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Excellent work. We've done almost the same in a 1880's tenement flat, top floor so put 100mm pir + moisture barrier across each ceiling as we go (bedroom, hallway and bathroom so far, kitchen + front room will be next years project), front room and bedroom and back of bathroom walls done in 50 or 65mm PIR depending on usage and barrier. Yes makes your rooms a bit smaller but the bedroom + bathroom the difference is insane. Can heat the bedroom on a <1kw wall heater to a furnace easily and will maintain temp over night nicely. Bathroom is in middle of building, but has a cold wall into the common stairwell and another wall thats into the next building, but roof line is only a few metre above and does a pretty good job of conducting heat away. Bathroom is now heated by around 400w of heated floor + waste heat from a current generation unvented cylinder. Previously it was like walking into a smelly wet cave.
External insulation is likely easier/costs less as you don't have to almost entirely strip interior of building out and has bigger gains, gets around a lot of cold bridge issues.
Making an older stone building entirely air tight is difficult and then you get a whole host of other issues, moisture instead of getting driven out of the stone starts making its way into the building etc.
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BrickMan
Unfortunately not as we're in a conservation area, external would've been higher performing and easier.