I've got frost buildup on my vents, and around the window and doors. It's -29F without the wind, but 30 chickens just put off too much moisture. There were a couple of mornings when I went out and opened the door and it was an instant fog-bank in there... left the door open for a little while to let some of the moisture out and unlatched the window (it has to be latched to be tight against the sill) and now it seems to be ok. Everything's frozen in there... solid. But it's warm enough for the girls... above 0F, probably about 15F.