Why isn’t the whole coop insulated?
To insulate the whole coop would create functional problems for the smaller doors. The warmer roosting area and daytime direct sunlight is sufficient refuge from the cold. If a hen needs to be isolated in the brooder during extreme cold weather, the brooder is low enough to be covered with two large comforters at night. During the day, comforters can be lifted to expose the south vent and let sunlight in. The brooder vent can be covered with sheet plastic or a couple of layers of bubble-wrap if you like.
