How to exctract external faces (facades) from a "complex" mesh?


I Have this group of mesh, I need to extract only the external faces so I can work on the facades (for architectural geometries).
in the Rhino file, I have the “test” mesh and the mesh to which I want to apply this rule.
anyone can tell me how I can do this? (I already have filtered the vertical faces and built “random” geometries on it).
Facade Airspace.3dm (12.1 MB)
Facade (23.6 KB)