I have open soffits on my coops. We get down to minus 16(ish) a few times a year with many nights at or below zero. We can run for weeks never getting above 20°.
In addition to open soffits I included as many windows as possible on my coops. I often open them in winter if we are above 30°. Right now we are at 22° and there are windows open.
So you can see my coops.
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This one is 8'x14' and has 11 windows as well as open soffits. I have kept up to 24 chicken in this one.
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This one is 6'x8' and has 3 windows as well as open soffits. Currently housing 8 chicks 9&10 weeks old.
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This one is for just 3 bantams. The soffits are not open BUT you can see through the back vent. It is 1' by 4' and on the front right at the top is a small additional vent. The small rectangle under the big vent is another permanent vent.
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Taken during construction. I keep 7 ducks in this one.
This is my duck house. It is 5'x8'. While the soffits are not open the bumps in the roof are. It also has 5 windows and 3 additional vents measuring 2 at 6" by 12" and one at 6" by 3'. Right now three windows are open on this coop.