Hi all

This is my first post but I have been lurking threads ever since we moved into our new old home back in May 2022.

I've done a lot of decorating in the rooms that just needed updating and a bit of loft work on airtightness and insulation so far but I have now started the first major renovation works in our house.

Quick bit of background
A neighbour informs us that the house is of 1902 heritage, so right on the Victorian/Edwardian period border. It's a semi-detached 5br 2bath with a facade typical of the era. Mostly the walls are 2 brick solid but we do have a cavity wall on the side of the rear projection. At some point I'll share a post with the overall long term plans to renovate, including EWI, IWI, MVHR and more.

Before all that I'd like to hear input on my most pressing issue.

I've ripped out our family bathroom of 1980s vintage, unfortunately the internal plaster and lath work was not sound and didn't survive the removal of tiles and plasterboard and the external wall interior plaster suffered a similar fate. With everything stripped back the sins of the external wall have been exposed. There are small holes, cracks and great big gaping chasms in the internal leaf. Whilst I've been removing the debris I've been contemplating the best steps to improve the thermal performance of the room.

This was my original plan for the wall and I think the safest approach

1. Expose as much of the wall as I can - i.e. Remove ceiling and take up floor (done)
2. Repair brick work, removing broken bricks and replacing them with better condition imperial bricks that the previous house owner has stored in our garden. Use lime mortar (fortunately the Traditional Lime Co is a short drive from us)
3. Plumbing first fix - the waste pipe is moving to the corner of the room but apart from that major penetration nothing else should be changing.
4. Airtightness work around penetrations old and new, is foam enough or do I need to employ tapes etc.
5. Parge coat of lime product (just plain lime and sand or a thermally efficient one, please share opinions!) to improve airtightness and provide a level surface for the next steps
6. Airtightness tape applied to joist ends
7. Attach ~80mm wood fibre boards with buttered on lime product and plastic non-bridging fasteners
8. Work on the detail around the window, considering using more synthetic products here, I think that, due to the way the reveals have been constructed, I don't need to use aerogel but I still don't want this being a major thermal bridge.
9. Final plaster finish - now I'm a bit stuck at what to do here, I'm trying to use vapour open products but I also want to tile the bottom half of the wall as this is where the bath is going.
10. Tile
11. Paint upper half of wall - how well do clay paints fare in a bathroom?

Is there any problems with that?

Since its a cavity wall, is this a safer place to use something like a phenolic board (I have a pile of Kooltherm 70mm from over-ordering to insulate my sloping ceilings)? As its a bathroom it is probably a source of moisture input into the walls a lot of the time, so perhaps being vapour open wouldn't be an advantage here?

I've seen in other posts people struggling with posting photos but I shall do my best to follow up with images of the wall and problem areas. I'll also try and share a picture of the layout as the external wall is not actually a significant portion of this room.

Cheers,
Shaun