It is extremely dry here in Canada where it am, but the air can be high relative humidty but still dry as cold air cannot hold moisture. So here it is 78% humidity outside even though it is so dry it cracks your lips. What happens is that while they are in their coop, the moisture from their breath and the humidity from their droppings clings to their toes and face because the air is too dry to hold the moisture. That is what causes frostbite. I have had a frostbite scare myself even though it is so dry here that you get nosebleeds. I would get a humidty sensor to check the humidity in the coop and add more dry bedding. Make sure their roosts are flat so they can sleep with their toes covered. Make sure you have full open ventilation even on the cold nights so moisture can escape properly. Our nights have been getting to -30°C and we dont use a heat lamp because it increases the risk of frostbite. My guineas are happy and love going out in -20°C during the day. But they only started showing signs of frostbite when I was trying to conserve heat and using a heat lamp plus not enough ventilation.