If it’s a gypsum skim coat then it should be pretty vapour permeable. As I understand it, gypsum plaster is not considered to be a great finish in traditional buildings because “breathability” has three attributes: being (i) vapour permeable, (ii) capillary active and (iii) hygroscopic. According to this paper, (see table on p.13 for a summary) gypsum plaster has a similar vapour permeability to lime plaster. But because it has high capillarity, gypsum plaster soaks up water easily. I think that its low hygroscopic capacity and medium speed of hygroscopic take-up mean that it then dries out slowly compared with lime plaster. I’m a bit nervous to advise on something I’m only just finding out about, but I think that retaining a gypsum skim coat should be fine if its in the sandwich between the original wall and “breathable” internal insulation. But the paint should probably be removed if it may be a vapour barrier
That is an interesting paper. It was published almost 20 years ago so I would hope that it has been well peer reviewed I wonder what the consensus was. I would be nervous about putting a less permeable hygroscopic layer in the midst of a permeable structure. Stripping paint from the existing wall skim before attaching a new layer is likely to damage the skim anyway, so the problem may resolve itself.
As an afterthought, in @pottyone72’s case we don’t know for sure what is under the skim. It is known to be lime plaster at certain locations. Whether other plaster has been totally replaced is unknown.
I have wondered about this .
@pottyone72, this is relevant to your comments about damp and mould in fuel poor households:
Roadmaps for fuel poverty and retrofit: we know what needs to be done so why can’t we do it? With Tania Jennings (Chair of the Association of Local Energy Officers, London)
Zero Ambitions PodcastListen on Apple Podcasts:
Zero Ambitions Podcast: Roadmaps for fuel poverty and retrofit: we know what needs to be done so why can’t we do it? With Tania Jennings (Chair of the Association of Local Energy Officers, London) on Apple Podcasts
or find the episode for end of November’23