Dealing with salt damp on chimney

*SDG*

I have an old terraced house and am pretty certain that I have damp on the corner of my chimney breast caused by salts from inside the chimney leaching out into the plaster. The salts are attracting atmospheric moisture and making the wall damp.

I have been advised to either:
- cover the affected area with tinfoil and wallpaper over. Presumably the foil will stop the salt attracting moisture and therefore stop the damp?
- knock the damp plaster off and replaster with a sand and cement mix

Tin foil is obviously the cheapest and least disruptive option. But will this simply cause the salts to migrate elsewhere and cause damp to reoccur?

It's highly tempting to use this budget option but will this just end up costing me more in the long run by causing further damage to walls/decoration?

Any advice would be greatly appreciated!

phill99

You need to look into why you have damp and cure the cause, not the symptoms.

Is the chimney breast blocked off? And if so, is there a ventilation grille in it? If this is the case, the air (which obviously contains moisture) can't circulate in the chimney and turns to condensation.

If you have ventilation, moisture is getting in from somewhere else and needs to be investigated.

Once the moisture source is dealt with, allow the area to dry out and then treat with a stabilisation solution and devorate in the normal way.

*SDG*

Thanks for your reply.

We have already investigated it and have concluded that it is most likely to be coming from salts leaching out of the chimney and the salts are attracting atmospheric moisture causing the damp.

I'm under the impression that the wall won't dry out until we stop the salts attracting moisture by the tinfoil barrier method or removing the contaminated plaster.

To give a bit of background.... The chimney is open with a vented rain cap, it's an internal wall with no pipes etc. to leak from, and the chimney stack and any flashings etc. appears to be sound. There is also a 7ft deep cellar beneath the house so can't imagine it being rising damp, and the house was built with a damp proof course. The damp patches on the wall contain salty deposits.

Do you think that there could be another cause to the damp? If it is salt damp would tinfoil do the trick or is that wishful thinking?

SG27

The salt is there because of the damp not the other way round.

dj1471

How old is the house? Is it mid or end terrace?

I'm currently buying a house with a damp chimney breast and have received the following (very good) advice:
1. If fireplaces are bricked-up ensure there is ventilation. Without it the flue will get damp. Likewise ensure the cap on the flue is good and not letting rainwater in.
2. Walls in old houses need to breathe. DO NOT use tin foil or a cement-based plaster, this will make the damp worse. If re-plastering use a breathable plaster.
3. Have the chimney swept. This will ensure its clear to help with ventilation and remove any accumulated soot which can attract moisture.

Where there's a cellar the DPC is at ground level not cellar level. Ensure there is nothing bridging the DPC (e.g. external rendering, which shouldn't go all the way down to ground level).

phill99

SG27 wrote: »

The salt is there because of the damp not the other way round.

Correct.

Salt only leaches out as a result of moisture in the plaster.

You need to do some more investigations!

*SDG*

Ahhh, I see. So if I knock the plaster of and re plaster is the salt issue likely to come back?

I'm a bit baffled then about what is causing the damp in the first place. As I mentioned in my previous post there are no leaks/rising damp issues, and the chimney itself should be well ventilated as all fireplaces are open (but not used) and it has a vented rain cap on the top. The house is a mid terrace, built around 1900.

Could condensation be causing the salt to come through the plaster? I do run a dehumidifier constantly at the moment to try and keep the damp patch from getting worse, and as far as I know there are no condensation or damp issues elsewhere in the house. The only affected place is the corner of one of the chimney breasts at skirting board height.

dj1471

When was the chimney last swept? There could be birds nests or other obstructions preventing proper ventilation.

Have you looked at your neighbour's side of the wall? Do they have fireplaces back-to-back with yours? What state are these in? Is it a cavity wall? If so could water be getting into it on your neighbour's side (from the roof or pipework)?

*SDG*

Yes I've asked the neighbour and she says he's never had damp. She doesn't have central heating though and has gas fires in all of her fireplaces in the house that are back to back with mine.

I've not had the chimney swept and I doubt whether the previous owners did (they too had gas fires in the fireplaces- I removed them and opened them up when we moved in about 2.5 yrs ago).