Why are they in a crate? I hope they have an actual coop and this is a temporary setup. They can't sleep in a crate long term. Also, foggy surfaces is a huge red flag, especially if you are expecting temperatures to drop below freezing. That's how you get frostbite - not from the cold itself, but from humid air that forms condensation on the chickens themselves, which then freezes on their fleshy bits. If there's condensation on the plastic, that means there's condensation on the chickens, and you don't want wet chickens in the cold. They need a proper coop with lots of ventilation to get rid of the moisture. They don't need heat.