I'm thinking about adding insulation to the wall between the house and an unheated conservatory. The existing wall is 1970 cavity block, with a thick rough-cast cement render and with retro-fit blown in glass fibre insulation
Additional insulation could be 25-50mm internal, &/or 50-100mm external.
If we used PIR, 50mm would give something like U=0,16 & 100mm U=0.11.
I think the (semi-)external option would be better? It would certainly be easier!
However, being original 1970 construction (aside from the CWI, & the conservatory, come to that) there's no vapour barrier inside, just plaster board, then a gap, then the first layer of block. I've never seen any sign of interstitial condensation (I cored a hole through the wall last Christmas). If we were to add vapour barrier to the inside it would probably be little additional effort to add a bit of insulation at the same time.