treatment for "salt damp" on chimney breast?

What is the best treatment for "salt damp" caused by hygroscopic salts on a chimney breast?
First, hack out the affected area of plaster and a good margin around it, and check the water isn't coming in through the bricks. I'm only talking about the case when water isn't still coming in.
But then?
Is it better to use a bond coat of SBR and then a cement render, perhaps itself with a splash of SBR in it, and then plaster or emulsion on top? I thought SBR wasn't breathable.
Or is it better to use a special "renovating plaster" that is sold precisely as breathable?
I don't quite get whether you want breathability or the opposite.
Advice would be welcomed.
Thanks in advance!
Harry
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Many thanks for this.
There's no obvious water coming in through the roof. The chimney has been blocked up and unused for maybe 15 years, but the one next to it and sharing a stack with it is used. The stains are in the kitchen, so there is a lot of moisture inside the room. I haven't noticed the stains get smaller when the day outside is dry, but maybe that effect is less likely to happen in a kitchen.
It is possible that the wet is rain getting in through the chimney pot. It has got a cowl but the winds can be strong. I am not sure how it would then get through the chimney wall but doubtless there are some unevennesses.
I will hack back to the brickwork and report back :)
When you say condensation, where would the condensing moisture come from? Inside the kitchen or down the chimney?
Thanks again!
Harry
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Bing! there's your problem. Chimney must be vented top and bottom. If it's not, it will slowly fill with condensation - 15 years is quite typical period for it to end up soaking wet inside. When you get to look in the flue, you will probabaly find it's so wet water is visibly running down the inside.
You need to get it vented properly, and then leave it to dry out before assessing what decorative repairs are required. This will take a long time - a minimum of a week per inch thickness of the walls, so allow 3 months unless it's very thick brickwork, in which case it will take longer.
If the chimney is on an outside wall, the lower vent can be to the outside, to avoid drawing air from the room. Another option is from the subfloor. Failing these, you'll need to have it vented from the room.

Rain going into an open pot will not be a problem providing the flue is properly vented. (It's better to provide a vented cap if you can, but that's rather more to reduce the frost damage in exposed stack brickwork.) However, capping the top off without venting it is worse than leaving it open.

Several sources. From air in the flue if the ventilation is poor. Moisture which comes through the brickwork. Rain entering the pot. Generally, there are several ways in, but ventilation is the only way out.
--
Andrew Gabriel
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On 1/31/2017 12:26 AM, Andrew Gabriel wrote:

+1
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On 31/01/2017 00:26, Andrew Gabriel wrote:

Our flue in the kitchen is like that.
It's got a cap at the top with breathing space, and a vent at the bottom.
I suspect what happens is warm moist air from the kitchen goes up into the cold flue and condenses out (the bottom of the flue is at ceiling level). I can't really see that the ventilation is helping at all...
Trying to decide what to do long term. We need a cooker hood (the PO seems to have lost it - apparently there was one 15 years ago) and it seems to me that if we vented that into a pipe running the entire height of the chimney then the chimney would be warm, and probably dry out.
And there would be lots of condensation in the pipe :(
Andy
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