If Sickening Radiance were cast over a house and covered the entire building, how much effect would it have on the people inside if every entrance were closed? I'd say it goes through windows, but would it be reasonable for it to go through the cracks in doors?
Depends how many cracks it has
A spell’s effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn’t included in the spell’s area. To block one of these imaginary lines, an obstruction must provide total cover.
Which Sickening Radiance modifies with:
The light spreads around corners, ...
So glass will block the effect but any cracks in windows, gaps under doors, chimneys, ventilation ducts etc. will let the light in.
Dim, greenish light spreads within a 30-foot radius sphere centered on a point you choose within range.
Light comes through windows.
It seems this is magical light and somehow it's also a bit weird because it spreads around corners.
Spreading round corners is also rather light-like in that reflected light is likely to do that.
Nothing is said about cracks but they're kind of "obviously" not corners.
The way I'd interpret this is to say the effect spreads like light would do through glass and small openings like cracks.
Coming down a chimney and back out seems like it'd complicate things so I'd say no to that and a ray propagates "round" one corner unless the opening is large.
I'd also probably give someone "struck" by a narrow band of light coming underneath a door less damage.