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I stripped a wall of gypsum lime, olastic membrane & brick, where the original chimney had been cackhandedly knocked away.
I agonised over how to make good, and ended up with a vapour layered plasterboard on a 2x2 stud, insulated with real wool, but ensuring a 2" void for *some* ventilation from below.
My problem with the gypsum was that it allowed much more efficient wicking of moisture through the underlying lime plaster.
NotThamesWater
I'm starting to sympathise with your point. I've gone down a rabbit hole and it seems like unless I remove the gypsum plaster on the inside the breathability will be compromised. Bit of an extreme conclusion but it makes sense I guess if vapour is meant to pass from inside to out.
I was planning to insulate a back bedroom internally, but now unsure what the hell I should be using to do so.