Farmhouse, insulation damaged, how to repair correctly?

Yes, me again with my never-ending construction site: farmhouse!

Now the following problem has arisen. While tearing down the wallpaper, underlying surfaces have been revealed that I hadn’t even dared to think about before. In one corner there is even a hole about half a meter in diameter and about 3 cm deep, revealing a clay/straw mixture. Can someone give me a tip on how I can repair this disaster?

So if I imagine it the way you describe, the only thing that immediately comes to mind is a complete renovation! ;) Alternatively, there is the option to cover the wall entirely with drywall sheets. Otherwise, I would think of the fabric material. Either as a mat, also called plaster mesh. You can do quite a bit with that, although drywall sheets would be more professional and durable in the long run.

You can only find a craftsman who understands these techniques. Because if you simply close it with gypsum, it may be that nothing will hold anymore. With old building materials, you have to be careful because they are often not compatible with modern materials. You can only patch with the same material!

Plaster damage

Clean the substrate of all loose and non-load-bearing parts and repair with matching clay plaster. :D