Help with Directional Light

My directional light ignores my walls and roof, making the room look outdoor lit. I'm guessing since the mesh normals are pointing opposite of the directional light, shadows won't work. Is there a way I can tell Unity/Directional lights NOT to ignore particular meshes like walls and roofs?

I tried changing the Mesh Renderer to Cast Shadows: Two Sided, and that seemed to work, but would that be the correct or most optimal way?

Cast Shadows: On



Cast Shadows: Two Sided