I use the quality slider at "medium" position to get a basic light into interiors. It add a diffuse light form the environment. If you careful use a matching interior skybox map you can fake a general interior light. Works quite well.
If the sun is down and you have no interior lighting, where would the light come from?
Personally I find getting the balance of interior lighting really hard without either blowing out the detail in the bright areas or hiding the detail in the shadows. I think it's easier to turn off the auto contrast and auto exposure until you have the lighting in roughly the right place, then turn them on again... but I can spend 50% of my time trying to light a scene properly (not just for a single render; for a walk-through).
.... If you ...use a matching interior skybox map you can fake a general interior light...
hi Micha, what is an "interior skybox map" ?
what is an "interior skybox map" ?
If not mistaken I think he means this:
Right, I mean a 360° HDRI of an interior. But maybe it's not so important. The idea behind is that the 3D floor gets the lighting from the environment upper part, the 3D ceiling get the lighting from the environment ground ... . So, you can choose or adjust the mood of the lighting. Additional I would set some lights to enhance areas of interest.
Still puzzled. If I set an HDRI it is outside the model. So how would this help lighting inside an area without windows (like for example inside a production facility)?
I would like to show a big model without having to set up artificial lights all over the place. I know it is not as realistic but an option for an even overal light would be very helpful.
Best you give them a try. 
You will see, the light come through the closed walls. It's a nice specialty of the "medium" quality. In "draft" mode you gelt more light and less shadows, more unrealistic, but maybe useful too some times.
Keep in mind that Iris VR doesn't use any global illumination for their lighting. It's intent is more to experience project scale vs realism.
For overhead HDRI light maybe these would be good, I use them for product lightning for stills rendering.
I did some tests with different HDRI and found, the influence of the choosen HDRI is quite low. So, only the general color mood counts.
Here an example - no lights and emitters, a big black cube around the scene. The skybox brightness is at 57000 Lux. I enabled the sun and edit the direction/ brightness for a more warm mood (no direct sun is visible, but it affect the scene). It's anything not so logical, but finally a basic setup without lights is possible. It's a pity that the functionality is dependent from the quality slider, maybe the low quality bring some unwanted side effects. But if it work ... 
Is there an option for showing closed interiors (without windows) without the need for adding artificial lights?
Just like in Sketchup. E.g. the competing product IrisVR does not need artifical lights to show the inside of buildings.
Maybe I am missing something.
Thanks a lot for your request. Would the current workaround Micha suggested already be sufficient for you, otherwise I can also file a dedicated corresponding feature request as well.
