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Greenhouse exhaust fan CFM chart

The short answer

A 6 by 8 ft greenhouse needs roughly 550 CFM of exhaust capacity in full sun at sea level, which is one complete air change per minute. The National Greenhouse Manufacturers Association method is CFM = floor area x 8 x Fhouse x Felevation x Flight x Ftemperature, and the short-house correction matters: a 12 ft long greenhouse needs nearly three times the airflow per square foot that a 100 ft commercial house does.

Base rule
8 CFM per sq ft
Short-house factor
sqrt(100 / length)
Passive vent area
20 percent of floor
Sizes charted
12 footprints

Researched from published specifications and verified owner reviews · Updated 2026-08-16

A closed greenhouse in full summer sun will pass 120 degrees Fahrenheit before mid-morning, and at that temperature tomato pollen becomes non-viable, lettuce bolts and seedlings cook. Ventilation is not a comfort feature, it is the thing that decides whether a summer crop exists.

Sizing follows the National Greenhouse Manufacturers Association method. Multiply floor area by 8, which is the figure that produces one complete air change per minute in a house of average height, then apply four correction factors. The one that catches hobby growers out is the house-length factor: it is a square root of 100 divided by length, which means a 12 foot long house needs 2.9 times the airflow per square foot that a 100 foot commercial house does. Short houses leak proportionally more through their end walls.

NGMA sizingA x 8 x F

Floor area in sq ft, multiplied by 8, multiplied by the house-length, elevation, light and temperature-rise factors. The result is the exhaust fan capacity in cubic feet per minute.

Figure 1. Required exhaust airflow by greenhouse size · 12 rows
Figure 1. Required exhaust airflow by greenhouse size
SizeFloor areaCFM, full sun, sea levelCFM, 40 percent shadedCFM at 5,000 ftPassive vent area needed
4 x 6 ft 24 sq ft 775 475 950 4.8 sq ft
6 x 4 ft 24 sq ft 950 575 1,150 4.8 sq ft
6 x 6 ft 36 sq ft 1,175 700 1,425 7.2 sq ft
6 x 8 ft 48 sq ft 1,350 825 1,650 9.6 sq ft
6 x 10 ft 60 sq ft 1,525 900 1,850 12 sq ft
8 x 8 ft 64 sq ft 1,800 1,075 2,200 12.8 sq ft
8 x 10 ft 80 sq ft 2,025 1,225 2,450 16 sq ft
8 x 12 ft 96 sq ft 2,225 1,325 2,675 19.2 sq ft
10 x 12 ft 120 sq ft 2,775 1,675 3,350 24 sq ft
10 x 16 ft 160 sq ft 3,200 1,925 3,875 32 sq ft
10 x 18 ft 180 sq ft 3,400 2,025 4,100 36 sq ft
12 x 20 ft 240 sq ft 4,300 2,575 5,200 48 sq ft
NGMA method with an allowed temperature rise of 4 degrees F from intake to fan. The shaded column applies a light factor of 0.6, which corresponds to roughly 40 percent shade cloth. Passive vent area is the standard one fifth of floor area, split between ridge and side openings.

Why an exhaust fan with no intake barely works

This is the single most common ventilation mistake in hobby greenhouses. A fan can only push out what it can pull in. Mount a 1,300 CFM shutter fan on a sealed house with no intake opening and it will move a fraction of its rating, run hot, and make far more noise than air. The intake needs to be at least 1.25 times the fan's own opening area, positioned at the opposite end and low down, so incoming cool air sweeps the length of the house before it leaves. A gravity louver shutter is the standard part, and it opens automatically under the fan's own suction with no power of its own.

Watch out

Intake area is not optional

A powered fan with an undersized intake can pull the structure itself into partial vacuum, which on a film-covered tunnel visibly sucks the cover inward and on a rigid house strains the door seals. Size the intake at 1.25 times the fan opening area at minimum, and place it low at the opposite end from the fan.

Passive venting before powered venting

The last column of the chart is the passive alternative, and on houses under about 100 square feet it is often the whole answer. Standard horticultural practice is a total vent opening of roughly one fifth of floor area, split between ridge vents and side vents so warm air can leave at the top while cool air enters at the bottom. That stack effect needs no power, makes no noise and does not fail in a blackout.

Almost no hobby kit ships with anywhere near a fifth of floor area in vent opening. A typical 6 by 8 kit has one roof vent of about 3 square feet against a requirement near 9.6. The cheapest correction is a second vent, and the cheapest way to operate either is a wax-cylinder opener such as the Bayliss XL Autovent Automatic Window Opener, which lifts the vent as the house warms with no electricity involved and never forgets on a workday.

The three stages of greenhouse cooling, in order

  1. Shade first. Blocking light before it enters is always cheaper than removing the heat afterwards. 50 percent shade cloth over the roof cuts the solar load roughly in half for the price of one fan.
  2. Passive venting second. Ridge and side vents on automatic openers, sized toward the one fifth of floor area figure. No running cost, no failure in a power cut.
  3. Powered exhaust third. Sized from the chart above, with a matched intake, on a thermostat. This is what handles the worst two hours of the worst days.
  4. Evaporative cooling last. Only worth it in dry air, only above roughly 100 square feet, and it adds humidity you may not want.

Ventilation equipment by house size

Passive openers for small houses, shuttered exhaust fans for larger ones, an intake louver that most builds forget, and internal circulation, which is a separate job from exhausting heat.

Matching a real fan to a CFM figure

Fan ratings are published at zero static pressure, which is a fan in free air with nothing in front of it. A shuttered wall fan pushing through its own louvers and pulling through an intake screen is working against static pressure, and the delivered airflow is typically 15 to 25 percent below the free-air rating. Add that margin when you select, or size from the manufacturer static-pressure curve if they publish one.

Figure 2. Typical shuttered wall fan capacity by blade diameter · 8 rows
Figure 2. Typical shuttered wall fan capacity by blade diameter
Fan diameterFree-air CFMRealistic delivered CFMSuits floor area up to
6 in inline duct fan 250 to 400 200 to 320 25 sq ft
8 in inline duct fan 400 to 800 320 to 640 50 sq ft
10 in shutter fan 800 to 1,100 650 to 900 60 sq ft
12 in shutter fan 1,100 to 1,500 900 to 1,200 85 sq ft
14 in shutter fan 1,600 to 2,100 1,300 to 1,700 120 sq ft
16 in shutter fan 2,200 to 2,900 1,800 to 2,300 160 sq ft
18 in shutter fan 3,000 to 3,900 2,400 to 3,100 215 sq ft
20 in shutter fan 4,000 to 5,200 3,200 to 4,200 290 sq ft
Ranges span single-speed and two-speed units from several manufacturers. The final column applies the NGMA figure for a house of typical hobby length, so it is conservative for long houses and appropriate for short ones. Always read the specific unit specification rather than the diameter alone.

Circulation is a different job from exhaust

Horizontal airflow, which is air moved around inside the house rather than out of it, is the ventilation job most hobby growers skip entirely and the one with the clearest disease consequence. Still air inside a leaf canopy becomes saturated because the leaf is transpiring into it, and a saturated boundary layer on a leaf surface at night is the precondition for botrytis. Air movement of even 50 to 100 feet per minute across the canopy breaks that layer up.

The commercial standard is horizontal airflow fans running continuously at low speed year round, including all winter when the exhaust fan is off. On a hobby scale a pair of small clip fans on a propagation shelf, or a quiet 6 inch clip fan above a seedling tray, does the same thing. The secondary benefit is mechanical: seedlings grown in moving air develop measurably thicker stems and transplant with less shock, which is why commercial propagators run fans over trays deliberately.

Note

The winter ventilation nobody does

A sealed greenhouse in winter accumulates humidity from transpiration and from the soil, and relative humidity above roughly 85 percent is where fungal disease takes hold. The standard practice is a brief vent in the late afternoon, on a warm day, to exchange the moist air before the temperature drops. It costs a small amount of heat and prevents a category of problem that heating cannot.

Work your own figure with the ventilation CFM calculator, which applies all four correction factors to your dimensions and elevation. The reasoning behind the numbers is in ventilation basics, the humidity side in humidity and disease control, the summer strategy in cooling a greenhouse in summer, and the passive-versus-powered decision in vent opener versus exhaust fan.

Related on this site

Common questions

6 answers

+ What size exhaust fan do I need for a 8 by 12 greenhouse?

A 8 by 12 ft greenhouse has 96 square feet of floor and needs roughly 2,200 CFM in full sun at sea level using the NGMA method, once the short-house correction is applied. In practice that means a 14 inch or 16 inch shuttered wall fan, or two 12 inch fans. Add a gravity intake louver of at least 1.25 times the fan opening area at the opposite end, or the fan will not achieve its rating.

+ How many air changes per minute does a greenhouse need?

One complete air change per minute is the standard target for a greenhouse in full summer sun, and that is what the 8 CFM per square foot figure produces in a house of average height. In spring and autumn, roughly one air change every two to three minutes is adequate, which is why a two-speed fan or a speed controller is worth having rather than a single-speed unit cycling on and off.

+ How much vent area does a greenhouse need without a fan?

Roughly one fifth of the floor area in total vent opening, split between ridge vents and side or base vents. A 6 by 8 house with 48 square feet of floor therefore wants about 9.6 square feet of opening. Almost no hobby kit provides that: a typical single roof vent is around 3 square feet. Adding a second vent is usually cheaper and quieter than fitting a fan.

+ Does elevation change greenhouse fan sizing?

Yes, significantly. Air at altitude is less dense and carries less heat per cubic foot, so the same fan removes less heat. At 5,000 feet the required airflow is about 21 percent higher than at sea level, and at 7,000 feet about 32 percent higher. Fan manufacturers rate their products at sea level, so a fan sized from a sea-level chart will be undersized in the mountain west.

+ Should the exhaust fan be high or low?

High, at the gable end, with the intake low at the opposite end. Hot air rises, so drawing from the top removes the hottest air in the house while cool intake air enters at plant level and sweeps the full length before leaving. A fan mounted low pulls cool air straight out and leaves a hot layer sitting above the crop, which is exactly backwards.

+ Do I need a circulation fan as well as an exhaust fan?

Yes, they do different jobs. An exhaust fan removes heat from the house. A circulation fan moves air within it, which prevents the still, saturated pockets that cause botrytis and damping-off, and stiffens seedling stems so they transplant better. Horizontal airflow fans typically run continuously at low speed year round, including in winter when the exhaust fan is off entirely.

Working out the figures for your own house and season? The Greenhouse Build & Growing Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.