If your chicks are only 8 weeks old & brooder raised, then you're not spoiling them by helping them stay warm. We get those frigid below zero nights & although our coop is not heated, our flock had the opportunity to gradually get used to the outside temps naturally. We don't go from a 60'F day in Oct straight to a -10'F night in Jan. As the weather chills, the chickens build up a tolerance. In the winter, I don't bring mine inside because the sudden temp change could send a bird into shock. There are plenty of broody hens who decide to hatch & raise chicks in the snow. I find those chicks feather out very quickly, but they also have mama caring for them. Brooder chicks mature a little slower. You will have to take your cues from their behavior. Are they always under the lamp & heater when outside during the day? If so, then don't leave them outside at night. If they're spread out & don't seem bothered by the day temp, then your porch, heater, & cuddling with siblings should help keep them warm through the night.
You probably already know this, but moisture is what makes the cold dangerous. If water spilled in the coop or there is poor ventilation, the water vapor in the air crystalizes on combs & wattles giving frostbite. They need protection from gusty winds & drafts, but be very careful not to block air circulation.
