The ventilation requirements I provided are for roosting at night.
Straight run is NEVER 50/50. It's how the hatcheries get rid of a lot of their unpurchased males. I suspect it's much closer to 90/10.
Leave all high ventilation open and fully secured with 1/2" hardware cloth year round.
If there is ventilation directly across from roosting space that may allow strong enough drafts to open feathers in below 40 temperatures, you need to be able to close the ventilation or baffle it to direct it...