Crunching numbers for adequate ventilation

[Sunshine Flock]

It's hard to believe I once had a math scholarship in college. Simple math escapes me anymore, but I think I've done this correctly.

As of yesterday, I now have 2.20' square of ventilation in the chicken coop, a small jump up from 1.67' square for my 33' square structure and six chickens.

This isn't a significant increase when placed up against the one square foot per chicken minimum requirement. But just like @Egghead_Jr and @TwoCrows said, if you create convection venting in the eaves, which requires a sloped roof front to back, you require significantly less venting.

Egghead Person, you said you have fifteen chickens and less than two square feet of permanent ventilation. According to the square foot per chicken rule, you should have fifteen square feet of ventilation at a minimum. We see this mentioned in the forums all the time.

Despite all of my research on this, I don't recall ever having read about convection venting with sloped roofs. When I removed a board I mistakenly assumed was structural and opened up the eaves on the west side of the coop (the down slope of the roof), which is exactly opposite the east side (the high point of the slope), the result was significant.

And when I walked into the coop this morning, the smell from the leaves, pine needles and droppings was gone. Yesterday the chicken coop smelled like a petting zoo, and today absolutely nothing. Not even a teeny hint that poop monsters reside in that structure, and the movement of air was refreshing.

Now, I do recall reading that proper ventilation can completely eradicate unpleasant smells inside a chicken coop. But I didn't realize how easily you can accomplish this with simple convention venting.

When I removed that board, I doubled the venting I already had on the east side, and added 75" square. The angle of the roof beams and where they rest on the supports created an additional inch in venting height.

But all totaled, I only have 2.20' square of ventilation, much less than the recommended 6' square for six chickens — and amazingly I don't need anymore!

If someone doesn't have a sloped roof, they'll likely need to stick to the square foot minimum, right? Is it the slope that creates the air movement? Can you replicate this by placing the vents opposite each other, with one side lower than the other?

Can a roof's overhang adversely affect air convection if it's too large?

When I build a retirement coop for the flock as they age, I'm definitely going to build it with a sloped roof, and I'll face the slope north to south for even better air flow. That's the predominant movement of air where I live.

Thank you for the feedback. This was revelatory.

 

[Egghead_Jr]

Two windows or vents directly across from each other are passive venting unless there is an inlet lower. Windows across from one another are only aided by wind which works in summer but not what you'd want in winter. Convection will only work on a flat roof if there is inlet low on coop opposite side of top outlet. The small amount of area needed to get the natural vacuum going wouldn't make for draft on birds if they were under the outlet. It's just a whole lot easier with a one slant roof, gables aren't bad just require to gable vents which is added work and cost.

An excellent example of passive venting would be to look up Wood's style coops. These are constructed so the open end is south facing. The coop must be 10' plus deep to keep drafts off birds and give them ample usable coop space. Really a great coop design but not something a slap it together back yarder would do. They are a large permanent structure. Windows in dormer made from second slope. Opened in summer to create convection and cool down the coop. Really are awesome coops and something I'll be building in the near future. For my smaller and portable coops I rely on convection.

 

[TwoCrows]

You can get away with less vent square footage if you have good air movement without being a draft. It all boils down to how well air is moving whether it be windows or screened off vent holes, even roof fans. Your main objective is to get all the smell and moisture out of the coop but not freeze your birds. If there is no smell in the coop first thing in the morning and nobody is wet or developing frost bite, then you are on the right track with your ventilation system.

 

[Howard E]

Not sure where you got your 1 SF per bird number? As per the main (first) reference below, it is 1 SF (actually, it was 9/10ths SF) of ventilation space per 10 SF of floor space. I tend to round it up.

https://www.backyardchickens.com/threads/historic-chicken-house-info.1142519/

In the link above are references to a number of poultry husbandry books written and published and in wide use over 100 years ago. A time when almost all farms kept chickens. A time when it really did matter.

I find it fascinating that even way back then, all this had been researched to the Nth degree. For example, not only did they have specs for how many SF of glass area was needed per SF of building area, but they even went so far as how big each pane of glass should be....what was optimal. But today? We don't consider or use any of it. Each person pretty much sets about reinventing the wheel and likely as not, it always comes out square.

Edit: BTW, I do not want this to sound like I'm being judgemental. Most folks would have no clue any of this old historic information exits, so have had no opportunity to study and consider it in what they have done.

I would make one more observation......and that from the perspective of someone who has read several of these old books, the word "cute" is never mentioned in any of them. Economical construction, "rat proof" and other environmental issues leading to safe, happy and productive birds (fowls) were big concerns, but cute was not on their agenda!

 

[Egghead_Jr]

There is actually a formula for air flow on a single slant roof from 3/12 and 6/12 pitch. My very first coop I used the minimum based on 0.5 cubic feet air per bird per minute. I can tell you it was not enough. Had to open up a tad more as I was likely less than 4 inch rise in 12 inch run pitch. But to give you an idea of how small those openings were to start I was using a 1/2 inch drill to make a line of holes. But if you indeed keep 18 degree to 30 degree slope it doesn't take much at all.

Less slope obviously doesn't draft well and too much pitch will vent through too fast. As in the air that enters is so fast it exits before circulating with coop air. If you stay in the slant range it inlets and circulates with coop air the entire length to catch moisture and ammonia before exiting.

 

[Sunshine Flock]

I think I need to get on the @ChickenCanoe and paddle downstream for once. Flow with the current, enjoy the eddies.

Stupid life sucking Asperger's.

I love science and am totally in awe of what natural, free, unaided convention venting has done for my chickens. Now is the time to get this stuff figured out. I'm buying raw land in Idaho in the next year or two and building my own forever cabin. Knowing exactly how I'll need to approach constructing a chicken coop is going to save me a lot of effort.

 

[Egghead_Jr]

oops...wrong thread...

@CarolinaSunshineFlock Might as well take this space to remind you that hardware cloth is needed on those vent areas. I imagine you have weasels in your neck of the woods too. They are why my very first coop years ago I was trying to get away with 1/2 inch holes to vent. Those little suckers will squeeze into a one inch opening and kill your flock in a night. Hardwire up those vents!

 

[TwoCrows]

Love all the physics talk here! Honestly, ventilation is a science each chicken owner has to calculate unto their own conditions and environment. What works for one, may not work for the other due to even the slightest difference in coop design, direction it is placed, flock size, where they roost, other environmental factors.

Great discussion here guys and gals!

 

[Sunshine Flock]

oops...wrong thread...

@CarolinaSunshineFlock Might as well take this space to remind you that hardware cloth is needed on those vent areas. I imagine you have weasels in your neck of the woods too. They are why my very first coop years ago I was trying to get away with 1/2 inch holes to vent. Those little suckers will squeeze into a one inch opening and kill your flock in a night. Hardwire up those vents!

I used fender washers and 1/2" hardware cloth to secure the vents last night. But since this is on the lower side of the sloped roof, with the way the 2x4 rafters rest on the support (what's that thing called?), they make contact with the wood on the exterior side, not the interior as it is with the up sloping side.

Does that make sense?

So once I finished screwing down the hardware cloth, I stood there admiring my work, only to realize I could see light where I couldn't see it on the east side. The entire span of the 2x4 (resting flat, not on edge) is a crawl space, between each section of hardware cloth, for clever little weasel creatures to squeeze through.

The interior was easier to secure. The exterior requires sharp angles in the hardware cloth, which can comprise its strength. I've snapped welds before.

I'm doing double duty on all hardware cloth on the coop: one layer on the outside and one on the inside for the windows. I'll do the same with the eaves.