Light is coming through meshes!
How did you setup the room? Is it a cube with it's normals reversed, like this?
Then you should try "Shadow Two Sided". You'll find it in the Details Tab under Lighting.
To see the result, you need to build the lighting.
EDIT: Ignore my question, I failed to see you mentioned modular walls, still, "shadow two sided" could be what you are looking for.
I guess you're messes are thin and light is coming through them. Try to give them some thickness or make a shell outside so that it covers your structure and then put a lightmass surrounding your scene and then try. It should work
answered Aug 27 '17 at 03:11 PM
Follow this question
Once you sign in you will be able to subscribe for any updates here