I've had chickens 4 1/2 years and never added heat during winter with temps down to -15 Fahrenheit, -8 F inside coop.
The coop I built is 18 square feet and had 5 Golden Comets in my first Flock.
Currently I have 5 ISA Browns occupying it.
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Ventilation is key to happy dry chickens.
Inside look at vents.
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Outside look.
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Closed windows allow ventilation below vents by positioning hinges as shown.
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I also did not fill groves between siding and window frames.
Roosts are located opposite side away from windows and vents.
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I keep feed and water on heated base during winter in both coops 24/7.
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I added vents to my bought coop (22.5 square feet) above windows.
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The coop came with a 3" vent on both sides towards the front.
I always leave this window open, except during a windy Noreaster, (windows face east).
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Roosts are opposite windows.
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Notice narrow roosts. Never had frost bitten toes through 2 winters with Barred Rocks with lows down to -7 F, -1 F inside coop with up to 7 Barred Rocks.
A recent photo of 3 occupants.
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Of course I have winter hearty full size hens. I've read they can survive temps down to -20 F, -28 C with proper draft free ventilation.
Barred Rocks are duel purpose with smaller combs and wattles.
My Golden Comets and ISA Browns are hybrids with large combs and wattles.
A photo of the last Golden Comet taken last July at 4+ years old. No frostbite on this comb.
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I lost all GCs to egg laying problems. I lost 4 Barred Rocks, one to prolapse, one culled for bad behavior and two to a predator while free ranging.
None were lost to cold weather. GC