Chicken Coop Ventilation Without Drafts

Chickens breathe out a lot of moisture, and their droppings release ammonia. A coop that holds that air in gets damp, smells, and is hard on the birds' lungs. But a coop with a cold wind blowing across the roost is no better. The goal is air exchange above the birds, not air movement across them. This guide covers how much opening area to plan, where to put it, and how winter and summer differ.

Ventilation versus draft

Ventilation is replacing stale, moist air with fresh air. A draft is moving air at bird level, especially across sleeping birds. You want plenty of the first and none of the second. The difference is mostly about height: openings near the roof ventilate; openings at roost height make drafts.

Utah State University Extension describes the winter pattern: bring cold air in near the roof so it warms as it falls, and let only enough in for adequate exchange. The University of Kentucky's ventilation article on extension.org says the same: in winter the fresh air brought in should be just sufficient for adequate air exchange, entering high and warming as it drops, while moist air has a way out.

How much opening area

Most extension pages describe ventilation in words. The University of Kentucky's engineering guide for suburban and urban poultry housing (AEN-161) is the one we found that gives numbers:

These are the figures the CoopSizer calculator uses. The minimum inlet is a small number on purpose: it is the winter minimum, the least air you should let in when it is cold. The 18 percent window area is for warm weather, when you want as much natural airflow as you can get.

Worked example: a 4 x 8 ft coop

In practice: a screened strip under the eaves on both long sides, closable in part, plus one or two windows on the south side that open wide in summer and close in winter. Put the winter openings higher than the roost bars.

Worked example: a 6 x 8 ft coop

Where to put the openings

• High, above the roosts. Winter inlets near the roof, so incoming cold air warms before it reaches the birds (Utah State, Kentucky).
• Upper vents on the leeward side. Utah State suggests placing upper vents on the side away from the wind.
• Away from prevailing winds. Virginia Cooperative Extension says to put openings on the side away from prevailing winds, usually south or east.
• A ridge or eave gap. Warm air rises and leaves through a ridge or eave opening. The Kentucky ventilation article calls this the chimney effect: air keeps flowing as long as outside air is cooler than air at bird level.
• Screened with hardware cloth. Every opening is also a way in for predators. Oregon State says openings should be too small for predators; cover them with 1/2 inch hardware cloth (see predator proofing).

Winter versus summer

Utah State puts it plainly: ventilation needs in summer are different than in winter. It also gives the reason winter is tricky: for every 18 degree F rise in air temperature, the air's water-holding capacity doubles. Warm, moist coop air that meets a cold wall condenses. That is the damp you see on windows on a cold morning.

In winter, close the big summer windows, keep the high inlets and the ridge gap open, and watch for condensation. The University of Kentucky's frostbite article says to increase ventilation if you see condensation on windows or uninsulated walls in the morning. Moist air plus cold is what causes frostbite. See cold climate coop design.

In summer, open everything you can. The University of Arizona's extreme heat guide on extension.org suggests cracking open coop windows and installing roof wind turbine vents, and making sure shaded areas are ventilated. Oregon State adds that fans should not blow directly on the birds.

Signs your ventilation is not enough

• An ammonia smell, strong enough to sting your eyes (University of Maine).
• Dampness or ammonia build-up (Virginia Cooperative Extension).
• Condensation on windows or walls in the morning (University of Kentucky).
• Wet, caked bedding, especially under the roosts.

If you notice any of these, add or open high vents before you do anything else. Do not close up the coop to keep it warm; that traps the moisture.

Do small coops need fans?

Usually not. Utah State says small houses can usually be ventilated without fans, using windows and vents. Oregon State says passive ventilation is adequate for most backyard coops. Virginia notes that windows or vents on one side usually give enough ventilation for small coops.

Quick checklist

1. Winter inlet: floor area x 6 / 25 square inches, minimum, high on the wall.
2. Ridge or eave gap: 3 in per 10 ft of width.
3. Summer openings: about 18% of the floor, mostly on the south side, closable.
4. No openings at roost height on the windward side.
5. Every opening covered with 1/2 in hardware cloth.
6. Check for smell and morning condensation every week in winter.

The calculator works out all three areas for your coop size and switches the emphasis for cold or hot climates.