It’s actually been warming back up again, so I think it’s not as bad as when I first posted.I'm glad to hear they survived the bitter cold temps we had. Heat lamp and tarping the coop was a great idea! Your coop is almost identical to mine and I have 4 chicks at almost 12 weeks old. Once they get bigger I think my coop will be a little too small. We are planning to extend the run another 2 ft x 8 ft on an "L" from the existing run over the Christmas holiday so they won't be so cramped during the day when they are confined.
The coop will be too small; I plan to add a second set of two nesting boxes on the one side when they start lying. I have a fenced yard so once I am sure they are fully aware of the coop as home and safe spot and a little bigger so my dog doesn’t lunch them they will have free run of the whole yard in the day.