As long as they are out of the wind and draft-free, they should be able to withstand temperatures down to 0F atleast without any problems. Chickens are remarkably hardy. My bantams do just as well in the cold as my large fowl chickens.
My silkies are doing great in our weather. When it hit 0F we put clear plastic shower curtains around their runs. (yes, there is ventilation at the top) They have the choice to be in or out, and they choose outside as soon as their doors open. They are so puffy right now that you can barely see their beaks. They've been out there in -20F so far, no problems.
My silkies have a light bulb that comes on (thermocube) when it goes below 35. We have very wet winters with freezing fog- it is impossible to keep the inside of the coops dry. You can get frostbite in those conditions at higher temps.
