I have an 8 month old pullet Sultan and she is kept in an insulated coop with 3 Cochins and a Sebright. I know the Sultan isn't as cold hardy as the rest and I am wondering at what temp should I start putting on the heat lamp. She has already stopped laying eggs...about a month ago. There is plenty of ventilation in the rafters and the windows are closed at night. I also close the pop door at 6:30pm or sooner. I just don't want her to be cold.