[Fedora]

They were OK once they closed the door! The freeze apparently didn't notice the great big windows all around the room, or the chimney leading down to the fireplace...

The windows and chimney had been coated with anti-freeze. (Trying to earn a No-Prize here :)

[Bear_in_RoseBear]

You know, actually, I don't think that chimney opened to the outside: when the father figure and his sidekick arrive near the end of the movie, they see the library nearly covered in snow and ice, still and lifeless; they make their way inside and manage to find the room with all the survivors in it, fire burning merrily in the fireplace. Smokeless fire, nice trick!