Pls help settle a black mould argument

[Osprey67]

My ragey googling isn't helping.

One bedroom has some black mould (not horrendous) at the top of the window. It's a corner bedroom, so two external walls.
It's the only bedroom that does not have a radiator under the window. It's a larger room in the house and the rad is on the opposite wall to the window.

DH thinks it's all about ventilation. I'm not disagreeing (however, this room is well ventilated despite his protests to the contrary. The window is open nearly daily).

But I think the bigger issue is the room is not getting enough heat in the places that matter (the window, we've just swapped out a really shit radiator).

Any thoughts on this? Plus any official links that tell me I'm right would be appreciated!

Its probably both. Opening windows is OK as long as its not humid outside. A dehumidifier is your friend.
We had mould in our bedroom. North facing corner room with single skin brick work..I dry washing inside (no tumbler) and the ensuite has no fan. We always used to awake to horrendous condensation.
We externally insulated the walls and re-rendered (the render was blown so we had to do the whole house anyway) and I have invested in a good dehumidifier for the washing. I also open the bathroom window after each use (and keep door between that and bedroom shut). Mould has stopped growing and condensation improved massively.
Sorry OP but no radiator change involved....

It could be that the wall isn't insulated and water is condensing on the cold wall out of the hot warm air from the rest of the room, or that the gutters or roof are slightly leaking in that area and so the wall are wet from the outside.

Quickest way to check is to tape some cling film onto the wall and see if the moisture beads form on the outside or inside of the cling film.

If it is that the wall is cold then you could either run the heating more or move the radiator, which would warm the wall up so that water won't condense on it, or you could insulate the wall. Insulated wallpaper is probably the cheapest and easiest way but internal or external wall insulation will be better if you can face the disruption.

Thanks both. I'll digest your posts and look to see what we can do.

You could also consider using insulating paint. We were redecorating anyway so adding a few layers of it to those bits around the window which seem prone to condensation was no big deal. I can't vouch for how well it works because we only did it recently but the reviews were good.

Insulating paint? Hmm.

It’s a cold wall that’s getting water vapour on it. Is it cavity wall or not? This is difficult to deal with if it’s single brick. The wall is getting cold and damp so eliminate both if you can.

It can, and likely will, be affected by both ventilation and heat.

But I'd also check the guttering and roof outside to check there isn't water hitting the wall.

And check the loft insulation runs right into this corner - as far as possible without blocking any eaves ventilation.

You need air circulating. Open windows

Ventilation is the most important thing, that sort of mould won't grow if there's decent airflow. Do you have heavy curtains or a blind there? Ideally you want it all as open and airy as possible. Have you checked your gutters/downpipes/roof? Is there a metal lintel that might be acting as a thermal bridge?

We don't have central heating so radiators aren't a factor in our situation.

We have a bedroom that has two cold corner walls. No problems with black mould when it was being used by one child. Swapped bedrooms to give the kids our larger room (they actually wanted to share). We then started getting problems with black mould. We had to paint the walls with antifungal paint. We reached the conclusion that the output of 2 adults breathing at nightime as opposed to one child was what made the difference. We then kept the window open at night and it helped a lot.

I now have that room to myself (snoring husband moved to another room) and we have no problems at all

To answers some questions:
Not sure re cavity wall. When we bought it said no, but when drilled to the outside for a satellite dish it was definitely there!
It just has a roller blind, but the mould is up behind the fittings of the blind.
Shall get DH to check the gutters/ insulation in loft.
Shall look at paint at a suitable time.

I've learnt alot! Thank you!

Paint does nothing. Heat and ventilation are needed. Check guttering and any driving rain getting in via poor external wall.