Hi there, nice to meet you! What kinds of birds do you have?
It's important to have good ventilation to let excess moisture from droppings and breathing to escape. Moisture in the air that freezes can cause frostbite, and uncirculated moist air in a closed coop can cause respiratory issues.
I don't let my birds sleep in the nest box, it makes for dirty eggs and crusted poopy bottoms. If they're stubborn about it, block the nests before dusk so they have no choice but to sleep elsewhere. You may have to place them on the roost for a few nights to teach them. After dark you can unblock the nests so they're available first thing in the morning.
Here's a few links you may find helpful:
Chicken Coop Ventilation - Go Out There And Cut More Holes In Your Coop!
Wintertime Prep Checklist
The Perch Experiment
Enjoy your yummy fresh eggs!
