Insulating over stone walls
We recently purchased a 100 year old house in Southern France - Marseille. Overall weather is nice, but a few cold spells. The house was completely renovated about 10 years ago. The east and west outside walls were insulated with an interior layer of 3 inch cellular cement blocks.
The North wall has nothing. The walls are 50 cm thick stone mixture finished with flat cement plaster. This wall is our backyard and is never seen. I would like to simply cover it (outside ) with 30 mm polystyrene sheets and plaster over this. I have read that portland cement makes a good adhesive.
Any thing I need to be aware of? vapor barriers, etc? Thanks loads!!