Damp plaster

[electrospark]

Hi, looking for some advice. I have a damp patch about 600mm square on an upstairs exterior wall. It's been fine up until now but with heavy rain we've had beating on the back of the house it seems to have made its way through a crack I think around our window. I've sealed the crack with silicone but the damp patch doesn't really seem to drying out and it's also grown a small layer of fur. Think the plaster has blown also as sounds hollow when you tap it. Is it best to knock out the plaster and start again or do you think I could paint some damp sealant over it and repaint?
My only worry is the damp will come back? Any advice?

Thanks

 

[Micilin]

You are right to address the source first.

I don't think the silicone worked. Is there anything else above or around the patch on the out side, or more cracking? Does the render on the outside sound hollow when you tap it?

 

[Guitar_man]

Remove all the render that sounds hollow. You might find there is none. If its just a crack in the render, chop out a channel along the crack and rerendering it with a sand cement mix with a dab of pva. When the frost hits the water behind the render and freezes, that's when your render will pop off. Silicone won't provide a waterproof seal to the wind and rain.