Efflorescence mystery

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We recently had a downstairs room skimmed and I put a mist coat on after two weeks when it all looked dry. That's when I noticed patches of damp across the top part of one of the walls below the picture rail, 2m wide x 1 m deep. I left it for another week and now it's obvious that it's efflorescence.

From what I've read it seems there must be a source of water permeating the brick, causing salts get to the surface, but I can't figure out how. Before the kitchen extension was added 20 years ago it was originally a fireplace on an external cavity wall with the chimney breast on the outside. Now all of that external wall is indoors on two storeys. There are no water pipes in the wall and below the efflorescence down to the floor is fine. The air is circulating freely in the chimney breast into a large void below the floorboards and there is no sign of damp or efflorescence on the other side of the cavity wall which is painted bare brick. Up above is a wallpapered bedroom which looks pretty sound and above that is a boarded attic space, so I can see that there are no leaks coming from the roof. So how can it just be affecting this one particular area? It surely can't be rain coming down the chimney and hitting this one spot can it?

The old wallpaper before it didn't seem damp although the wall was a mish mash of layers of paper, one coat plaster and a layer of what looked like peelable paint (maybe eggshell?). Does anyone have any suggestions of what else might be causing it and how I can fix it? Is it possible I just painted the plaster too soon and it will dry out eventually? It's fine on all the other walls. Thanks.
 
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I'm wondering if the lack of response means there's no obvious answer to this.

Since I want to use Dulux Trade Matt on the wall I called up Akzo Nobel technical department to see if I could use their Alkali Resistant Primer on the wall as I wasn't sure if already having a mist coat on it would make it pointless. They didn't throw much light on it either way but the bloke in the Decorator Centre said I could. So that's what I'm about to do. Any thoughts? Thanks.
 
damp high on a wall near the ceiling may be condensation. How is the room ventilated? Is wet washing or a shower nearby?

Lift the floorboards and sniff for damp, especially near any radiator pipes.

Are there radiator pipes above?

Some photos would be nice
 
Thanks - there are no sources of water nearby. The room is 4m x 5m with an archway into the kitchen. Directly behind is a food cupboard and a painted bare brick wall which is not showing any sign of damp. It's all well ventilated with the pipes coming in to the kitchen sink from the other end. There is a radiator to the right but no damp in that vicinity. We've had the floorboards up recently when we were sanding and one is still loose so I've had just my nose down there. It's about a two foot void which ventilates freely into the chimney breast. It was an open fireplace which has been just been plasterboarded. We didn't put an airbrick in because of the free flowing air below. It is definitely ventilated but as you mention condensation could it have changed the moisture balance?

I'm attaching a photo but it might be hard to see. Thanks very much.
View media item 95669
 
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what's the scale of that photo?

stand back and snap the whole wall please

is the wall made of old bricks?
 
As far as I know they're the orginal brick - 1930s. Here's the whole wall. Could it be that the ventilation below isn't as effective as the open fire place was and so caused condensation? Thanks
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that's a very big damp patch. Am I looking at a chimneybreast?
 
Yes, but it never protruded into the room it was on the outside of the house. The chimney breast itself is about 40 inches so this damp is in the bricks either side too.
 
right.

Disused chimneys suffer internal condensation unless they are ventilated top and bottom. Rain can also penetrate from the top.

How is this chimney (it probably has two or more flues) ventilated?
 
I can't say I know , how can I check? As I mentioned it's vented into the floor void and there's a chimney pot up top with a curved terracotta cover, I assume to stop birds. Whether it's blocked above the fireplace I don't know for sure but we used to get grit and stuff coming into the hearth especially after high winds. We've been here three years and there's always been an open fireplace. There's a gas tap nearby so I assume there was a gas fire installed at some point. There was a fireplace in the room next door which is open, so if I had to guess, they were both open.
 
I posted the above last summer about efflorescence appearing on recently plastered walls after a mist coat of Dulux Diamond Matt. I subsequently applied Dulux Alkali Resisting Primer, albeit to the mist coat rather than the bare plaster, then finished the job. Over a period of months the surface in the area to the right began to roughen up and eventually the paint lifted to the point where I could peel large pieces off. I've now taken a scraper to it and it's coming off in one piece in most areas.

There are no water leaks so I can only imagine that there is/has been condensation in the chimney even though it's ventilated into a large void below the floor. There were some crystals where the paint split but otherwise the plaster underneath seems in good condition and doesn't appear to be damp, so do you I might have painted it before it was fully dry or is there still likely to be some underlying problem which will re-occur if I paint it again? Thanks.


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My guess is that it was painted before being fully dried out - it certainly seems dry now, though.

The most important part of treating efflorescence is to apply your alkali resistant primer directly to the bare plaster, once the wall is totally dry and all salts have been brushed off. Adding any form of moisture before treatment (filler, emulsion, or even wiping over the bare plaster with a damp rag), can cause the salts to reactivate and, in time, start to break through the new decoration. Now is the ideal time to use the ARP, while the surface is dry and free of salts, but efflorescence is notoriously difficult to stop and even this method is never 100% guaranteed.
 
Thanks, I'll do that. It'd odd because the other walls are fine and it was all done at the same time.

I read somewhere that wiping the plaster with turps first is a good idea. Do you agree or should I just brush it and avoid any moisture contact altogether?
 
I always avoided moisture and in the majority of cases things worked well. As I mentioned, it's not an absolute cert to work and I did get the odd couple of jobs that had some minor re-occurrence, which had to be be re-done, but they all turned out well in the end. Some people recommend using white/distilled vinegar to neutralise the alkali, but I never tried that method.
 

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