Welcome to BYC. Do you have any pictures of the inside of your coop? My chickens have always survived with temperatures near zero or below for the short time in winter that it gets that low here. They need some ventilation at the top of the coop to release humid air from breathing, but they need to have no direct wind blowing on them during such low temps. Do they roost? Are they wide roosts? Make sure they have plenty of dry bedding, and they need to have thawed water all day long. You can use electric heated dog water bowls or bring water out to them a couple of times a day. If you have a basement or warm garage, you can move them temporarily but make sure the temps are not too warm for them.