I guess it depends on the types of chickens that you have. If the chickens are hardy then no. i live in northern Ohio and the chickens that we have do good in this weather. I have read that if you heat the coop and keep you chickens locked up all the time they don't feather like they need toin the winter, and that can cause lots of problems. I only use a heat lamp with the new peeps. other then that a nice cozy coop with ventalation. Good Luck
gibsonbuzz~ We live in MI. We have an insulated coop with 4 windows that I can open/close and ventilation at the top. We do have 28 chickens too, so that helps keep things warm. I did leave a heat lamp on during the day when the chickens were able to go outside. They seemed to do well...no frostbite.
I'm betting that a coop inside the garage won't need heat. There will be two walls (garage, coop) to separate it from the cold. Just for reference, my insulated garage tends to hold around 20F in the winter. But you might want to think about how it's going to get daylight. And consider dust control cuz there's going to be ALOT of it. Also, my chickens won't go outside when there's snow (which is about 5 months of the year) even with a covered run.
I live in northeren Maine my coop is insulated but no heat,I have golden comets black sex links EE`S and a penciled hamburg rooster.The only thing I had a problem with was the ventilation so I put bigger vents in last week and so far the hummity is good but its going down into single numberd this week so I `ll see how this works for me.
I did hang a heat lamp and turned it on when it got below 20 F
I told the Warden (wife) that it was to keep their drinking water from freezing, but it was more for my peace of mind than the girls comfort. They seam oblivious to the cold.