The first wall we had insulated, about 1984 or 85, was done by a small builder with absolutely no attempt to stop warm moist inside air from getting behind the insulation. At the time I did not realise the imortance of this. It used insulated plasterboard mounted on battens screwed directly onto the wall; at least the battens used were pressure treated with preservative. And it was in the bathroom, where humidity is sometimes very high. About 25 years later we completely remodelled the bathroom and took the opportunity to rip out the old insulation and put in much better insulation with a proper vapour barrier and internal, warm side, battens.
I expected to find a pile of rot behind the plasterboard, but it was OK. I think the explanation is partly the pressure treated battens, but also the poor standard of insulation (18mm of polystyrene!!) meant that the wall behind was still fairly warm. The better your insulation, the colder the wall behind will be, so the more important to seal the insulation really well.