When we know it is going to drop below zero, we've been closing the ventilation window. The coop is fairly large and quite dry. The chickens are always locked in the coop at night, with the pop doors accessing the run, closed down.
Imagine my surprise when I went out to check on them one -10 degree morning, and saw a lone chicken out on the roost in the run! DH had not checked the run when he shut them in. The Welsummer managed the -10 night just fine, without anyone else to provide body heat.
Originally, I thought I might need to provide some heat when it hits -25, but after that experience I think they will be fine if they can huddle together with the window closed. It looks like we'll find out how they do this weekend.