mjbphotos
Moderator
Interestingly, it does NOT ring "hollow" from the opposite side. So I guess there's insulation on one side only, and obviously not enough. It's also interesting that the hollow sounding side is plastered, with presumably the same plaster as the solid sounding wall next to it, and the total thickness of the wall is at least 25 cm. No idea why exactly it's constructed in the (unknown) way it is. Can only make assumptions based on that since it's an outer wall, it's probably required to be fireproof.
How would an outer wall have plaster (sheetrock) on it? If it doesn't 'sound hollow' from the other side, chances are there is a layer of wood there too, not insulation on that side of it (only). If the wall is that thick, it's either 2"x8" construction,or a 'double wall.