I didn't mention acclimatization. This is an important distinction when moving chicks or even adult chickens from one temperature zone to another. Being adequately feathered is a critical point, but so is being acclimated to the temperature change.
Chicks at six weeks are pretty much feathered...
When it's below freezing, it's not against the law to turn on a little heat.
That said, it's very easy for humans to mistake their comfort zone for their chickens'. Whereas we are most comfortable at 70-75F, chickens find that oppressive, even young 6 -7 week old chicks. They are decked out...