This rather depends upon external finishes, stud wall thickness and whether the building can be considered exempt of the Building Regulations. Regardless of insulation type and thickness, always include a 500 gauge polythene vapour barrier internally between finish (plasterboard or timber panelling) and a breather membrane externally. If the external finish is a timber boarding, this should be on a minimum 25mm thickness counter batten fixed over the membrane (if vertical boarding is proposed, a second counter batten should be fixed vertically prior to the horizontal batten used for fixing the cladding). If he structure is exempt and cost is a driver, use mineral fibre quilt / batts between the studs to the same thickness, otherwise consider rigid board insulation, such as Celotex or Kingspan. For plasterboard finish with a higher performance, a thermal laminate can be fixed over the vapor barrier instead.