Big Ol' PageVENTILATION
Go out there and cut more holes in your coop!
Now! Really truly!
Because chickens are amazing producers of moisture, ammonia and heat, that's why. Small but mighty! (Mighty messy anyhow).
1) Ventilation removes dampness and humidity from the coop. Chickens generate scary amounts of water vapor, partly through breathing out (same as we do, that's why a mirror fogs when you breathe on it), and largely through pooing (chickens do not urinate as such - all the water they would be peeing out if they were any other sort of animal is contained in their poo). They process a lot more water than you might think. All of this water tends to make the coop air humid. High relative humidity (especially in cooler temperatures) makes chickens more susceptible to respiratory disease and increases the chance of frostbite. Chickens can stand considerable cold without frostbite if the air is dry; not so much if the air is clammy.
2) Ventilation removes ammonia fumes from the coop. Unless you sit there all the time, ready to whisk each plop of poo away to the compost pile the moment it comes out of the chicken, there will be some ammonia being released into your coop's atmosphere. It does not take all that much ammonia to cause subclinical damage to the tissues of the chicken's respiratory tract, which makes the chicken more vulnerable to any respiratory 'bugs' that may be floatin' around the environment. Basically if your nose can smell ammonia, there is enough of it to be harmful to lung tissues.
3) Ventilation usually helps keep the coop from getting too hot in summer. Chickens' bodies perform best below about 75 degrees F; over 90 F they start to have real problems, suffer heat stress, and if it gets too hot they can die, especially larger-bodied and heavier-feathered breeds. Proper ventilation will at least keep your coop from getting any hotter than the outside air.
When do I need ventilation?
Always. Yes, even in cold weather. Yes, even in northern cold weather. Realio trulio. There may be a night now and then when it's so vastly cold you close things down, or if you're having a hurricane you may close the vent flaps and windows so that the weather stays outdoors, but those sorts of things will be rare exceptions, not the rule.
Types of ventilation
Passive (natural) ventilation means that you have openings that air flows through with no help from you or the power grid -- just the natural action of wind and the tendency of warm air to rise. Passive ventilation includes an open window, a ventilation slot, a louvered gable-end vent, that sort of thing.
Passive ventilation is the easiest, cheapest, safest, and most foolproof method for the vast majority of backyard coops, in my opinion. Build lots of it.
Wind turbine ventilation means those spinning turbine things, about the size of a basketball, that you mount on a building's roof. When the wind blows, it spins the blades and they suck air actively out of the coop. This can move a goodly amount of air, but only if the wind is blowing. When the wind stops, it becomes a smallish hole in the roof, period.
Active (mechanical) ventilation means using an electric fan, generally plug-in although small solar powered units do exist. This allows you to get greater air movement with smaller holes in your coop walls, but with several important drawbacks. You really ought to get a fan designed for dusty and outdoorsy environments (designed for barn or workshop use), which costs more - a house fan will very quickly clog with dust and stop working or die altogether. Even appropriate fans need to be cleaned regularly or their performance becomes poor and they can become a fire hazard. Also, if your power supply fails, so does your coop air quality (solar units usually run only when the sun is actually shining on them, so are no use at night).
Opening the door a couple times a day to walk in and out of the coop does not count as 'ventilation', sorry.
So how much ventilation do I need?
More than you probably think. More, proportionately speaking, than you're used to seeing on a house, or doghouse, or garden shed, or things like that. It is really impressive how much water vapor (as well as ammonia and heat) even just a chicken or two will emit, round the clock, day in day out.
So the best answer is probably "as much as (or slightly more than) you can reasonably build". Honestly, that is the simplest, easiest, most foolproof way to go. It is ever so much better to have more than you need than to need more than you have! Especially if "needing more than you have" comes down to a trip out back with the reciprocating saw to hack big ugly holes in your nice pretty trimmed-and-finished coop in the depths of January. Plan ahead.
If you really want me to suggest numbers: if summer heat is not a big problem where you live, then you will most likely be fine if you build at least 1 sq ft of vent opening per chicken, or (if you want a lower but therefore less-conservative number) 1 sq ft of vent opening per 10 sq ft of floor area. In a hot area, you will need more for summertime, possibly just having one or more walls being totally hardwarecloth. If you have unusually few chickens for your size of coop, or live in a very dry area, you may be fine with less ventilation; if your chickens are very crowded, your climate wet, your coop full of poo, or the bedding is wet for any reason, then you may need more than the above numbers.
All vents should have doors/flaps/covers/what-have-you so that parts can be closed down when not desired. Unless you're in a climate that stays fairly warmish year-round, covers should be draft-proof. Either they should fit very snugly, or be weatherstripped in places the chickens can't peck, or (sometimes simplest) the ones you're not going to ever use in cool weather can just be "decommissioned" at the end of the summer, panels bolted over them, and any gaps sealed til Spring in some manner the chickens won't peck at. In areas where cool weather is not all that cool and only lasts a few months, you can reverse the concept -- just build one or more walls entirely of wire (on studs) and simply cover 'em with plywood or plastic for your so-called winter.
If you live in a hot climate, you need large areas of ventilation that can be opened up on all 4 walls, and really it is best if one or more walls can be pretty much removed entirely so they're just screen (like hardwarecloth). In a climate where it never gets really all that hot, you can probably skip the whole-wall-coming-off part... unless you are in a desert-y area with giant temperature swings from day to night, in which case you may still want something of that sort. But even up North it is far-and-away best to have the ability to fling open the hatches and get lots and lots of fresh air. If nothing else, this will be of great assistance to you in drying the coop out if you should ever find yourself needing to hose down or disinfect the inside!
Securely screen your vents, whatever the size, with something like hardwarecloth that predators can't rip off, climb between, or grab handsfulls of chickens through.
"What if I just use a hole-saw to put a buncha 2" holes in the walls and screen them, that'll be good, right?" Unfortunately, a 2" diameter hole is about 3 square inches of total area. To put this in perspective, a square foot is 144 square inches. You would need almost 50 holes to equal one square foot of ventilation, and a typical coop is going to need MUCH more than just one square foot of ventilation! So, no little round holes. You want actual decent-sized openings, like 6" x 4' or 1'x3' or like that, on most if not all of the walls.
Ventilation yes: drafts NO
While ventilation aka air exchange is necessary and good, having cold air aimed right at your chickens is BAD. (I'm talking about in cool weather, here, not your 'pleasant cooling breeze on stifling August day' which would of course be good.) Small "air leak" type gaps can also cause condensation and frost, which nullifies much of the value of what ventilation you have. So you need to design your ventilation intelligently.
Ventilation that you'll be using in cool/cold weather (i.e. all year-round) should be high up above chicken level, at the tops of the walls, ideally protected from rain and wind to some degree by roof overhangs. You can put vent slots, long and relatively narrow, atop all four walls. (By narrow I mean like 4"-8" wide or something like that, not an inch or two width of 'arrow slot', unless it is a small coop for just a couple few chickens.) Vents near the roost are good in hot weather but bad in cold weather. Ask yourself "will a chicken experience a noticeable breeze on the roost in the winter?" If yes, arrange things so you can shut down those vents when temperatures drop.
You'll want additional ventilation for warmer weather, that can (should!) be lower down where the chickens can catch some breeze. Windows work; giant removable wall panels work; that sort of thing.
Do the coop 'people door' and pophole count? Sort of. I mean, yes, they do provide ventilation when they are open, but remember that they will not always be open and you need to be able to provide sufficient airflow even when they aren't. I would not suggest counting on them towards your basic ventilation needs.
Manage your ventilation intelligently -- you will want to change the amount that's open according to the weather, although as mentioned you don't want to shut it all down except in very rare instances. Sometimes you'll want to close upwind vents if it's getting too windy in the coop on a windy day.
In a really windy site, you may want to build some sort of baffle or hood for some of the usually-upwind vents (the high year-round ones) to blunt the force of the wind.
What about winter? Don't I need to close the vents to keep the chickens warm?
NO. Well ok, yeah, you will close some of them down, relative to summer conditions; but you still need a goodly amount of air exchange going on, so you cannot shut your ventilation off.
In some ways ventilation is actually more important in winter because cold air can't hold nearly so much water vapor before it gets saturated i.e. really damp and humid and clammy, i.e. you're trolling for frostbite and respiratory disease.
So yes, your vents will be letting in cold air, but you know what, that's OK as long as it is not breezing down directly at your chickens. If you're concerned about the chickens getting too cold -- although most standard-sized breeds are fine down to freezing and significantly below, as long as the air is dry and relatively still and they have an appropriate-width roost and plenty of food -- then insulate your coop. And yes, insulation is quite useful even with vents open (for some reason this issue comes up often); would you think it pointless to wear a winter coat just 'cuz you had no hat on? What insulation does is reduce heat loss from the coop so that you can afford to admit more cold air without making the place too cold.
In a super-cold climate, and let me say that I do not consider southern Ontario Canada where I live to fall into this category (!), you may want to think about arranging for your vents to be taking air in from a somewhat thermally-buffered source... a predatorproofed flue run along the ground a ways and covered in insulation, or a translucently-enclosed space that the sun warms, or the building's attic, or a larger barn, or like that.
Some links with useful further information about ventilating chicken quarters:
They're mostly aimed at big commercial barns (poultry and otherwise), but there is a lot that applies just as well to our little backyard coops, so take a look:
Check out this great discussion with pictures: Do I have enough ventilation?
Coop & Run - Design, Construction & Maintenance Forum Section
Chicken Coop Ventilation - Go Out There And Cut More Holes In Your Coop!
Why is ventilation such a big deal? Because chickens are amazing producers of moisture, ammonia and heat, that's why.