I'm not sure if I'm the best one to answer this but, I'll give u a few tips to give your rooms some atmosphere. This image is actually from my first map and the first room i made.
1. If you have a window in your map. You should almost always use spot light to create some shadows. Don't overdo it though, just a subtle shadow can be enough.
2. Dust... Not necessary but can look nice with billboards. I used ps_light_dust.ps ParticleSystem
3. I have 3 point lights in the picture. One in front of each window and one at the middle of the room. Play with the color and range to gain the best looking effect.
4!!?!. just place a LOT of stuff in the room. Theres all kinds of stuff in the entities list.