Here in Wisconsin, we routinely stay in the below zero to teens area for temperature throughout January and February. We also will dip into the -20's with -40 wind chills, at least once during winter and often multiple times. We never provide heat, to the contrary we never close our shed up and keep a west facing door open year round.
Chickens are extremely cold hardy, I cringe when folks talk about heating them, they can heat themselves quite well. Adding supplemental heat can actually cause more harm as it can mess up their ability to acclimate, and cause big troubles if that heat is ever shut off.
I also read how some breeds are more cold hardy than others, which in my experiences hasn't been the case. I've had lots of different breeds, heavy breeds, light breeds, crested, silkies, frizzles, and d'uccle. I've never had a chicken not survive winter, they all do fine, I haven't noticed a difference in how a breed does during winter. The exception is roosters with large combs and wattles will often get frostbite and lose their comb points, but they get over it without intervention and don't get frostbite the following year.
Provide fresh clean, unfrozen water, block prevailing winds, provide proper roosts, put down some hay or straw to stand on outside, warm mash on really cold days, and south facing windows for sunshine and your birds will do fine.