As long as they have shelter from any direct winds or wetness they can handle pretty low temperatures, we can sometimes get down to -20's with -40 windchills, my chickens are in an uninsulated shed, they do shiver on those days but I get them some warm water and food and I get them scratching around and they are fine, I have never had a chicken freeze to death.
Depends on the breed. Normally feathered, standard sized, or small-combed fowl will handle cold better than Silkied/frizzled, bantamized, or large-combed fowl.
For the average DP breed hen, down to about 15-20F comfortably without a heater.
My standard large fowl birds handle negative F's just fine....no heat is needed as long as dry and out of wind.
Even had a broody with chicks last winter....lowest was -12F.