You're right Chaos. But when it hovers around 8F for a while I'm getting a few frozen solid with a longitudinal crack.
I should be able to check nests every couple hours but I haven't given it due diligence.
ETA
In fact, it is best that eggs cool down quickly to a point after they are laid - just not below freezing.
Ideally, for hatching eggs, there should only be 2 temperature changes. A cool down when laid and then slowly brought back up to incubation temp on set day.