I have recently noticed that an outside corner of a top floor bedroom has become damp. It seems worse at ceiling level but is evident at floor level too. It is damp enough to have caused staining and some black mould under the paper. It is not near a window or other bridge.

The roof is insulated with no sign of damp in the roof space and a roofer has checked tiling with all being sound and the wall is uninsulated cavity construction. There is no sign of damp in the room below. I have removed the wallpaper and dried out the wall and ceiling but the damp returns overnight. I have had no construction work done and nothing has been changed in the 12 months other than general redecoration at which time there were no problems. The nearest guttering is several feet away. No sign of damp on exterior wall.

The house double glazing does not have trickle vents. It looks like a condensation problem so I'm thinking poor ventilation may be the root cause but I'm not sure why its suddenly become a problem. Coincidentally there is an air brick on the ground floor directly beneath the problem so I'm wondering that if that ventilation source has become blocked whether condensation could be tracking up the interior wall and into the ceiling plaster. I'd be grateful for any advice on what the problem might be and any suggestions to resolve it. Cheers.

I'd check the vents were clear fist, as it is normally the easiest fix.

Identifying the exact cause of damp is often difficult, but if you say that the roof is 100% watertight then the marks on the plaster are either caused by condensation or they are transmitting from the wall to the ceiling.

Perhaps it would be a good idea to get a dehumidifier and see if the problem imporoves.

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DIY | Donnas dream house

I have been running a Dehumidifier for a couple of days and had a poke about in the airbrick as best I can without bashing it free. Following that the ceiling and wall appear drier but the ceiling is very cold in that one spot where the damp appeared and I have since noticed that there is a vent tile in the roof over the cold spot so now my best guess is that cold winter air blowing through the vent tile onto the ceiling below (only about a foot) has created a cold spot causing condensation to appear within the bedroom. I will lay a thicker layer of insulation and/or some kind of baffle in the loft area beneath the vent tile and and ensure there is good venting throughout the cavity wall. That should do it hopefully but I still have no idea why its suddenly become a problem when nothing else has changed. Cheers.

Good luck with it, please let us know how you get on with it

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DIY | Donnas dream house

I think I have solved this problem. The roof space has vent tiles set low down on the roof as you would expect. Each of these vent tiles has a cowl on the exterior face and a sleeve on the inner face. Above the area of the damp appearing on the ceiling there is such a tile and on closer inspection the bottom of the sleeve is very near the ceiling surface and the loft insulation has been disturbed, presumably when stored stuff was moved, causing the vent tile to blow cold air directly onto an uninsulated surface. The weather has been particularly cold in recent weeks and the ceiling surface has been getting very cold indeed and we have had the heating cranked up - so condensation has formed giving the damp patch in this one spot which has spread to the adjacent wall. I have now repaired the damaged insulation, dried out the ceiling and tested humidity after a week or so and its at 50% so all now seems well with no signs of reoccurrence.

Just to be on the safe side I got hold of a damp meter and stuck it in the ceiling but I dont really understand the reading. It seems to be indicating 13%. Is that good, bad or average? I didnt have reading when all this started.

Cheers