Damp coming in through bedroom window - who do I need to look at this?

[MaidofMuswell]

We live in a typical 1930s semi and our bedroom is at the front with a large bay on to a quite wide road so v exposed to weather. In winter especially patches of black damp / mould not sure exactly what the difference is appear in both the window frame and wall beneath it. The exterior brick walls are coated in a sort of pebble dash render that's been painted but presumably isn't weather proof. Does anyone have an idea what sort of tradesperson I'd need to look at this - a builder? mason? utterly clueless as is probably clear!

It’s usually poor ventilation that causes that. Can you leave your windows on vent? If not, make sure you open them for a bit each day.

Agree with post re ventilation.

Use a mould killing spray to get rid of it and keep window open a bit even in winter. Yes really, even it seems like madness.

I have a 1930s house and it’s a nightmare for black mould. Never had it in any other house I’ve lived in.

does it have a cavity wall or is it solid wall?
in victorian buildings the bay windows are thinner (one brick thick) than the normal wall which usually is double brick thick.
that can cause condensation on the cooler wall beneath the window.

We have a 1930/40 semi and have a similar problem the last time we remortgaged they sent someone round with a device to test for damp. She did the walls that get mold on them and we don't have a damp problem it is just a ventilation problem and the damp air hitting the cold wall.

We just use bleach a lot unfortunately and always ventilate the house as much as possible.

This is super helpful re ventilation thank you! Yes 30s house, didn’t know they had this reputation!

Condensation. You need to sleep with the window a crack open every night (or just ventilate the room properly every morning, for a good half hour).

Yes I agree that it's unlikely that damp is coming in, more that it can't get out when windows are kept closed during the winter.

Which is good news, as it far less costly to fix!