Ventilation in an Open-Sided Coop

Two numbers share one word, and only one of them is about holes

Ventilation is two quantities. Rate is how much air passes through the building in an hour. Speed is how fast that air moves across a bird. A sealed coop in a cold country fights for rate and has to ration it, because every cubic metre it lets in is heat it loses. An open-sided kulungan gets rate almost free and hardly ever gets speed.

Here is how free. DA Administrative Order No. 12, s. 2002, the Code of Practice and Minimum Standards for the Welfare of Chickens, puts a ventilation capacity of 8 cu m per kg of bodyweight per hour in its Appendix I. Fifty manok at 1.7 kg apiece, which is the weight where the two Philippine stocking limits meet, is 85 kg of bird and 680 cubic metres an hour.

Now hang that on a wall. One open long side of a coop 3 m long with 1.8 m walls is 5.4 square metres of mesh. To carry 680 cubic metres an hour, the air has to cross it at 0.035 metres a second. Three and a half centimetres. Hindi mo mararamdaman iyon sa likod ng kamay mo.

Three caveats travel with that sum and none of them rescues the rate. The order states 8 cu m/kg/hr for broiler breeders and this page has applied it to a backyard batch. It is a design capacity for a fan system, not a measurement of a bakuran. And it assumes every cubic metre of moving air goes straight through the opening, which no real building manages, so the honest reading is that the true figure is some multiple of 3.5 cm/s and still nowhere near a breeze. Kaya hindi kakulangan ng butas ang problema sa kulungan mo. Ang tanong ay kung may hanging dumaraan doon.

The other number is not like that at all. Aviagen’s hot-weather note says circulation fans in open-sided housing “should produce an average of 3 m/sec (591 ft/min) air speed at the level of the birds”. The same document sets air speed at chick placement at 0.15 m/sec. One document, one batch, and a 20-fold range across six weeks. No fixed hole can be right at both ends of that.

What PAGASA’s wind normals say about your bakuran

The climatological normals carry a wind column that nobody reads, and it is the column that decides this question. Column 13 is the prevailing direction on the 16-point compass, column 14 the mean speed in metres per second.

Science Garden reads 1 m/s in all twelve months, which is 33.3% of the speed Aviagen asks for. Cabanatuan, in Nueva Ecija, rounds to 0 m/s in May, June and September. Across all 48 station-months here, 77.1% are below 3 m/s.

Read that as a ceiling and not as an estimate. A PAGASA anemometer stands in the open at standard exposure; your kulungan sits behind a fence, a mango tree and somebody’s wall, at knee height. Whatever the station records, the bakuran gets less. Kaya kapag may nagsabi sa iyo na sapat na ang hangin sa lugar mo, tanungin mo kung saan niya sinukat.

That is also why the mister every foreign guide recommends is usually the wrong purchase here, and why moving air is the lever that still pays.

Ang hangin ay bumabaligtad dalawang beses sa isang taon

PAES 402:2001, the Philippine Agricultural Engineering Standard for broiler housing, is short and specific about ventilation and it never gives a size:

This is not advisory literature. DA Administrative Order No. 06, s. 2004 enforces the PAES “as integral parts in the design, construction and maintenance of agricultural buildings and facilities”, and the Board of Agricultural Engineering adopted the same volumes as the code registered agricultural engineers sign against.

Two of those clauses depend on knowing which way the wind blows, and in this country that is not a fixed fact. PAGASA names the two monsoons, and the station table above measures the switch: at Science Garden the prevailing wind is N or NE from October through February and SW from June through September. At Iloilo it is NE for seven months and SW for five, an exact 180 degree reversal. Davao runs N from November to April and S from May to October.

So the downwind side of your kulungan in January is the upwind side in July. A fixed opening cut on one gable is correct for half the year at best. Section 8.6.1.3 is the standard admitting as much when it asks for controllers that adjust opening sizes “as weather changes hourly and seasonally”. Ang mura at hindi elektronikong bersyon niyan ay ang tarpaulin na binubuksan at sinasara ng tao.

The east-west rule survives the reversal, and that is not a coincidence. A building whose long axis runs east to west has its two open long sides facing north and south. Amihan arrives from the north or northeast, habagat from the south or southwest, so both monsoons strike a long side at roughly 45 degrees and neither strikes an end wall. Turn the building to face the amihan square on and you have optimised for the cool half of the year and shut the hot half out. Libre ang desisyong iyon habang nakahiga pa sa lupa ang mga poste, at hindi mo na mababago kapag nakatayo na ang bubong.

The American rule is an area rule, and here it is nine times too small

Search the query and the first screen is Idaho, Ohio and Ontario. The rule they trade is one square foot of ventilation per ten square feet of floor, and the best of them argues with it: The Featherbrain calls that figure “totally bogus” and then reasons entirely inside the winter frame, recommending closed windows in cold months and warning that humidity raises the risk of frostbite. Her own line is “Summer drafts are good. Winter drafts are bad.” Nothing in the piece mentions the tropics, and it is not written badly. It is written for a different building.

Put the two rules on the same coop. Take a bakuran box 3 m long, 2 m wide, 1.8 m to the eave.

9 times as much. PAES 402 defines open-sided housing as “long and narrow type of houses wherein at least one-half of the front and the back of the house are open”, and section 8.6.1.2 points the same way when it puts the inlets along the long sides. That is the closest thing to an area rule in any Philippine document, and it is a fraction of a wall rather than a fraction of a floor. Section 7.3.1 says what the rest of the wall may be made of: “chicken wire or bamboo slats”.

The difference is not a matter of degree. An area rule for a closed box asks what fraction of the envelope may be given up. The Philippine definition asks what fraction of the envelope may remain. Magkaibang tanong iyon, at magkaibang gusali ang sagot.

Ridge vent or no ridge vent, and the width that settles it

FAO and TNAU appear to contradict each other, and it is worth resolving because both are right.

FAO puts the maximum width for an open-sided building at 8 m (26 ft), defined as the width “under conditions of a slight breeze, which allows air movement across the shed at the height of the bird”, wants the end walls closed to force air across that width, and then says flatly: “Centre ridge ventilation is not recommended, as it discourages airflow across the full shed width.” TNAU says open-sided houses in tropical countries should be “not more than 22 to 25 feet” wide and that above 25 feet “ridge ventilation at the middle line of the roof top with proper overhang is a must.”

Twenty-five feet is 7.6 m. The two documents are one rule seen from either side of the same number: below about 8 m, cross-flow reaches the middle of the house and a ridge opening only steals the pressure difference that drives it; above it, cross-flow dies in the centre and the ridge is the only way out for the hot air.

PAES 402 section 8.1.1 sets a commercial pen at 10 m to 12 m wide, which is past that line in both documents, which is exactly why its section 8.6.1.1 requires a ridge or chimney outlet. The standard is internally consistent and it is describing a commercial house.

A bakuran coop is 2 m to 3 m across. It is a quarter of the width where the ridge argument begins. Cross-flow through the two long sides is the entire mechanism, and a monitor ridge on a coop that narrow is a nice roof rather than a ventilation decision. Kung may pambili ka ng dagdag na yero, ilagay mo sa overhang. One metre of it, as both PAES 402 section 7.1.1 and FAO ask for, buys shade over the open side and keeps the rain out of it, which is the thing that makes a keeper hang a tarpaulin and forget it there until April.

Ammonia is the half of ventilation you cannot see

The other job the air does is carry gas away, and this is where a health limit exists in Philippine law. This section is not veterinary advice; if birds are showing eye or respiratory signs, that is a call to an LGU veterinarian and not a reason to cut another vent.

Two Philippine documents give the same figure. DA AO 12, s. 2002 section 8.4: “Ammonia levels in the poultry houses shall not exceed 20 ppm at the bird level.” PNS/BAFS 312:2021 section 6.9.2 repeats it for layers, “if it exceeds 20 ppm at layers’ head height”, and section 6.9.1 states the purpose: to prevent build-up of “heat, humidity, dust, and noxious gases to levels that are harmful or can cause pain or distress”.

What 20 ppm buys is margin. The Merck Veterinary Manual puts damage at “25–30 ppm”, which “can damage the mucociliary apparatus of the upper respiratory tract”, and at “50–75 ppm” reports “decreased feed intake as well as caustic burns and ulceration of the cornea, which can result in blindness”. Merck also names the source, and it is not the bird: “Ammonia gas is produced by the metabolism of uric acid through bacteria that can thrive in wet poultry litter.” Basang litter ang pinagmumulan, kaya ang unang lunas ay tuyong sahig at hindi bagong butas.

Nobody in a bakuran owns an ammonia meter, and the substitute has a flaw worth knowing about. ATSDR gives ammonia an “odor threshold = 5 ppm”, which is four times below the Philippine limit and would make the human nose a decent instrument. Except that the same page warns ammonia “causes olfactory fatigue or adaptation, making its presence difficult to detect when exposure is prolonged.” The person who cleans the kulungan every morning is the person least able to smell it. Ang kapitbahay mong dumaan lang ay mas magaling na sensor kaysa sa iyo.

One detail in the standards is doing real work. Both write the limit at bird level, and ATSDR notes that anhydrous ammonia “is lighter than air and will therefore rise”. A keeper standing up is sampling air a metre and a half above the litter that made the gas. Yumuko ka muna bago ka magdesisyon.

The one test you can run for the price of a thermometer

PAES 402 section 8.2.2.6 is the most useful sentence in the whole standard for a backyard keeper, and it is a pass-or-fail test with no equipment beyond two thermometers:

Temperature in the poultry house shall not be permitted to become higher than the outside temperature during summertime.

Section 8.2.2.5 asks for thermometers “in strategic locations inside the poultry house” to make that checkable. This is the point of the whole design: a coop that is hotter inside than out has failed at ventilation regardless of how much mesh is in the walls, and one that matches the outside air has done everything an unpowered building can do. It cannot go lower without a fan.

Hang one thermometer at bird height inside and one in the shade outside, and read both at two in the afternoon in April. Kung mas mataas ang loob, may problema ka sa bubong, sa lapad, o sa dami ng manok, at hindi sa laki ng butas. How many birds are in there is the lever most keepers refuse and the one with the clearest mechanism, and what the roof is made of is the other.

What nobody publishes

Four gaps, and they are why this page is arithmetic on other people’s numbers rather than a table of Philippine measurements. Iba ang may sukat, at hindi ito bakuran. It belongs beside the rest of the register.

• No Philippine document gives an opening area. PAES 402 fixes the position of every inlet and outlet and the size of none of them. Section 3.4’s “at least one-half” is a definition of a house type, not a ventilation specification, and this page has read it as one.
• No indoor air speed has been measured in a Philippine backyard coop that I could find on 22 August 2026. The 3 m/sec figure is a breeding company’s fan specification; the 1 to 4 m/s figures are station anemometers in the open. Nothing published sits between them.
• PAGASA’s normals are monthly means. Cabanatuan’s 0 m/s in May is a month averaging below half a metre a second, not a record of calm hours. The frequency of dead-calm afternoons, which is the number that decides whether a fan is worth buying, is not in the normals at all.
• The rate in DA AO 12 is 24 years old and written for breeders. No newer Philippine figure for a ventilation rate turned up anywhere, in any unit.

What usually goes wrong

1. The mistake: Sizing the openings against a rule of thumb from a US or UK coop guide.

   Do this instead: Open at least half of both long walls, per PAES 402 § 3.4, and stop calculating.

   That rule is a fraction of the floor for a sealed box; the Philippine definition is a fraction of the wall for an open one. On the same 3 m by 2 m coop the two answers differ by nine times.

2. The mistake: Cutting a fixed vent on the side the wind came from when the coop was built.

   Do this instead: Make the openings adjustable and expect to change them twice a year.

   PAES 402 § 8.6.1.1 puts the outlet on the downwind side and § 8.6.2 puts fans opposite the prevailing wind. PAGASA records the prevailing direction reversing between amihan and habagat at all four stations checked here.

3. The mistake: Leaving the rain tarpaulin down after the wet season ends.

   Do this instead: Take it up in the dry months, or hang it so one person can raise it in a minute.

   A curtain hung in July and still there in April removes the only ventilation the building has, in the month with the highest mean daily maximum on the record.

4. The mistake: Judging ammonia by whether you can smell it.

   Do this instead: Ask somebody who has not been in the coop that day, and crouch to bird height before deciding.

   ATSDR puts the odour threshold at 5 ppm but warns of olfactory fatigue with prolonged exposure, and both Philippine limits are written at bird level rather than at standing height.

5. The mistake: Buying a fan before checking the two thermometers.

   Do this instead: Read inside and outside at two in the afternoon first. Fix the roof, the width or the stocking density if the inside is hotter.

   PAES 402 § 8.2.2.6 says the house shall not be hotter than outside in summer. An unpowered building that matches the outside air has done all it can, and a fan added before that point is paying to move hot air around a hot box.

The reason this reads so differently from the American literature is that the American literature is solving a real problem that does not exist here. In a Wisconsin winter every cubic metre of fresh air is heat leaving the building, so the whole craft is buying the minimum you can survive on. Walang init na ipagtatanggol dito. Nothing is being conserved, nothing has to be rationed, and the design question is not how little air you can get away with. It is how to keep air moving on the afternoons when the station anemometer reads 0 m/s and the birds are already at the edge.

Questions people actually ask

How much ventilation does a chicken coop need in the Philippines?

No Philippine standard states an area, and the American rule of thumb of one square foot of vent per ten square feet of floor is not the rule that applies here. PAES 402:2001 defines open-sided housing as a house where at least one half of the front and the back are open, which for a 3 m by 2 m coop with 1.8 m walls works out at about 5.4 square metres of mesh against the 0.6 square metres the American rule would give. That is nine times as much opening, and it is a fraction of the wall rather than a fraction of the floor. The standard fixes where the openings go and never how big they are.

Which way should a chicken coop face in the Philippines? Show the answer

East to west along the long axis, and three independent documents say so. PAES 402:2001 section 4.4 states that the building shall be constructed in an east-west orientation. FAO says an east-west orientation of the length of the building helps to minimize exposure to direct sunlight in the tropics. Aviagen gives the same instruction for open-sided broiler houses. It also happens to be the orientation that survives the monsoon reversal: the amihan blows from the north or northeast and the habagat from the south or southwest, so a building with its long sides facing north and south takes both of them diagonally rather than taking one square on and the other end-on.

Is ammonia dangerous in a backyard chicken coop, and how would I know? Show the answer

Two Philippine documents give the same limit. DA Administrative Order No. 12, s. 2002 section 8.4 says ammonia levels in poultry houses shall not exceed 20 ppm at the bird level, and PNS/BAFS 312:2021 section 6.9.2 repeats it for layers at head height. The Merck Veterinary Manual puts damage to the upper respiratory tract at 25 to 30 ppm and corneal ulceration at 50 to 75 ppm. Nobody in a bakuran owns a meter, and the substitute is imperfect: ATSDR gives ammonia an odour threshold of 5 ppm but also warns that it causes olfactory fatigue, so the person who spends most time in the coop is the least able to smell it. Ask somebody who has not been inside that morning.

Does a backyard coop need a ridge vent? Show the answer

Probably not, and the reason is width rather than heat. FAO caps an open-sided building at 8 m wide so that a slight breeze still moves air across the shed at bird height, and then says centre ridge ventilation is not recommended because it discourages airflow across the full shed width. TNAU says the opposite, that ridge ventilation is a must above 25 feet, or about 7.6 m. Those are the same rule read from either side of the same width. PAES 402:2001 sets a commercial pen at 10 to 12 m wide, which is why it requires a ridge or chimney outlet. A bakuran coop 2 to 3 m across is nowhere near any of those numbers, and cross-flow through the two long sides is the whole mechanism.