Dampness either side of chimney

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Hi there I recently bought a semi detached house and on the Home report there was no dampness. After re wiring re plastering everything the house dried out 100%. After the chimney wall was decorated a few months later we encountered a outline of dampness which then turned out to be quite a bit on these two areas. The roof has been checked and is fine, cavity insulation is fitted with a cavity on one side of the chimney with rubble bridging from external to internal brickwork as for the other side the insulation is just damp. The dampness has never gotten any worse as we left the wallpaper on for 5/6 months. I know the next step will be to remove the damp insulation but I still don’t know what’s causing this as it’s not gotten any worse. I have bricked up the chimney in the inside and sealed it all and feeding the cables through the chimney breast to tv and fitted a vent on outside but chimney isn’t damp at all
 
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Did you vent the flue top and bottom when you bricked it up?

It's that upstairs, downstairs and where is the chimney in context of the rest of the house?
 
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Yes it’s vented top and bottom. The chimney runs right up through the gable end I have tested the walls upstairs in the bedroom either side of the breast and it’s dry just seems to be two sides of the breast as if it’s right in the corners in the cavity driving me insane I have a de humidifier in place but if the insulation is damp then I don’t think it will do much until I get it removed.
 
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How old is the house? Sometimes salts from the original open fires leach into the masonry then into the plaster. As the salt is hygroscopic it attracts moisture from the air. I had the same in my house. Salt damp
 
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Built in 1945 mate I just find it strange how it’s outside the chimney breast either side
 
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I found the same issue with my kitchen chimney breast. It might not be the same for you, but after investigation I concluded that previous owners had the kitchen plastered and then painted before the plaster had dried out properly. The moisture in the plaster was then trapped behind the wall made even worse when it was all wallpapered over.

As a result, brick salt (efflorescence) was drawn out from the bricks and started making similar patterns on the plaster/paint as you can see in your picture.
 
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