Old stone house advice

6 Feb 2021
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United Kingdom

So I have recently brought a c.1800 stone cottage and am at the start of the extensive work it needs doing to it.
The area which is a bit of mine field is the topic of cement rendering and keeping walls breathable.

Now the house is basically split into 3 section as I've illustrated in my masterpiece

1) original house - lime mortar stone wall. This is the red walls in the picture linked above.
2) first extension - cement mortar stone wall (was also cement rendered on the inside which I've now stripped down and it was pretty wet). This is the green walls in the picture linked above.
3) Second extension - 1950's block built extension. This is the blue walls in the picture linked above.

Now the entire house is Tyrolean rendered on the outside apart from the curved wall which is exposed stone on the outside.

So my main query is about how to internally insulate and externally render the wall. In my mind we have 2 options:
1) Keep cement render outside and then waterproof the inside of the walls, then add internal insulation and plaster.
2) Strip Tyrolean render and replace with lime render, and keep the 1950s extension cement rendered. Strip the rest of the inside (as this is currently cement rendered) and add 'breathable' insulation.

Has anyone had experience with stone built houses that can give advice on this situation?

I've read a lot about not allowing stone walls to breathe can lead to damp to build up and deteriorate the walls which is obviously something I want to avoid!
Also given parts of the stone walls have cement mortar, does this effectively mean that those walls already can't breathe properly?

Thank you,

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