I have 8 silkies, 5 sussex and 1 banni. They live in my 8x10 inclosed shed and come out during the day into there fenced yard. The shed has no windows but it does have vents at the top. I have layered the floor with straw and they seem to like it but I'm afraid that they are cold and when it gets colder they might freeze. How do I keep them warm? Can I use a heater in the barn with them? Please help.