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. These patches disappear or reduce quite quickly as soon as the weather warms up.

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? My chimney is not ventilated I believe, or capped. 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.

Anyway, would any of these solutions help?
- I have seen suggestion that DampStop lining paper over the affected area is a way to mask the problem, but would that just make things worse?
- salt neutralising treatment applied to wall, (i.e. over the existing paint) then prime (+ possibly stain block) and repaint
- combination of the above
- clean chimney flue, add vent bricks in chimney

Or are all the above just dancing around the issue and the only solution is to take it back to the brick, neutralise the salt, and replaster?

Also where is the moisture likely to be coming from?
- moisture in the air in the room is being attracted to the surface of the wall by salts in the plaster?
- moisture in the air inside the chimney is being attracted by salts/soot etc through the brick and plaster of the chimney breast and coming out through the wall and showing on the surface?

Thanks in advance!

 

[Cubist]

Would it be fair to say these damp patches are appearing at and above head height in a ground floor room? If so, then the most likely cause is warm moisture laden air rising to meet the cold interface of the chimney breast surface and condensing out there. This at least should be your starting point before moving on to other root causes that may suggest much greater investment and scale of intervention. Damp is an elephant best eaten a bite at a time.

 

[DanielWestern]

Thanks for the response..
Its a first floor room, and yes I would say head height and above. Attached a pic which will hopefully give more of an indication where the damp is in relation to the fireplace. Apologies its not better but its a small room and difficult to get a good angle to take a pic...

If it is a case of "warm moisture laden air rising to meet the cold interface of the chimney breast surface and condensing out there" - which it may well be - what would be a good course of action?

 

[Cubist]

I should have been clearer and pointed out that this can occur on upper floors of a building too. I've been puzzling over how to prove this conclusion simply and easily without specialist kit but the only thing I've come up with is an idea that relies on sheet of plastic and a square of kitchen towel. Hopefully other members will come forward with something better but in the meantime - tape a kitchen towel to one side of a piece of plastic while leaving a good border and tape the whole thing to the wall immediately over one of the areas that gets damp while making sure the plastic is in good contact with the wall. If the towel becomes wet after the heating system and room have been used as normal AND the wall behind the plastic is drier than the surrounding areas, that should be nigh on conclusive.

In terms of treatment - check and ensure that any air-bricks are clear of obstructions and that any trickle vents in the window frames are open and clear. In the absence of air-bricks or trickle vents then leaving a window slightly ajar may help.

Frankly, this can be a tough nut to crack as much depends on how the house is heated and what the rooms are used for. It can even be affected by the occupants bathing and cooking habits and whether they are prone to leaving internal doors open.

Hope this helps and that a fellow member steps forward with a better suggestion.

Cheers and good luck.

 

[DanielWestern]

Thanks - thats a really good idea about the kitchen towel and plastic, will definitely give that a go to have a better chance of figuring out where the moisture is coming from - from inside the room directly or from inside of chimney out through the bricks/plaster and into to the room. Why didn't I think of that!

We have a French door directly to the left of that chimney breast which has trickle vents open all the time, so should be good air flow there. No air bricks in the room though. The room is not used that much, occasionally as an office but its not slept in, neither are clothes dried there, so there is no more humidity in this room compared to others, actually less.

My gut feeling about what the outcome of a towel/plastic experiment would be is that the towel AND the plastic will be dry - as if its hygroscopic salts in the wall drawing in the moisture from the room then the plastic will be forming a barrier between the salt-contaminated plaster and the moist air, so no condensation will occur - but that in itself would be telling.

 

[Cubist]

Bugger!

Seems I wandered off without closing my browser and the First Lady briefly took my place and saw my suggestion. En passant she observed that yours truly is guilty yet again of, in her opinion, Over-Engineering! Her superseding, and rather supercilious in my view, suggestion is to replace the Paper Towel and Plastic with only a sheet of Clingfilm.

Grudgingly, I have to concede she may be right!

Re your reply. If the door is usually closed when the room is not in use you could try leaving that open/ajar for a while. That could alter the typical air-flow/convestion currents enough to have an impact. Also, I hope you are found to be wrong regarding the final root cause as, in the worst case, it can be a miserable, dirty and possibly expensive repair to make for little payback.

 

[DanielWestern]

Haha - clingfilm - even better! My thanks to the First Lady..

Will try leaving the door ajar as well to see if that helps.

Yeah - fixing salts in the plaster is a real pain in the arse, so I am hoping I can alight on a less onerous cause/solution first...

 

[Cubist]

Well that's really 'rubbing salt in the wounds', Ta very much!

Note to self - when leaving the room, even when home alone, clear the search history AND close the browser!

 

[DanielWestern]

Haha, sound advice for us all..