Thanks.
Yes, old house. The damp is visible (stain) and stud wall feels clammy to touch. Showing up as a damp stud wall, so plasterboard and wallpaper.
Once we know we have no more guests turning up this summer I’m ripping the stud wall off to get behind it. Damp seems to come down the chimney stack – the stud wall is damp where it joins the stack.
Definitely not condensation – we know what that is like, and have dehumidifiers all over the place to deal with that.
As to the specific question, I have no idea … Lots to learn, I think!