It sounds like your girls aren't roosting if they only have 2 1/3-3 ft of height, so if the walls are draft free and they have bedding on the floor to keep their feet warm you may not need any additional heat during the short freezing temps. Their body heat may be enough to keep them warm but it depends on the overall square footage of the coop.
My neighbor has a large coop and run, without additional heat, and her chickens were happily outside scratching around in the snow and ice we had a couple years ago (Portland).

The link below is my favorite place to discuss chickens, ducks, etc. I used to have quail and the members there are happy to share their knowledge!

Here is a link that might be useful: Backyard chickens

Thanks annz! Yes, we have bedding and draft-free walls! They do roost about 8 inches up. The hen house itself is about 5x4, pleanty for four birds!

Our winters can go way below freezing, and I have no heat in our coop, but mine is a large (about twenty by forty) poultry building. They will need some sort of ventilation in any but the most horrifically cold days you won't get there. With adequate bedding, they should be fine.