I think there is an easier way to do it in the settings tabs, maybe try leaving the environment tab on but switch it to a bitmap or something, but a cool trick I used to use was create a 2-sided material and use it as a ceiling. You can create a transparent material for the front and just use a default one for the back. That way sun accurately comes in the windows but not the ceiling. Google “vray sketchup 2 sided materials” and you will get some ideas of how to do it.