I'm unable to create a cube that encompasses the entire building, which is a rectangular cube. On two of the faces, I see the slight gray coloring of the cube, but on the back two faces, there is no such coloring.
 
When I extract textures, all I get are black sides for these two back faces. Why is that?

Hi,
one possibility is that the normal is directed toward the interior of the cube, instead of the exterior. To display it, click on the diaply normals icon. To invert the normal, select the face (you should be in front the normal to do so), then press the I button in the keyboard. The normal is inverted.
 
regards
Realviz support