I was wondering if and when we should use a heat lamp for our 8 girls. It has not been much below freezing yet, but at what point will they suffer from the cold? Their coop is draft free, although it is open near the top for ventilation. Our youngest is 5 months, so they are all a pretty good size. Not sure of all the breeds (most were adopted locally) but all are standard sized. As you can probably tell this is our first winter with chickens as family members, and I would hate for them to be too cold!!! Thanks.