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
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 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
Thanks for your help,
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
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
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..
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