I build only what I need. I don't even built the roof, I simply imply its existence with a custom light gel and square spotlight. I fill in light in selected spots with large radial lights with low settings (usually under 5). It's better to use multiple radials to acheive a softer look. Now look out the windows. There wasn't much sky visible in the scene, so I actually used a sky image (created in Bryce beforehand . . Bryce is a paint program, you knew that right??) and applied it to an elongated cube just outside the windows. I then light the "backdrop". Voila! Sky without bothering with sun and atmosphere setting that would get in the way! Like I said, it's like a movie set, or stage play. Show only what you need to show. :)