Insulating original external wall

[jayc89]

Our extension will cover what was the original rear external wall of our house. The original house is 215mm solid wall construction, whereas the extension is cavity wall with an expected u value of 0.17. 

 

I'm concerned about thermal bridging where the two join. Should the original external wall, which will become an internal wall, also be insulated?

[tonyshouse]

I sometimes recommend the first 600mm should be insulated. Better plan would be EWI