Our house is a ground floor flat, in an 1830's cottage. The bedroom has two damp patches on the wall, at head height; they correspond to the position of two cast-iron air vents on the outside wall. The bedroom has been damp before we bought it, as the wall below the picture rail has been de-plastered and covered with 2x2 and then plasterboard (although from outside, it looks as if there was a layer of hardboard first?). I think a historic damp problem might be why the vents were installed. As the air vents (8" x 6") are under an arch, no rain can reach them, and there is no sign of water running into them. Consequently, I assume that the damp is caused by moist air coming through the vents and condensing on the back of the plasterboard: the damp is only around the vent (i.e. on the inside, you can see the outline of the vents on the wallpaper).
Would it be best to (initally temporarily) block up the air vents? If I do that, should I open up a couple of ventilation grilles into the bedroom, to ventilate behind the plasterboard?
thanks, Nathan