Find the area of interior and exterior wall surface

[dexgear]

Hello everyone, I’m trying to find the total surface area of the interior and exterior walls including structural columns in order to find the area of wall paint.
I use solid.ByUnion to combine walls and columns into one geometry and then change it into surfaces. Then I use Surface.FilterByOrientation to get the vertical surface. Now, I don’t know the way to separate the interior surface and the exterior surface. Please help me.

[dexgear]

I try to use Room to intersect. The exterior faces are OK but the interior faces still have problems. I don’t want the small surface around windows and doors. Any ideas? or Any solution without using Room.