Rising dampness on internal wall

Got a small question regarding rising dampness (currently ~2ft high) on an internal ground floor wall...
The wall itself is around 100 years old, and was ~2 years ago covered by 10-20mm plaster and then two layers of plasterboard, with the first plasterboard foil backed (no idea why):
|------------old brick--------|-plaster-|#-p. board-|-p.board-|
The '#' denoted the foil backing.
Oddly enough we have started getting dampness on the wall, with a dampness meter (when poked into the plasterboard) reading >90%. But more oddly the old brick wall behind the plasterboards is virtually dry! If this was rising damp from the ground shouldn't the brick be wet and the outside plaster/plasterboard progressively get drier??
One dampness expert says "we believe that it is contaminated wall plaster in these areas" (no further explanation) and recommends a 600 treatment for less than 2 linear metres of wall...
Another explanation was that the plaster is drying out and expelling water, and due to the foil backing the moisture has nowhere to go except to the edge and then out/up. But shouldn't the brick wall then be equally wet as the plaster?
I first suspected my plumbing but that seems to be all in order...
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'Rising damp' is usually a myth put anout by those who would like your 600. Don't give it to them. The moisture is probably coming from inside the room, breathing, washing, cooking etc, and condensing on the wall, hence the underlying brickwork is drier. Remove all the non-breathable materials, especially the foil, but also the gypsum plaster, which disintegrates on wetting and is not very breathable, would be best removed and replaced with lime plaster, as per the design 100 years ago. Improve the room's ventilation and remember that 'dampness meters' are notoriously unreliable.
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cs wrote:

to prevent the necessary flow of damp in the wall, and thus cause your damp problem. Victorian house walls handle damp quite differently to modern buildings, and if you dont maintain that permeability youre likelt to get damp problems.

well I never

those measure conductivity, theyre not reliable measures of damp. Youve got metal foil on there right?

Almost every case of 'rising damp' is either condensation or penetrating damp.

Who says theyre an expert? Did they say so? :) Seriously its not going to cost 600 for 2m, and no 'treatment' has anything to do with solving the problem. Just remove the problem and replaster with lime, which allows drying better than gypsum.
I take it youve checked your external ground levels are below the internal floor, your gutters arent blocked and drains arent flooding.
NT
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On 28 May,

Condensation?
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Looks like someone's completely misguided attempt at solving a damp problem they obviously didn't understand.

His expertise is in getting 600 out of people. He either has no clue about damp, or is also a liar.
You have almost certainly got a condensation problem. It might all just go away by ventilating the room (that area in particular), but it's difficult to work out some way to charge you 600 for doing that.
What you should really do is rip off all the plasterboard, make sure the room ventilation is good, and then leave it for a few months to reach steady state, before reassesing the situation. You might just be able to reskim the original plaster at this point. Don't put up any decorations which restrict passage of moisture through the wall though and don't put things in front of the wall which restrict air flow or insulate it from the rest of the room -- 100 year old walls don't work that way.
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You could start by standing a mirror against the wall (or a tile). If that gets wet, you probably have condensation, which is common with solid ground floor kitchens/sculleries/bathrooms. Don't listen to anything a damp company tells you.
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