One-sided geometry lighting issue

I’m creating a walk-in house, where you start outside and make your way in. All the rooms are their own object, including the “Outside Wall”. They are all one-sided with at least a 25cm gap between anything that isn’t the connecting doorways.

Here are some picture for perspective:


^^^ Current room placement is just showcase for how it will look in unreal, but I understand how to export properly. Anyway the UV is the lightmap on configured for channel 2 on the room highlighted.

What I want to make clear is that everything is 1 sided. The outer walls represent the exterior of the house, but have the window borders. Everything has a specific vertex snap point so it all seems seamless when playing in unreal, and it does. But the problem occurring has to do with the lighting:


^^^ This is one of the rooms with the outerwalls attached. Puzzled, I tried to see if it happened with all objects:


^^^ These are the rooms by their selves, rotated with the windows facing the sun so the light hits a flat surface.

From right to left: Baked lighting, Direct Lighting, and a rotated room with the doorway facing the sun. Something weird about it right? It’s almost as if it’s perfect!

I searched far and wide for a solution. I tried:

  • 128+ lightmap AND mesh resolutions
  • changed the world settings lightmass configuration (Indirect Lighting Quality 1.0, Smoothing 0.75…etc)
  • Double Sided mesh
  • Two Sided lighting
  • UV just that polygroup alone, let the sun shine through to a different mesh surface, same result

I tried everything I could find. Does UE4 not like one-sided geometry?

EDIT: Found something interesting that I didn’t really pay attention to. This is the outside of the room, with the 2 sided material option (I turned off everything else in sequence to see what triggers it). What could it mean?