Global lighting appears "grainy" in an interior scene using Ray Trace rendering.
The Solar light is lighting the entire model area. The resolution of the solar light is very coarse in this case. This will generally pose a problem when rendering an interior scene where the solar light is coming through openings and Sky lightingis used. This is even more pronounced when the sky shadows are jittered. This is because much less light is coming through the openings versus the light that is hitting the outside of the model.
Add Sky Openings to the openings that the solar light is passing through. This will cause the light to be much more detailed.