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You should figure out target value for your insulation (there are calculators based on amount of unheated walls you have for a given room). Then start looking at the depth of your joists, clearance under the joists, access, cost, remembering that kingspan and celotex are both utter bastards and involved in Grenfell towers/cladding scandal and so on. That should help make the decision.
atk
M4xime
Okay that makes sense. No I was more thinking around whether you'd need to add another layer to prevent condensation etc ...
Also, some people seem to use rock wool and some PIR boards, is there a better option?