Recurring Damp Stripe above Window

[RealPolitik]

I have a damp problem - an arch-shaped stripe of damp (approx. 6" x 3') keeps appearing on an internal wall about six inches above an upstairs window.

The room was reskimmed about 15 months ago and there was no problem until about 5 months ago - it's a wall under a gable end with no gutter above it. The roof isn't old and is in very good condition (both tiles and mortar), window is uPvc and in good condition. There's an air brick into the roof cavity above it, but the shape and location of the damp doesn't seem related to it (and the ceiling is dry)

I can't see any problem with how the frame is sealed, but there have been a few dried trickles of dirty water from around the wooden plank in the opening above it (a remnant from the original wooden windows - similar dimensions to a skirting board, horizontally across the top of the window opening).

I've scraped off the efflorescence and painted with damp seal several times, but both damp and evidence of dripping/trickling water is getting worse. The house is currently empty but the central heating is set at 8 degrees.

Does anyone have an idea of what could be causing it (assuming the window frame is properly sealed)?

 

[Static]

Crack open a window for a couple of months and let things dry out..

 

[RealPolitik]

I'm sure it would help, but it's locked up and left at the moment, with the central heating thermostat set low.

I'm not sure why it has only recently become an issue - it's been largely unoccupied for over a year.

 

[freddiemercurystwin]

It sounds like condensation, in which case you need to increase ventilation. Can you add a trickle vent in the top of the frame?

 

[JohnD]

"an arch-shaped stripe of damp (approx. 6" x 3') ... about six inches above an upstairs window"

Depending on the age of the house, it may be that there is a brick arch or joggled brick lintel, where the bricks run front-to back of the wall, so forming a cold bridge. The "wooden plank" might be hiding something, such as a gap under an arch . Pull it off and have a look. Older houses often had wooden lintels, which are liable to rot.

You do not say if it is a cavity wall and, if so, if insulated.

If the room was replastered a year ago, it should have dried out by now provided it was well-ventilated.

Your airbrick (?) into the loft sounds unusual and undesirable. Is it blocked with insulation topside?

You say the roof is new, so I will guess it has been refelted. Go into the loft and see if it is damp from lack of ventilation.

Also examine the outside surface of the wall with binoculars and lok for bad pointing or render.

 

[noseall]

As Freddy has said it is likely to be attributed to condensation via cold bridging.

 

[RealPolitik]

Thanks very much for all the opinions.

There's no evidence of a brick arch externally at least, the new plaster dried normally. The house dates from about 1900 - if there is a cavity, it's a very narrow one.

The whole terrace of houses have a similar air brick (all only on one house's side of the shared gable end), but there's no access to the roof space where the air brick is. No evidence of bad pointing (flat roof under the window so easy to access).

A trickle vent might be an easy thing to try.