I have to agree with Jack on this one, no heat needed!! 
I live in SE Idaho so our temps get just as cold if not colder than yours and I don't use heat in my coop. This year we added on to our coop and used the Woods style on it. My first year of keeping chickens I used a heat lamp and it was always damp inside my coop. That is worse for your chickens than the cold. If you always keep your chickens above freezing and you happen to lose power for an extended period of time (God forbid), that would be detrimental to your chickens because you have not let them acclimate to the cold. Provide lots of ventilation without drafts and you and your chickens will be just fine.

I live in SE Idaho so our temps get just as cold if not colder than yours and I don't use heat in my coop. This year we added on to our coop and used the Woods style on it. My first year of keeping chickens I used a heat lamp and it was always damp inside my coop. That is worse for your chickens than the cold. If you always keep your chickens above freezing and you happen to lose power for an extended period of time (God forbid), that would be detrimental to your chickens because you have not let them acclimate to the cold. Provide lots of ventilation without drafts and you and your chickens will be just fine.