Damp under bay windows

[MrsSilenceDogood]

There is damp on the interior wall where the bottom of the bay meets the top of the wall. The damp only appears to be at the corners. The window photographed is on the first floor, but I also have the same problem with window on the ground floor. How can I fix this? Can I fix it from the inside? As I am a beginner, a quite detailed explanation would be amazing.

 

[blup]

It could be ingress of moisture from outside, the rain has got into the render/brickwork and not dried out. Its also possible that the room itself isn't kept war and damp has developed, although this would usually manifest itself as black mould. Inspect externally under the windows to see if there are signs of damp or gaps that would let moisture in. The remedy may be to chop out the plaster and let the wall dry out, replacing with breathable plaster.