Cavity wall, damp membrane or lime plaster

21 Oct 2021
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United Kingdom
I'm currently renovating a terraced house built in 1854. It's built with solid stone external wall (i.e. no cavity). The internal walls are brick. It's been taken back to brick and nearly ready for re-plastering.

I'm undecided on how to go about re-plastering the external walls. Options:

1. Re-plaster straight onto the stone with Lime
2. Build a cavity wall, insulate, board and plaster.
3. Install a breathable mesh membrane over the stone, top with insulated plasterboard and skim over the top.

Options 2 or 3 appeal to me the most because I think it would be good to insulate and also less hassle for decorating (i.e. no special paints).

My questions are:

If option 2, what materials to use with and how big a cavity do you need to make the required air gap? It's quite a small space so worried about losing more. Not sure if this will even be possible where the staircase abuts the external wall.

If Option 3, do you need to render the external wall first before applying the membrane to make it flat? I've been told you don't by two damp proofing companies, but when I called permaguard about their products they said you had to render with sand and cement to make it flat... Didn't really make sense to me as I thought the walls needed to breathe.
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