Use a vapour barrier on the inside (warm side) of the insulation. This prevents the warmed and therefore moisture (including air breathed out by humans etc.) laden air from permeating out into the insulationg rockwool or fibreglass batts, condensing there and soaking the insulation. A suitable barrier is plastic film. It is also appropriate to vapour barrier around the back of electrical outlet boxes in the outer walls for the same reason. The outer 'waterproof' membrane should be of the type that will prevent the ingress of water but should be of the type that 'breathes' i.e. any moisture/vapour that DOES get into the wall cavity will be able to dissipate out through the outer wall. Our climate is similar but colder and the above is based on our experience here with a) Wood frame housing with finished (plasterboard) inner walls. and b) Wooden sheds which are basically unlined. Terry.