Here are some test about interior rendering. The first one is done with Final Gather set on "Medium" with 5 Diffuse Bounces. The is only one Sky portal mapped with a Kelvin Temprature Color with Intensity set to 3.
The second one is rendered with Irradiance Particles you can use with this free plugin.
There is 2 SkyPortal lights mapped with Kelvin Temprature Color and 2 photometric lights.