisture from the atmosphere. My question is, how to address this? I know there is the nuclear option, hack off the plaster and take some of the brickwork out and fix up, redecorate, but thats really not an appealing option if I can avoid it!

Also, I have seen some suggestions elsewhere that this kind of thing might be related to lack of ventilation in the chimney - would that be relevant as well? I don't understand how that would be the case here, as the damp patches seem surface level only and dry out quite quickly, I would have thought that if moisture inside the chimney was being drawn out through the bricks to the room (which is what I assume ventilation for the chimney would relate to) that the damp would be a lot more pervasive and slow to dry out, but I am a novice so not sure. I suppose its also possible that its just that the chimney breast is colder and moisture in the room condenses there in the colder areas, but it always seems to be in the same locations which makes me think its salt contamination...

Anyway, would any of these solutions help?
- some kind of damp proof lining paper
- salt neutralising treatment applied to wall, then prime and replaster
- combination of the above
- vent bricks in chimney as discussed above

Any ideas? This seems to be a common issue but I haven't yet come across anyone saying they fixed it by doing xyz..

 

[foxhole]

Is the chimney capped?
Is it ventilated ?

 

[DanielWestern]

I believe it is neither capped nor ventilated. Attaching a pic here if that helps.

 

[davidm1969]

I had same issue in an upstairs room where chimney breast had been removed.

Remember if hydroscopic salts from plaster/chimney it's getting moisture from the air in the room so solution is easy stop the moisture getting to the plaster. Paint over and 1m either side with an oil based paint like zinser coverstain, 2 coats, then redecorate on top of that.
Worked a treat

 

[opps]

I had same issue in an upstairs room where chimney breast had been removed.

Remember if hydroscopic salts from plaster/chimney it's getting moisture from the air in the room so solution is easy stop the moisture getting to the plaster. Paint over and 1m either side with an oil based paint like zinser coverstain, 2 coats, then redecorate on top of that.
Worked a treat

AFAIK, and I am no expert, the old soot absorbs the moisture from outside and then dumps it through the plaster over the cold months.

Years ago a regular customer had damp patches on the chimney on the third floor. The room wasn't used. He had the house repointed, had the chimney flaunches done and fitted caps over the pots. 3 months later, I redecorated the room. I primed the chimney breast with Zinsser BIN. Filled where required. Sealed the filler with SBR and then lined/painted the walls. By the time winter turned up, the damp patches were back.

He was advised to hack off the plaster and use sand/cement.

 

[davidm1969]

AFAIK, and I am no expert, the old soot absorbs the moisture from outside and then dumps it through the plaster over the cold months.

Years ago a regular customer had damp patches on the chimney on the third floor. The room wasn't used. He had the house repointed, had the chimney flaunches done and fitted caps over the pots. 3 months later, I redecorated the room. I primed the chimney breast with Zinsser BIN. Filled where required. Sealed the filler with SBR and then lined/painted the walls. By the time winter turned up, the damp patches were back.

He was advised to hack off the plaster and use sand/cement.

I was advised to try using zinser coverstain by a local builder (this was by the west coast where they had lots of driven rain and damp issues) and found same advice on surveyor.tips website (page no longer works unfortunately).

At the time I also posted here and whilst there were plasterers of course advising the hack the plaster and slurry /waterproofing method, at least one other person said no harm in trying the coverstain first and of course if it didn't work do more extreme versions like the slurry and hacking plaster (does same ie waterproofing). All I can report is what worked for me, possibly I was lucky and only had a mild version of the problem. I have seen others advise to do what your customer ie sand/cement and others to also use a DPM. And lots of people explaining that even an unused sealed off chimney should be vented (at top and bottom) to allow airflow and prevent moisture building up within it.

I'm guessing if salts are in the plaster it will absorb moisture wherever it comes from which could be inside the chimney from above, from a leak, from cold bridging or from inside the room. Hence people ask whether capped or leaks etc

 

[foxhole]

Chimney needs capping and venting to cure problem before you address decoration .

Looks to be timber? fixed to the side of the house this will draw rain and soak the walls and prevent drying .

 

[mrrusty]

Been there, done that, got the t shirt. Cap the chimney first - imagine a bucket stood out in the rain - that's how much water is going down the chimney. Water + combustion products = acids. acids + building products (lime etc.) = salts.

If the salts are in the brick already (they are) they will be drawing water vapour out of the room and making "damp" patches. like the picture here where the salt has migrated through plasterboard dabs. This is an internal wall -part of a room which to my personal knowledge (family have owned the house since it was built in 1902) had a coal fire every day for 100 years.

I have tried damp-seal paint to isolate the salts and had some success just like @davidm1969 above. Zinser BIN didn't work (I didn't try zinser coverstain). Ronseal damp-seal paint did to some extent about 95%. I have also on another chimney stripped to brick, sealed with SBR slurry and fixed plasterboard with foam adhesive which isolates (not water based dabs) and reskimmed. This so far (couple of years) has been a 100% cure