I have made a shack in the creation kit, using an already existing one as a template (Ysolde's I think). It's got big gaps in the roof planks so I would like the light from the outside to be simulated, because right now it just shows the void. I've been searcing for a while and I haven't found the answer how to apply it.

I know it should work because there are a lot of interiors in Skyrim that does this.

Grateful for any help.

Go to World, then Cells and select your interior cell.
Then look in the 'Lighting' tab. This is where all the lighting is handled, other then hand placed lights.
It is easiest to select a vanilla 'lighting template' like 'FarmlightingTemplate'
Tick 'show sky' to show the sky.
Then select the weather for your region. If you are not sure which, check out the settings in the nearest vanilla interior.
I believe that if you select 'use sky lighting' it lights the interior of your shack itself, with daylight. But that generally is far to bright.

Thanks a lot for your help.