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Most important things to keep in mind when designing a coop for winter are 1. Space as they will be spending more time inside than usual and that can make birds more testy with each other (you want 4sqft of coop space per bird) and 2. Plenty of draft free ventilation. You want at least a square foot of ventilation per bird but more is better especially if it gets very hot. If you don't have enough ventilation moisture can build up in the coop and cause frostbite. It is not the cold itself that's a problem in winter, it's the moisture. Unfortunately omlet coops aren't the best coops in this regard as they are small with poor ventilation. They work best in very mild climates for very small flocks that get along well in close confinement but even then they aren't great