What roosting hens fill the air with overnight
A shut coop is not empty air. Every hen on the roost breathes out warm, moisture-laden air all night, and the droppings accumulating beneath the perches release moisture and ammonia steadily into the same small space. Because that air is warm, it rises — gathering at the roof, exactly above the birds. Left with nowhere to go, it condenses on cold surfaces, drips back onto bedding, and keeps the whole house damp; the ammonia fraction hangs around the hens' heads and irritates the airways of birds who will spend a third of their lives breathing it. This is why a coop can feel stuffy and stale on a still morning even in the middle of winter, and why 'sealing the hens in warm' so often produces the sniffles and listlessness the sealing was meant to prevent.
Why the openings belong above the birds, not beside them
Ventilation and draught are not the same thing, and roost height is where they part company. Openings placed high — in the gable ends, along the ridge, or as a gap under the eaves — let the rising warm, wet air leave by the route it is already taking, and let cooler replacement air drift in gently above the hens' heads, diluting the space without blowing across them. Openings at roost level do the opposite job: they let wind rake straight through the birds' feathers as they sleep, parting the plumage they rely on for insulation, while the damp air up top stays largely undisturbed. A hen tolerates very cold air that is still and dry far better than milder air that moves — the goal is a coop that breathes overhead while the roost itself sits in calm air.
The frostbite connection
The high vents earn their keep most counter-intuitively in a freeze. Frostbite on combs and wattles is driven less by how cold the night is than by how damp the air is: moisture settling on exposed skin in still, saturated air freezes it far faster than dry cold does, and keepers who shut every vent 'to keep the hens warm' often open the coop to blackened comb tips. A coop vented above the roosts stays dry inside, and dry is what protects the birds — hens in dry, draught-free housing with full crops handle hard frosts that would damage birds in a damp, sealed box. If your coop windows mist up inside overnight, that is the moisture that should have left through the high vents, condensing in front of you.
Checking your own coop
Stand inside the shut coop at roost height on a still morning and use your nose: a healthy coop smells of bedding and birds, faintly; if the air catches your throat or stings your eyes, ammonia is building and the hens are breathing it all night — add high ventilation before you change anything else. Aim for openings at the highest points of both ends where you can, so air has an in and an out, and cover every one with strong welded mesh, because a vent is also a door to anything hunting at night — the gap must pass air and nothing with teeth. Then resist closing them in winter. If a helper or family member does the evening shut-in, the instruction 'vents always stay open' needs to be written where they will see it. KinsLe's Family Sharing keeps those coop details — which vents stay open, which door gets the extra fastening — on the flock's shared information, so the hens get the same careful shut-in whoever does the round.