I do not plug in a heat lamp until it's under 15 degrees, but then I do. One important thing I think is to cover the bulb part of the lamp with protective wire covering. My chickens fly off their roost to the floor (they don't hop down) so a chicken could easily hit the heat lamp. Feathers are EXTREMELY flammable. I know people keep them in the cold, but as one poster said the dryness is the biggest factor. I was at the feed store the other day when we were having daytime temps with highs of about 18. The clerk told me that one customer came in and said they had to put a chicken down because it was out in the run in that weather and got severe frostbite on its feet. Chickens aren't the brightest bulbs, and won't stay in when frostbite is a possibility. I keep my chickens in the coop during the day when it's that cold. They complain, but too bad. LOL