Apply Shell modifier, few minutes ago i have make same effect by mistake by forgetting to paste shell when i was editing UV, that is due to normal, direction and position(i will take a wild shot here and say) that this parameters correspond to normal, color and distance in vray... Anyhow in order not to kill render time as your scene is pretty much simple and doesn't require those settings to be changed in order to "pop up" tiny details you will need to give some thickness to your walls + you will lower your calculation for a fraction as it will be easier for engine to work properly.
