Condensation/mould/damp under double-glazed bay window....?

[WhatAFunnyPotato]

Hmm...posting here as I have no idea what kind of expert I need!

We have a double-glazed, four-paned bay window in our bedroom. In recent days I've noticed a wet tide-mark on the wall below it, and when I touch the wall my hand comes away wet. Mould is starting to form too.

I always air the room every day anyway, but am confused as to why this has started. Possibly the window needs replacing?

Any ideas? Who do I speak to - a builder? Or glazing expert?

Have you got a leak, either through the wall or window or a gutter or pipe near it?

Not sure - good point, though, must check!

The water seems to be seeping downwards from the base of the window, I think...

I would say that sounds like a crack in the frame or the joints in the frame or perhaps the mortar under the frame.

Or if it is a bay window, there are normally 3 sills on the outside to give the shape. The cement between the parts of the window sills may need replacing, allowing water to penetrate the wall below.

Thanks. Interestingly, our neighbours were underpinning their foundations a few weeks ago and we were told to watch the bay window in case anything happened as that's where we'd see any problems first. If there is a crack, it may well have stemmed from that work. Will get on to the party wall surveyor.

We have this in our upstairs bay window. In our case I think it is because the other walls in the house are cavity walls (recently insulated) but the upper part of the bay itself is tile-hung then has only a single skin wall, so it's very cold. It acts as a 'cold bridge' and condensation settles then forms mould. (Plus the dust that gathers there encourages mould I fear. Oops.) Seems particularly bad at the moment which I'm hoping is partly seasonal but we are working on ventilation solutions and might also try to put on some secondary insulation inside. Wiping down the condensation is probably something else we should do regularly but not sure it will happen.