Hi Kulkul,
Thanks a lot. It’s really exciting to learn how to achieve the same approach in different ways with dynamo 
I’ve taken your script to a step further for trying to get the internal faces of the walls with some openings. However I didn’t work as it was supposed.
As you can see the custom node “Surface.FilterByOrientation” works great. It filtered out all the vertical faces both external and internal faces of the walls and the side jamb faces as well. The challenge is coming afterwards to filter interior wall faces out propertly by using “List.FilterByBoolMask” node. As you can see the filtering is working a little bit weird.
Therefore I tried to combine your workflow with Vikram one. As you can see in this scenario with opening is working a little bit better than before. However, I didn’t manage to get rid off the side jambs in the opening.
Best regards,
Alejandro