the cope can not be air tight, just like a persons house, but, maybe it is becuase your coop is to drafty. some coops I see are very small, more like a box with an open door or front, that to me is not enought protection from the cold if you live in an area with winter weather. Also, put lots of pine shavings/hay on the floor, just like we use a rug on the floor to keep our feet warm in the winter. I give my girls cracked corn in the winter to help them produce body heat to stay warm also.