Damp patch around air vents in bedroom

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
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DAMP PATCHES?! ON THE WALLS AT HEAD HEIGHT?! JUST WHAT SORT OF SEX ARE YOU HAVING IN THERE?!
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Nathan Critchlow-Watton wrote in message

I'd block them up permanently and just use the windows. They probably date back to the time when coal fires would fill the room with smoke and you needed plenty of ventilation.
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being colder at that point.
--
Tim Mitchell

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Thanks for all your replies. Just to make it clear, the air vents are plasterboarded over, so wouldn't vent smoke directly from the room - although I've no idea why else they would be there!
Tim's suggestion makes sense, although the damp is on the "outside" side of the plasterboard: wouldn't the condensation form preferentially on the stone "tunnel" that goes to the vent, rather than on the back of the (nominally) warm plasterboard?
Either way, blocking it up for a while seems a good idea. Is there a simple way I can measure the damp level, to assess if blocking the vent off has worked? Should I open up a grille in the bedroom, to ventilate the void behind the plasterboard?
Thanks for your help, Nathan
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Nathan Critchlow-Watton wrote in message

The plasterboard is presumably a relatively recent addition.

So is there anything stopping the rain blowing against the PB from outside?
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Nathan, I think you are right about condensation forming on the back of the plasterboard. but I don't think it is coming from outside. It is much more likely to be due to vapour permeating through the plasterboard from inside and reaching its dew point within the wall thickness - so-called interstitial condensation.
The three methods of tackling this, as with all condensation, are 1) try everything you can think of to stop producing excess vapour. 2) provide ventilation to dissipate vapour-laden air and evaporate moisture 3) prevent vapour-laden air from being cooled.
The two vents were probably installed to help manage condensation and the plasterboard is now preventing this. Ideally, the vents should be opened up to allow more ventilation, but this is probably difficult for you. Blocking them up, though, could easily make the situation worse, so I would tend to leave them.
Probably the most effective way of tackling the problem would be to provide an effective vapour barrier to prevent the vapour getting through to the cold zone in the wall. Any condensation will then form on the surface. You don't mention any insulation in the wall, ideally there should be some behind the plasterboard but if so that's even more reason why there should be a vapour barrier on the warm side. As a start, to see whether a vapour barrier might be effective, is there any chance you could painting the wall with a couple of coats of proper old-fashioned oil paint (not permeable type paint like Dulux Weathershield). Ideally gloss, but undercoat would do. This could solve all your problems..
Peter
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