Ventilation is not draft, and the difference is what causes frostbite

The mechanism, because it explains everything else

A chicken is a small warm animal wearing a down jacket. Cold, on its own, is something it handles well — chickens sleep pressed together on a roost with their feet tucked into their feathers and their heads under a wing.

What they cannot handle is moisture. Each bird exhales water vapor all night. Droppings under the roost release more. In a sealed coop that vapor has nowhere to go, so the humidity climbs until it condenses on the coldest available surfaces — which, on a bird tucked into its own feathers, are the exposed comb and wattles. Wet tissue at below freezing is frostbite.

So the winter question is not “how do I keep the coop warm.” It is “how do I get the moisture out.”

Ventilation against draft

That is the entire distinction, and it is why “seal the coop” and “ventilate the coop” are not contradictory advice. Seal the low openings that blow across the birds. Keep the high openings that let damp air escape.

Where the openings go

• High, under the eaves. Warm damp air rises. Openings at the top of the walls or in the gable ends let it out at the point it collects.
• On more than one side. A single opening does not produce airflow; it produces a hole. Two openings on different walls give the air somewhere to come from and somewhere to go.
• Above the roost, never level with it. Set the roost bar low enough that the birds sit below the moving air.
• Covered with half-inch hardware cloth. Every vent is an opening in the predator envelope. See predator-proofing.
• Open in winter. This is the hard part psychologically and it is the whole point.

How much ventilation

We are not going to give you a square-inches-per-bird figure, because we could not find one published by an extension service and the numbers that circulate on forums are not sourced. What we can give you is the diagnostic, which is more useful anyway.

Go into the coop first thing on a cold morning, at roost height, and use your nose and your hands. If it smells of ammonia, there is not enough ventilation. If the underside of the roof is damp or frosted, there is not enough ventilation. If the air feels noticeably damper than outside, there is not enough ventilation. If it smells of nothing much and the surfaces are dry, you have got it right.

A hygrometer makes this quantitative if you want it to be — several of the thermometers we looked at read humidity, and a coop reading much higher than outdoors is telling you the same thing your nose is.

What about insulation?

Insulation and ventilation are not in tension, but insulation is much less useful than people expect. It slows heat loss from a space that is not being heated, which mostly means it slows the rate at which the coop reaches outdoor temperature. It does not stop it. And if insulation is installed in a way that closes off high openings, it makes the moisture problem worse while feeling like an improvement.

Insulation is also a place mites and rodents live if it is not sealed behind a surface. For most backyard coops, effort spent on ventilation and dry bedding returns more than effort spent on insulation.