Eliminating condensation on gable end walls

[Dusty]

Hi all

 

During the very cold days over winter, condensation has been forming on my 1st floor bedroom gable end walls. The construction is gypsum plaster over a solid brick and lime mortar gable wall which was cement pebble dash rendered in the 1960's. The cottage is approximately 200 years old.

 

I am hoping to insulate internally and have looked at PIR boards. I am limited on space so max board depth is 50-60mm. 

 

I am a complete novice and any advice would be greatly appreciated.