Dampness

[RSoles]

OK, 300 years old, Walls are made of stone. As far as cavities are concerned, there aren't any; just rubble infill.

What about the outside of the wall, is it bare stone and pointing or rendered over?

I have similar walls ( though only 200 yo) rendered and whitewashed.
If they are rendered have you had them painted recently or (god forbid) coated? Walls of this construction need to breathe so some paints will not be suitable, trapping moisture inside.

Cracks in the render can let in a surprising amount of water, especially from the West/South West.

I've only ever had problems where gutters or flashing had failed and let water in. Are you sure you don't have standing water in the gutters?

I have sash windows so condensation on the walls isn't a problem, though condensation on the glass is..... along with keeping warm....but that's a different story.

[Clippy]

Doesn't sound like condensation to me either as you have not mentioned an internal source which is generating the excess water.

What is the ventilation like in the house?

Do you regularly open windows in the house?

Does the house have any air bricks? If so, make sure they are not blocked.

[Gibbon]

I had exaclty the same symptoms and it was condensation. You don't have to have a steam room to produce it. Just exhaled air is moist enough, (breathe on a cold mirror). If the wall has a north aspect and is a solid wall and therefore cold then and there is little ventilation this is the most likely explanation. Dry lining wherein you effectively insulate the wall will solve this.

[wurzel]

Doesn't sound like condensation to me either as you have not mentioned an internal source which is generating the excess water.

What is the ventilation like in the house?

Do you regularly open windows in the house?

Does the house have any air bricks? If so, make sure they are not blocked.

No air bricks and the room is very poorly ventilated.

No render on the exterior wall, just pointing & it looks sounds.

[RSoles]

Sounds like condensation then.
How long ago was the staircase converted into a room?
Could you increase ventilation from the room into the rest of the house? An above door vent or just leave the door open as an experiment?

[Clippy]

Would suggest getting some air bricks installed around the house and also look at getting, if possible, some trickle vents for the windows.

This should help, if it is condensation, or point to damp.

[wurzel]

I've been using a company called Peter Cox, who seem to know their stuff.

Cheers, I got them in & hopefully it's all sorted now

[worzelGummidge]

Hire a professional old hand builder.
Easier said than done I know.
Anyway I had a damp problem in the front room for years. I changed the guttering and started to ask the neighbor about their damp issues... anyway I hired a old builder who has been about a bit and he went strait to the problem.
A very small - 5mm wide hole above the cornice thingy outside of the house on the wall.

If you get a professional then they just know where to look. The trick is to find them though..

[quackhandle]

Good call, had Peter Cox in to tank out my basement/cellar (my house built in 1876) wasn't cheap but they did a fantastic job.

They were even honest when they said they'd charge alot for their sparky and plumber and advised me to to get my own in.

qh

[Clippy]

Cheers, I got them in & hopefully it's all sorted now

Details man, details.

What was it and how was it solved?