Like the previous poster stated, unless you have a particularly delicate breed of chicken, most breeds are cold hardy. Their downy layer of feathers are very insulating and the scales on their legs, and the combs and wattles slowly acclimate to weather changes and are very tough.
Chickens are like most other birds you see in the wild~who don't have heated nor insulated little houses~and are designed to withstand cold and hot temps. We humans are the only problem...when we are cold we imagine the animals are cold. They generally are not~God gave them fur and feathers for a good reason~they are designed to live outdoors.
As long as they have shelter from the wind and rain/snow, they usually do just fine. Heating and insulating their building, no matter how many vents, can lead to too much humidity, mold and too much moisture in the bedding.
Really, its best if you use no insulation at all, nice deep bedding and some good open spots in the coop for fresh air flow. If it feels warm to you in your coop, then it is too warm for chickens.