Remember when you were small and your mother insisted you wear a sweater and you said why, you aren't cold, and your mother said, you wear the sweater because I'm cold? Well, you are the one dreading the nose dive in the thermometer, not your chickens.
In case you never noticed, chickens, by the time they've reached six weeks old, are sporting nifty down jackets that insulate splendidly from the cold. And I mean
cold! Chickens, if they could read thermometers and actually cared, would laugh at 20F.
The temperature really isn't what you need to be concerned about. Is the coop draft-free? Does it have ventilation so the vapor from the chickens respiration can escape? That's really the only danger - that frozen water vapor can't escape and it will instead settle on the chickens' combs and cause frostbite, especially in your state where I imagine humidity is usually present no matter the weather. (And I wouldn't recommend the addition of any water in the coop, warm or cold unless it's in a sealed jug to prevent evaporation.)
You want some cool air coming in from down low, the chicken pop door, and an outlet up higher for the warmer air to flow out of the coop. The chickens will fluff up, trapping warm air from their bodies, and they'll slumber peacefully in their coop. In the garage, they will be stressed because it's not "home".
The first time after I got chickens and it got really cold, I filled 2 1/2 gallon plastic jugs with hot tap water and placed them in the coop. I don't think it actually did much, but it made me feel better. I have one of those flat panel heaters that they make just for chicken coops. You might look into that if you have electricity to your coop.
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