Coldest I have run was at -15 Celcius but I won't do it again. The coldest I would do is -10 Celcius. Three layers is more than enough and the mid layer is a big variable that can be a synthetic top that is very thin up to a micro fleece. Last week I ran it was -5 Celcius and the run was beautiful in every meaning of the word. I find myself recently preferring cross country skiing when it is very cold.
