is exist way to take all external walls of building
an example
but im not shure that its method is alwayes correctly work…
Real building is more difficult of course 
properly set walls to interior or exterior in their function (type parameter), then sort all walls by that parameter.
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make a big wall outside the perimeter of the building and place a room called “outside” in between and see if the wall in question is roombounding for the room “outside”
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Like Marcel said if you create an external room around your building at the same height of your building you can find the walls that bound this.
This script is just a starting point I have only tested it on a single storey building it might need adapting for multiple stories.
finding external walls #5.dyn (19.6 KB)
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