Damp, bay window. Solutions?

[CoolHands]

Upper floor bedroom bay window attracts damp inside on the walls see pics despite radiator. I can only assume this is condensation even though a dehumidifier has been used regularly and all windows opened very frequently too. (Bathroom has no extractor which needs to be sorted as probably won’t help with the showers). It’s front of the house and doesn’t get any rain from outside hitting it (always blows onto the back of the house) so I’m sure it’s not external.

I’m thinking insulated plasterboard dot and dabbed inside? That was my main thought. Otherwise just saw local house having exterior insulated, would that be better (only on the bay area). I remember from having the room replastered once the interior wall quality was a bit st I’ve a vague memory it was like some kind of ‘board’ (probably asbestos knowing my luck )

tldr insulated plasterboard or exterior insulation if we have render knocked off?



Bit blurry:


Outside now needs crack filling and repainting etc:


Local house having exterior:

[wolfracesonic]

Definitely looks like a condensation problem. EWI should perform better than internal insulation but more expense and can sometimes look a bit odd. Is your’s brickwork behind the render, like in the pic? In my neck of the woods I’ve only ever seen timber studs with metal lathing fixed to it.