Welcome to BYC!
You should rarely need to add heat to your coop unless you have very young or very old or sick birds. Chickens can adapt to some brutally cold temps if given the opportunity. Make sure to have good venting in your coop as chickens poop and breathe all night long, creating a lot of moisture. This damp heat has to go somewhere and if you don't have venting in your roof or ceiling, this damp wet air is going to rise and fall back down on your birds as frost, giving them frost bite and being very cold.
So keep your roost bar low to the floor. Have them roost in relatively quiet air. All the moisture from the pooping and the breathing will rise up to meet this positive air flow of venting on both sides of your coop at the eaves and go right up and out, keeping the birds area with a tiny bubble of warm air and dry.
Now, I do have a cut off point for adding a small heat lamp. If it is going to be 30 degrees colder than the normal over night low, then I will add heat. So if your average over night low is zero and it is going to get down to -30 or more, then I would add a small heat lamp. Not enough to heat the coop, but enough to add some heat to their roost bar. You want them to be able to go outside on the coldest of days and not stuck in the coop because of the heated coop.
They say 1 square foot of ventilation per bird in the eaves. If it is going to be a really cold windy night, you can block off some of these vents to slow the air movement down. But you don't want to close it up tight. You are not trying to heat your coop. Just keep the birds dry, roosting in slow air and the moist air out of the coop.
Kelsie has left you with the Where Am I Where Are You thread so you can chat with other chicken owners in your territory to see how they do things.
Welcome to our flock!
and Welcome To BYC! A great place to start is the BYC Learning Center, lots of nice articles on all aspects of poultry keeping you'll find helpful 
