So true.... no way to know until it happens.
As to temps.... do you mean single digits F or C?
If you mean single digits F, that is cold, but not bad. How far north are you?
I definitely see an increase in feed intake when temps go under 10F. If you are far north, or tend towards many cloudy dark days, I would highly recommend putting in a light on a timer. 10 or 12 hours should be enough light for them to eat, but not so much that they are triggered to lay.
It needs to be light for them to eat, they need to eat to stay warm.
Back to ventilation.....
Oddly... I found that in my chicken shed, snow blew in through the open eaves, but did NOT come in through the vents lower down on the walls.
I didn't fully close my eaves, just stuffed crumpled up feed bags in most of them.
My main vents in my chicken shed are below perch level. However, the sleeping perch has a solid poop tray below it, and a solid wall behind and above, so it is out of drafts.
My chicken coop has a window and a vent that is open all year, above perch height.
My bantam coop has WAY more ventilation. The top one foot of the 4 foot side is fully open, and the man door in the front wall is usually open. There is also a back wall vent, 4 foot by maybe just 3 inches. Also that building was built with tiny scrap, so lots of air goes through.
However.... their perch has a solid back wall and a solid poop shelf below.
Those little guys do great.
A little snow blowing in is fine.
Snow drifts in the coop are better avoided.
For openings/vents that are suddenly letting in buckets of snow, I have stapled up scrap remay (that season extender white breathable garden cloth). Quick and easy to do, even in a blizzard, if you can find the remay and the stapler.