The basics of chicken coop ventilation
How airflow works inside a coop, why moisture is the real enemy, and the cooperative-extension numbers every flock-keeper should know.
Most backyard coops fail ventilation for the same reason: the keeper sealed everything against winter cold and never opened it back up.
Cold is rarely what hurts a flock. Trapped moisture is. Birds exhale water vapor every breath. Droppings release ammonia as they break down. In a sealed coop, both sit at roost height and cause the frostbite, respiratory illness, and pasty combs that owners blame on the weather.
Two numbers from the University of Kentucky Cooperative Extension Service do most of the work. In summer, target 5 CFM (cubic feet per minute, the unit for airflow) per adult standard hen. In winter, drop to about 1 CFM per bird. The summer number is for cooling. The winter number is for moisture removal, not heat exchange.
Vent area itself is sized off the floor, not the flock alone. Plan 144 square inches of total vent area for every 10 square feet of coop floor. Split that roughly 50/50 between low inlets near the floor and high outlets near the ridge. Air enters low, warms and absorbs moisture, leaves high. A coop with vents only on one side does not move air, it traps it.
Use the calculator on the homepage to convert your specific flock and floor area into a vent plan with inlet and outlet square inches. Then read the seasonal guides for the adjustments your climate needs.