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Does that room have any ventilation?
Is the humidity unusually high (~70% up)?
I wondered if the brickwork is getting damp somehow - crack in the render or borked seal between window and mortar - and it’s getting trapped behind the render but then it would fuck the plaster and it doesn’t seem to have done that.
Howard-
Yeah I would say the room is normally 70% and up.
On the wall it goes away in summer, but even during the 40 degree heatwave and no rain for 2 months there was a small (5mm) tide line around the window.
If it is condensation we need to increase ventilation but also keep the house warmer (insulation) so air can hold more water?
apc
Sorry long post, tldr; condensation or not?
We had a surveyor around for damp issue in upstairs spare room last year. He concluded it was likely condensation as there were no signs of penetrating damp. I’ve done a lot of reading on the topic but I am stumped, as was the surveyor tbh. It is all around the window frame and also random patches on the wall (pictured).
It was basically invisible during the summer
This winter however it’s formed a pretty clear diagonal patchy line. Not had heating on much but has also rained a lot. What could possibly be causing this? The only thing to note is next doors house casts a diagonal shadow down the exterior of this wall as well, roughly the same shape (top right gets sun, rest doesn’t)
Still think it’s condensation as taking back the paint reveals bone dry plaster but could be wrong.
Even when running a dehumidifier for a day or two the marks stay on the wall. There has never been any mould so I doubt the wall stays damp for long just the marks seem to stay. The current paint is very plasticky.
How do we fix or at least confirm it is condensation?
We want to get rid of external render for breathability, eventually. Another option could be internal insulation to make the walls warmer.