don't think warm, think dry! The drier they are, the warmer they are. A lot of people think they should close up the coop because we have been told since childhood to seal it up tight to keep the heat in. What you keep in is the moisture and a damp chicken is a cold chicken.
It seems so counter intuitive, but the colder it is, the less moisture the air will hold, and it settles down on top of the chickens. You need ventilation above their heads, and they need enough room so that they do not roost close to the wall or ceiling, they need about a 12 -18 inch clearance. Being close to the wall or ceiling, agains causes moisture to build up on them.
So open up the ventilation on the south or east side. No need to add heat for most chickens. If you do add heat, make it a permanent electrical source that has been installed by an electrician. Many a hung up heat lamp falls down when you have animals moving all around. Extension cords easily can start fires especially dropped on bedding.
Mrs K