can you celotex Victorian external walls? My house was built 1890 with original plaster in most places and no cavity. Surveyor said it was a big no to add any fixed board insulation as it will stop the walls breathing and cause damp problems.
Would have thought it'd be like any wall and if you create a good enough seal you won't need to worry about your current walls and damp.
Anyway, made a start on the walls here today, not as much progress as I'd like to have made, but few annoyances with the current rad pipework being enclosed by the water pipes and nailed down.....Put my foot through the floor with a minor slip, luckily it was above our porch so just into another cavity that i'd already filled with wool insulation!
Before
After
Nails...
https://imgur.com/sv0ImhK