As with heat, I suspect it depends on humidity.I've worked in minus zero conditions, and to be truthful you don't really notice the difference between a minus 5 or a minus 15 - AS LONG -as the wind is calm and it is a dry cold.
I have worked in Greenland and Finland in just a T-shirt under my overalls, but in Germany I needed a good lot of layers on for roughly the same temperatures.
Mind you, there's some great thermal gear about nowadays.