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If there's no air gap then you'd need a vapour control layer on the inside of the rock wool which would ideally tape to the equivalent in the floor/wall below.
I've just done internal wall insulation for a 3-floor victorian terrace. Kind of wished I'd re-hung the joists on thermally broken wall hangers given plenty of them were rotten at the rear of the property, but hindsight is 20-20.
Ended up going down the air gap, 100mm PIR, air tight membrane plus heat recovery route... should be toasty, dew point calculator was happy too. EPC will go from E to B (eventually).
JonoMarshall
@starfish&coffee
Rockwool is typically made to the depth of a 2x4 inch beam. On the inside of the building you strip the outside facing walls down to bare brick. Build a stud wall structure on the inside. The spacing of the beams being the width of the rock wool and the 4 inch side of the beam providing the depth. Then place rock wool between the studs floor to ceiling.
You cover that stud structure with OSB board and then attach thin sheets of plaster board on top of the OSB board.
Considering how much you've spent on the house this should be a relatively cheap jobby. The most hassle will be re-doing the window sills to go with the slightly thicker wall.
At this point you're no longer talking about tearing down Victorian houses etc. The rock wool does the job.