Hello, thanks a lot, the "single sided" wall sounds like it's exactly what I need.

I made an object from 2 cubes with single sided faces.
The outer cube's normals point inside, the inner cube's normals outside.

In preview, I see exactly what I expect, but in render mode, all faces become
double sided and I cann see the outside only. "Normals forward" in the materials setting is unchecked.

Best regards,
   Michael