Light leaking through corners of room when using real-time (Dynamic) lighting in Unity 5

Hey everyone,

I'm having an issue where light from the Directional Light is shining through the corners of the room. Please see images below. !

I imported the mesh from 3DS Max as an FBX. The walls, ceiling and floor are all one mesh, and are singular faces with their normals facing into the room. I've set cast shadows to "Two Sided". I've tried importing a simple box, and found I get the same issue. I also made a two sided shader, but this didn't fix the problem.

One fix that I did find was setting the the Normal Bias, in the Directional Light, to 0. The problem with that is this asset will most likely be used by other people in different projects. Because of that, I really need to solve the base cause of the light artifacting so that this asset can go on and be used in future projects without issue.

Any suggestions or advise would be great and much appreciated.

Thank you,

Wayne