Plaster looks damp but isn’t

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Hello

I moved to a new house and removed the fireplace surround - and then removed the wallpaper. On both sides of the fireplace have been two patches, at the bottom, that don’t feel damp at all, but change to a darker colour throughout the day. It looks damp, but doesn’t feel it at all. Doesn’t even feel particularly cold.

As I’m at work all day I couldn’t notice a pattern as to what was causing it to go darker - but, given recent weather, I always assumed it was water getting down the chimney.

However, today there has been no rain but I have had the wallpaper steamer out again and noticed the two patches appear again. But then, when I turned off the steamer and let some air in it changed back very quickly. I’m guessing it is just moisture in the air clings to those two particular areas - but, any idea why? And whether, when I get the room skimmed next week, I should knock those two areas off and have them replastered? Or are they not doing any harm?

Two photos attached, taken about half an hour apart…
 

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Should I just get those parts replastered, or will skimming over it be okay?
 
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I’ve now had the room skimmed, and there are a few patches - that perfectly mirror the holes in the plaster on my original post - that aren’t drying out. They did fill these holes before skimming, but they just aren’t drying out. The rest of the room has dried out, but as you can see on the attached photo there are a few areas that are still dark.

There is a damp proof course in place, and there is no water coming down the chimney as that was bone dry when the fire was taken out - the plumber said it was all good inside.

We are going to have a TV Stand up against this wall when we have finished decorating (hence leaving the skirting board off) so it isn’t a major issue really - but just wondering what people think? Shall I get a heater on it and try to dry it out that way? Or just put the mist coat on anyway and if that patch of paint dries a different colour then it doesn’t matter because it’ll be hidden away?

Thanks in advance
 

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How old is the house

Is it a concrete floor?

Did you clear the old rubble out of the builders opening behind the fireplace?

I think it's probably damp from below.

Show us the rest of the walls in that room, at skirting level.

What is on the other side of that wall?

When you say there is a damp course in place, do you mean chemical injection? Why was it needed?
 
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How old is the house

Is it a concrete floor?

Did you clear the old rubble out of the builders opening behind the fireplace?

I think it's probably damp from below.

Show us the rest of the walls in that room, at skirting level.

What is on the other side of that wall?

It's a 1930's Semi-Detached with Concrete Floor

Yes I cleaned the inside of the chimney out.

I'm not at home at present to take photos, but the rest of the room is dry at skirting level.

Other side of the wall is the kitchen/dining room - the chimney goes up the middle of the house.

I'm almost certain it isn't Rising Damp - 1) Because there is DPC, and 2) Because it previously used to come and go when the room was humid (using the wallpaper steamer etc), and there are no tide marks on the wall.

I do think hygroscopic salts are causing it, but just don't know how to get it to dry out.
 
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I haven't mentioned rising damp.

What sort of DPC is it?

Peel back the carpet please to see if the concrete floor is equally dry all over.

You say the kitchen diner is in the other side of the wall.

Is it the end with the kitchen sink and other plumbing?

Does it have a fireplace?
 
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I haven't mentioned rising damp.

What sort of DPC is it?

Peel back the carpet please to see if the concrete floor is equally dry all over.

You say the kitchen diner us in the other side of the wall.

Is it the end with the kitchen sink and other plumbing?

Does it have a fireplace?

I didn't have the DPC done - it was already there, but it is like a black, smooth tar on the floor. But, yes, it is all dry.

The other side of the wall is just the dining room bit - so it's no where near the kitchen sink/plumbing etc.

There is no fireplace on the other side, just a radiator on the wall.
 
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The bitumen coating you describe is used to protect floor coverings, especially parquet, from contact with a damp floor. It was also sometimes used to stick down plastic tiles. It is not a DPC. If the floor is damp, it will not prevent water being absorbed into walls.

Chimneybreasts tend to have been built without a DPC, even when the rest of the house walls have one. Sometimes this can be dealt with by exposing the base in the void below the floor (which you do not have) or by digging out beneath the hearth, so moisture can evaporate from the bricks and escape up the flue.

If the source of water can be found, e,g. a leaking radiator pipe, it can be remedied better than by concealment.
 
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To find out if the water is coming from the air or the wall, tape a piece of clear plastic tightly to it. If the water is coming from the wall, mist and droplets will form on the underside of the plastic. If it is condensation, it will form on the room side.
 
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Its likely the hearth has no damp proof course. No fireplace vent at ground level won't help as they'll be no air movement to carry away any moisture.

It doesn't look to bad. I'd be inclined to paint the areas that you're concerned about with anti damp paint and then decorate over the top. Its been like that since the house was built.
 
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