I live in central Minnesota... at what temp should I turn my heater on for the chickens.. I have 12, in a coop that's 12x12. It is insulated. I did not plan on closing the chicken door all winter.. I have a spce heater that I put in a wooded box with wire on each side, it has a thermistat for temp control. Or would a ligth be enough to heat..it's a shop light, right now it has "plant" bulbs in, which does not throw heat.. any advice for this rooky chicken "farmer" would be appreciated