# A Ventilation Equation

Discussion in 'Coop & Run - Design, Construction, & Maintenance' started by Egghead_Jr, Nov 7, 2010.

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Oct 16, 2010
NEK, VT
Being that I'm still in the process of building a winter coop, yes I've been slacking, adequate ventilation has been on my mind. So I decided to do some research this morning. After a few hours of sifting through university publications and the like I pulled out the info that is pertinent to us BYC'ers.

Chickens need 0.5 cfm air flow. A handy formula for gable and eave vent requirements is A=4.7Q/V. Your slope of roof should be between 4/12 and 6/12 (edit: took out the angles I'd figured in my head that were off. Pitch is correct still) to ensure proper mixing of fresh air and coop air.

Q= required air flow = # of chickens x 0.5cfm
V= wind speed
A= your minimum required square inch of opening in gable/ridge vent and that amount again in each sofit vent.

This formula works for each 10 feet of building width. As we are BYC'ers I doubt we have buildings wider than 10ft but if so than double the area for up to 20 feet, etc.

You can Google your state/region for average wind speed. Here in New England the average is 8 miles/hour in winter and 5 mph in summer. The lowest wind speed is your parameter for area needed.

Example:

I have 7 chickens. My coop is less than 10ft wide (gable side). My lowest average wind speed is 5 mph.

A= 4.7 (7 x 0.5)/ 5 = 3.29 square inches. Voila! my gable vents need to add up to a minimum of 3.29 and each eave vent the same.

I'd already drilled a pattern of 1/2 inch holes in the gable, 7 holes each side. Using the area of a circle...
(.5/2)^2 (3.14)14 holes = 2.75 square inches. Not enough!

If I re-drill the holes to 5/8 inch then: [(5/8)/2]^2 (3.14)14= 4.29 square inches. More than adequate.
Therefore I'll be drilling 7 holes with 5/8 drill each gable end and 14 holes under both eave ends.

I hope this wasn't confusing and is helpful.

You may think 0.5cfm is not a lot but a chicken is small, 10cfm is required for a hog and 35cfm for beef cattle. To circulate out the moisture and gases caused by one chicken doesn't require much and this is a easy way to ensure the minimum

Last edited: Nov 11, 2010
2. ### CargoChillin' With My Peeps

Sep 28, 2010
Farmington, NM
Wow. You will have to keep us posted as time goes by.

I personally like a lot more ventilation than what you have described. I'm a nerd though so I applaud your use of science.

3. ### patandchickensFlock Mistress

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Apr 20, 2007
Yeeeeahhhhh, well, what you say is maybe correct for commercial chicken barns.

However, there are some real problems with applying it to backyard coops.

For one thing, your chosen approach leaves you with insufficient ventilation 50% of the time (well, a bit less than 50% in the higher-average-windspeed months, but still, not *lots* less than 50% of the time). Or maybe even more than 50% of the time, depending on what the distribution of windspeeds is in your area (if you're in an area that sometimes gets very windy days, that can really skew the average...)

To me, having an insufficiently-ventilated coop HALF the time is not such a great idea. The coop needs to be ALWAYS, EASILY sufficiently-ventilated.

Secondly, wind does not pass nearly as well through small holes as through larger openings. For instance you will get less total airflow (I do not know the exact numbers) through thirty-two 1x1" holes than you will through a single 4x8" hole. Half-inch or 5/8" holes are even worse, because they restrict airflow even further.

And on top of that, those numbers are for commercial conditions, which are often pretty different than what you get in a backyard coop. Backyard coops can actually have *more* ammonia and moisture production per chicken than commercial coops, for a variety of reasons.

Thus, I have to say that FROM EXPERIENCE your numbers are way, way too low for reliably-adequate ventilation and reliably-healthy chickens. You are very likely to discover this the hard way, with your wee little drilled holes, unless you scrap them and put in PROPER AMPLE ventilation.

Sorry, I know numbers and theory and all that are fun (I am a biologist), but, they do not always adequately model how the real world works in situations different than what the numbers and theory were developed for ,

Pat

Last edited: Nov 7, 2010
4. ### elmoChillin' With My Peeps

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May 23, 2009
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Just wondering what wind speed has to do with it. Is the idea that your vents will be on opposite walls, so wind blowing into the coop will cause air to circulate out the other side?

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Oct 16, 2010
NEK, VT
"Wee little drilled holes."

Considering my # of birds, 7 in a 4x4 coop, the ventilation looks to me to be very adequate. As I stated previously this equation prompted me to actually drill bigger and more holes.

Wind velocity aids in circulation of air. Completely stagnant air would only circulate via bouyancy (hot air rising); Hot air exiting the gable/ridge vent causing negative pressure to pull fresh air in the eave vents thus circulating the air inside and moving the required min of 0.5 cfm (cubic feet per minute) out. Hence if wind is determined to be 1 MPH then the needed air venting area would be significantly larger. In fact it would be 5 times larger than my example. My 7 birds would require 16.5 square inches in gable and on each eave end.

Regardless, the equation may not suit everyone but I did feel it makes appearent the ventilation issue and adresses it with a minimum value and working knowledge of how venting works. I'll let ya know if the coop frost up any this winter prompting me to make more holes.

6. ### patandchickensFlock Mistress

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Well, it's up to you if you want to trust theory over peoples' actual experience. Eventually you will gain enough actual experience yourself that you will see what we mean here

IMO for 7 birds in a 4x4 coop (which is a substantial stocking density, there), I would not personally want to go with less than something like 2 square feet of ventilation -- and for summertime you will want more than that. Based on experience. Mind, in a very dry climate your needs might be a *little* less, or in a climate where winter temperatures do not drop below the mid 40s F. But actually even in those climates I would build more ventilation (like, a LOT more than just 2 sq ft) because you will need it in summer and quite honestly may well need it the rest of the year too. (It depends on all sorts of variables you are not considering, so cannot be predicted well for a particular coop)

Whereas your proposed ventilation is apparently on the order of 10-20 square *inches*, i.e. in the neighborhood of 0.1 square feet.

This is not a math problem, it is not physics homework, it is a matter of what observably WORKS. What you are describing most often does not work (I mean, you can keep chickens alive that way, but usually air quality will suck badly through at least some and often most of the year, often with health consequences to the chickens)

(Furthermore, I doubt very many people have coops exposed to the wind that the National Weather Service or your local airport are measuring. And what matters is the wind velocity component perpendicular to the holes you've drilled and in the particular location your coop is sitting in.)

BTW you do NOT need both eaves and ridge-or-gable-end vents, and indeed in hot or cold weather they are inappropriate and inefficient. (In hot weather because you get to the point where what you REALLY need is to have most or all of one or two sides of the coop being open; and in cold weather because if you have eave-to-ridge ventilation it causes drafts). You can most certainly get effective passive ventilation through just one opening (if you don't believe me, go shut your bedroom door and open the window about 6" and leave it that way overnight, and in the morning tell me how much air exchange didn't occur ); and that is what you want in a typical coop during a cold winter, because it is minimally-drafty.

You can certainly try your leetle drillbit holes. But, chances are very good you will be one of the people posting here in a month (or perhaps less) "why is my coop so stinky", "why is there condensation or frost all over the inside of my coop", and/or "what do I do about this frostbite on my hens' combs". People have TRIED this before you, and it has pretty well been established what is the track record of leetle wee holes versus bigger (adjustable) openings

Have fun,

Pat

7. ### chillmillerChillin' With My Peeps

Nov 7, 2010
Tonopah/AZ
idot wrong post my pc hate me b i like your idea though

Last edited: Nov 7, 2010
8. ### elmoChillin' With My Peeps

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May 23, 2009
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Quote:Where I live, I couldn't count on wind as a factor in determining adequate ventilation because most of the time there isn't any wind at all! I built 1 square foot of ventilation per bird up high over roost level, then additional ventilation (almost 3 square feet) per bird down lower that I can open or close as needed. This August during our 100 degree plus heat wave, this passive ventilation kept the coop no warmer than the outside air, which is what I was aiming for.

9. ### patandchickensFlock Mistress

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I'm sorry, I know I should just shut up and wander off, but this was bugging me last night, the more I thought about it.

I spent a few minutes googling and believe I have found where you got your equation (or at least, a comparable source): http://www.ces.purdue.edu/extmedia/AE/AE-97.html .

HOWEVER note that they say in several places that outlet openings must be a minimum of 4" wide to prevent frosting closed during winter. "To do this and still maintain an equivalent ventilation area, open only the ridge in every third truss bay to a width of 4.2 inches (3 x 1.4 inches)." Even aside from winter issues, the smallest vent opening width I can see them discuss anywhere is 1.4" wide by the length of a bay. In other words, they are NOT talking about a bunch of 5/8" holes drilled into anything Small openings really do have different airflow dynamics than large ones.

Second, what you've calculated is only minimum wintertime ventilation requirements (although, again, in a commercial-barn setting). The "book" values for SUMMERTIME minimum ventilation requirements are more than an order of magnitude higher, like 5-8 cfm per hen. If you therefore are going to have to get out yer Sawzall and rip some big holes in your coop in about six months ANYhow -- why not just do it NOW and have that ventilation AVAILABLE for winter use as well?

But those "book values" are really just not very relevant to backyard coops. Backyard coops operate under often VERY different parameters from commercial chicken barns. They are not climate controlled and thus are colder (sometimes a whole lot colder) in the winter, and hotter in the summer -- which affects how much ammonia and humidity are going into, and resident in, the coop air. And it is those two factors (indoor and outdoor temps, and actual NH3 and H20 inputs/contents) that really determine how much air exhange you need. Simple universal rules of "X cfm per hen" are ONLY correct for the particular set of assumptions for which they were derived, and your coop is likely to be far from those conditions.

Want math? Go read http://www.cps.gov.on.ca/english/plans/E9000/9700/M-9700L.pdf which is one of the better expositions I found (in like ten minutes of googling ) on the realities of trying to CALCULATE ventilation needs. One thing I like about it is that it makes fairly explicit what assumptions are being made about indoor/outdoor temperatures, and talks about the issue of calculating ammonia and humidity removal which after all is the real POINT of ventilation anyhow.

If you don't like math *that* much (er, I don't mean the o.p. obviously, but, anyone else reading this), or if you DO like math and understood enough of that webpage to grasp that you would need to know one big hell of a lot about your particular coop situation (on any given day) to correctly know how many cfm of air exchange will provide adequate air quality...

...it DOESN'T MATTER Because by far the simplest thing, which frankly is pretty much what commercial livestock barns do TOO only they're understandably more hyper about avoiding accidentally designing in a larger ventilation system than needed (because they use EXPENSIVE mechanical ventilation), is to just PLAY IT AS IT LIES. THey use technology to monitor air quality, but we can do just fine with observation.

So, coop too hot? Open more ventilation (up to the point where it's isothermal with outdoors, at which point you're basically stuck). Smell a bit ammonia-y? Open more ventilation. Getting drippy condensate or frost (except it's unavoidable on windowpanes when there is a very large temperature differential)? Open more ventilation. Seems fine, no smell or condensate, but it's cold? Try closing down the ventilation somewhat, til you reach the point where you DO get ammonia odor and/or condensate/frost, then open it back up a tad and there ya go.

If you want high-tech, use a hygrometer, but be careful because nearly all of them seem to be inaccurate right out of the box (salt calibration method can help you figure out a general correction factor) and I honestly do not know how trustworthy different hygrometers are in very very low temperatures. But with those caveats, you can perfectly well stick a hygrometer on the wall (after checking its accuracy) and then fiddle your ventilation to the point where, when you go out there first thing in the morning, humidity is no higher than 60-70% and you don't smell any ammonia. If those two conditions are met, your ventilation is sufficient.

Best practice, IMO and honestly it is what commercial barns mostly do too:

BUILD AMPLE ventilation capacity, to meet whatever your *maximum* requirements may ever be,
and then REGULATE whatcha got according to whatcha need.

Pat

Last edited: Nov 8, 2010
10. ### CargoChillin' With My Peeps

Sep 28, 2010
Farmington, NM