Obviously chickens sleeping in trees have great ventilation. But they have another advantage. They have the freedom to move around and get out of direct strong winds. If given a chance they can take care of themselves pretty well.
People further north than I am have solved frostbite problems by providing more ventilation, not locking them up tighter.
Lancaster my advice for winterizing in Oregon is to not worry about keeping the area where the chickens are warm. They don’t need that. Keep strong breezes off of them (at least the option for them to move to shelter if they want to) and their down/feathers will keep them warm, just like the wild birds that overwinter there.
Plenty of ventilation is important.
Don’t baby them. No matter the weather give them the option to go outside if they wish. Mine normally stay in protected areas if a strong cold wind is blowing but even with temperatures below zero Fahrenheit mine go outside as long as the wind is calm. It was four degrees above zero Fahrenheit when I took this picture. Yes, that is ice from their waterer.
If there is snow on the ground they tend to avoid it for a day or two until they get used to it. With these they were already outside when the snow fell so they never bothered to come in.