It's -20 here this morning. One of mine, the one with the bear coat (double-coated Lab/Golden) went out to pee, feet got so cold in 1-2 minutes, she was trying to hold up all four, laid down in the snow, was floundering, and I didn't know if she was going to make it back into the house without me bundling up, going out and physically dragging "herself" inside.
Yep..she worked her way up to it with some coaching and got back inside, but even heavy coated, acclimated dogs (with so much fur between her toes you can barely find the pads), bear close monitoring in this stuff. The 24/7 dog door is officially locked down until we get weather at least above zero. The natives are restless, but safe.