Single brick thickness bay window and mould

[Anticyclone]

1930s house with bay window in bedroom. Below the bay window the wall is only a single brick thickness unlike the rest of the walls which have cavities between two rows of bricks.

This means the inside wall gets very cold and damp from time to time, due to all the condensation from us breathing at night!

This then leads to mould which we end up having to wipe off every few weeks with anti mould spray.

Would a good coat of anti mould paint solve the problem? Or just temporarily cover it up?

There is a radiator in the middle of the bay which we try to keep on on the morning to help but now it's getting colder the problem is getting worse.

Anti-mould paint has no effect on condensation.

Better to dryline the bay with slab which is plasterboard bonded to expanded foam. Kingspan make one, I think Celotex too.

You need it to be several inches thick. A plasterer will know how to do it. If the radiator is tight to the wall it will need to be stepped out, as will any electrical sockets. Sometimes the inside is clad with old uninsulated plaster on wooden studs, which could be stripped off and the space insulated.

If the outside of the bay is hung with tiles or slates, they could be taken off and external insulation applied then tiled over, but this is usually more expensive and needs good detailing to throw off rainwater.

Thanks, will look into your suggestions.