Lath & plaster does not form a vapour barrier; deliberately not in most cases (there was perhaps some crossover when more moderm building techniques were still using lath and plaster) - old houses need to 'breathe' as they say, moisture needs to be allowed to move between inside and out. They can be warm & dry, but the way to achieve it is (paradoxically) porous materials.
Yes of course, but I was thinking more in terms of DIYers jamming a lot of non-breathable eps into old houses. Probably better to say consult someone who knows what they’re talking about before you internally insulate your old house.
