It got down to -15F here last night and is still about -5 now. I've always worried about a heat source being both a fire hazard and a possible risk for my hens' immune systems. One of my dad's friends had a flock that they heated the coop for. They all ended up with respiritory problems and died, and I've heard stories like this before. Apparently it's no good for them to have such a difference in the coop temp and outdoor temps.
We do have a fully insulated coop. Since this more extreme cold I've been bringing warm foods like oatmeal and soup broth out to the hens to drink a few times a day. Their heated waterer is having trouble keeping up, so this is hydration for them as well.
They do look cold out there, but I don't see anyone shivering a lot or anything. It's supposed to warm up tomorrow so I think I'll just keep up with what I've been doing.