Surface mould under bay window

[Pottymouth83]

The bay window in our upstairs bedroom keeps getting a small
amount of black mould under it.

It seems to be in areas that have small pieces of furniture in front of them (a linen chest and a chair). We’ve tried leaving a good gap between the furniture and the wall but it still comes back. It cleans away very easily (which makes me think it’s a problem on the inside, not penetrating through from outside?)

Is it just a ventilation problem? Why don’t we have the same issue with the downstairs bay window?

We have a similar issue in an upstairs bedroom, we have just bought a dehumidifier to see if it helps

It’s frustrating isn’t it? I’d like to put a window seat in but I obviously can’t as it would just get disgustingly mouldy behind it!

Why do people keep suggesting dehumidifiers instead of investigating and removing the cause of the damp?

Because this type of problem is generally down to condensation. A dehumidifier will help if nothing else will.

OP I had exactly the problem you describe. I assume it only happens upstairs as it's your breath at night, condensing on the coldest part of the room, when temps are lowest.

What solved mine is leaving a small window open, just a crack permanently in the winter. Since I've done that I have zero mould and very little condensation on the windows.

Oh, I hadn’t considered it could be us breathing in there all night that’s causing it (gross!) 😆 We don’t actually have the window open this time of year as it’s so bloody cold but I’ll give that a try and put an extra layer on in bed!

Because if it is caused by condensation (as seems likely) then you need to take some of the moisture out of the air. The alternative is to insulate the walls, but this is likely to be expensive (I speak from experience here, having just shelled out for external insulation).

Upstairs will generally be warmer (and therefore more humid) than downstairs. Dehumidifier will solve the problem short term but you need to look at improving ventilation as others have said. Ceiling vents (into loft) or window trickle vents should help.

Yep, its your breathing…

Weird isnt it…
Try crack a window a tiny bit, it will help