To insulate or not ...

[motorbiking]

A photo of the house would help. There are various configurations that were used at that time and the thickness of the walls is important. If you have say a 325mm rubble wall then the U value (heat loss rate) is going to be quite high. Just 50mm of insulated plaster will improve that by a factor of 4. Other factors such as windows and chimneys also make a big difference.

 

[jeds]

Yes, unfortunately, I'm still not 100% clear? It's not your fault Anna, these old houses were built in dozens of different ways. There were few rules then and people did what was local custom, and with what local materials were readily available. It is important though because incorrect advice can cost a lot of money and potentially cause problems.

As mentioned above, external walls are the main walls of the house that keep the wind and weather out. But external walls have an internal face, and this is what we need to know. The confusion is that people (we all do it) often refer to internal walls as walls between rooms - which are really partitions.

So, the external walls are stone but the internal face of the external stone walls; Is the internal surface lath and plaster or solid plaster?
If it is lath, the lath must be fixed to some sort of timber. That could either be timber battens fixed to the wall or a timber frame built up against the wall.
Can you tell which?