I am renovating a house from 1887. It is built with double brick walls with fixed binders. Due to heritage preservation regulations/restrictions, it is being insulated from the inside. Here, I am using hempcrete blocks.

However, in the bathroom – specifically in the shower niche – I don’t think this is a viable solution since the hempcrete will be enclosed by my wet room membrane.

The shower niche is located in a corner of the room, where one of the walls is an interior brick wall. It has been plastered and is perfectly straight and level. However, it is cold because it is directly connected to the gable, which is uninsulated. I would like to insulate this wall a little – for example, with 20-30mm of cork. The back wall of the shower niche faces the gable. Here, the internal shower wall needs to be 15 cm thick (measured from the back wall) to conceal a sewage downpipe to the first floor.

We need to be careful not to oversize this, so the back wall doesn’t risk drying out too slowly and becoming too cold.

A new internal partition wall needs to be built for the shower niche.
The entire shower niche needs to be protected with a wet room membrane and tiled.

How can I insulate the walls here and how to build up the 3 different walls
?