Total vent area should be 15-25% of the greenhouse floor area, and that fraction is of the floor, not the glazing. A heat balance shows it holds the interior within 5 K of outdoor air only when the wind blows at roughly 3-4 m/s; in calm air the buoyancy of the warm air alone needs about 71% of the floor area, which is why shading and wind exposure matter as much as vent size.
Greenhouse ventilation is the exchange of interior air with outdoor air, driven by wind and by the buoyancy of warm air, to carry away solar heat.
Overheating Is The Failure Mode, And The 15-25% Rule Hides A Wind Speed
Overheating kills plants faster than cold. A closed greenhouse in full sun can reach 50 °C or more, and manual venting is the first defense: open doors, windows or cold frame lids when the interior exceeds 25-30 °C. A passive wax-cylinder vent opener, salvaged from garden supply, expands as the temperature rises and pushes a vent window open with no electricity. The rule of thumb is that total vent area should equal 15-25% of the floor area. The working below shows what that rule assumes.
Level 4 here means the arithmetic behind a rule of thumb. It rests on textbook physics and recalled standard values, not on a measured tolerance, and each assumed input is labelled.
What You Need
- Vents at two heights: low side vents or doors, and high ridge or gable vents
- A thermometer inside and one outside in the shade
- A way to open vents remotely or automatically (wax-cylinder openers, cords and pulleys)
- Insect mesh, which cuts the open area and so needs the sum below redone with its free area
Method: Size The Vents From A Heat Balance
Step 1: Estimate the solar heat entering per square meter of floor. Take peak sunlight at 1,000 W/m² (the standard clear-sky reference irradiance, recalled) times a transmission of 0.9 (the roughly 90% of light that salvaged glass passes).
Step 2: Decide the temperature rise you can accept. Choose the allowed rise of inside air over outside air, here ΔT = 5 K.
Step 3: Find the airflow that carries that heat away. Q = P / (ρ × c × ΔT), where P is the heat power, ρ the air density and c the specific heat of air.
Step 4: Convert airflow to opening area for wind-driven flow. Q = E × A × v, with E the effectiveness of the opening (about 0.25-0.35 for oblique wind and 0.5-0.6 for wind blowing straight onto the opening, a recalled value from building-ventilation references), A the opening area and v the wind speed.
Step 5: Check the buoyancy (calm-air) case. Q = Cd × A × √(2 × g × H × ΔT / T), with Cd about 0.6 (a typical orifice discharge coefficient, recalled), H the height between low and high vents, and T the absolute temperature.
Worked Example
Per square meter of floor.
Heat entering. P = 1,000 × 0.9 = 900 W per m² of floor. This treats all of it as heat in the air, which overstates the load because plants evaporate water and the ground absorbs heat. It is the conservative case.
Airflow needed. ρ = 1.2 kg/m³, c = 1,005 J/(kg·K), ΔT = 5 K. Q = 900 / (1.2 × 1,005 × 5) = 900 / 6,030 = 0.149 m³/s per m² of floor. That is 0.149 × 3,600 = 537 m³/h per m² of floor. For a house 2.5 m high that is 537 / 2.5 = 215 air changes per hour.
Wind-driven opening area with E = 0.25 (oblique wind, the cautious case):
| Wind speed | A / floor area |
|---|---|
| 1 m/s | 0.149 / (0.25 × 1) = 60% |
| 2 m/s | 0.149 / (0.25 × 2) = 30% |
| 3 m/s | 0.149 / (0.25 × 3) = 20% |
| 4 m/s | 0.149 / (0.25 × 4) = 15% |
So the 15-25% rule is consistent with a wind of about 3-4 m/s (and with 2-3 m/s if E = 0.35: 0.149 / (0.35 × 3) = 14%, and 0.149 / (0.35 × 2) = 21%).
Calm-air (buoyancy) case. H = 1.5 m between vent centres, ΔT = 5 K, T = 300 K. v = √(2 × 9.81 × 1.5 × 5 / 300) = √0.4905 = 0.70 m/s. With Cd = 0.6: flow per unit opening area = 0.6 × 0.70 = 0.42 m/s. Each of the low and high vents then needs 0.149 / 0.42 = 0.355 m² per m² of floor, so the two together need 2 × 0.355 = 0.71, about 71% of the floor area.
On a 10 m² floor the rule gives 0.15 × 10 = 1.5 m² to 0.25 × 10 = 2.5 m² of vent. The wind-driven sum at 3 m/s asks for 0.20 × 10 = 2.0 m² and the calm-air sum asks for about 7 m².
What The Numbers Say To Do
- Open the vents wide, not slightly. A cracked vent is far below the area the sum needs.
- Put vents at two heights. The buoyancy flow depends on the height between them.
- Shade the hot afternoons. Halving the sun absorbed halves the airflow needed; see Shading Strategies.
- Count mesh as lost area. Insect screen blocks part of the opening, so the free area is smaller than the hole; enlarge the opening to compensate.
- Place the house where the wind reaches it. A sheltered position needs larger vents or shade.
Limits
- Assumed inputs: irradiance, transmission, ΔT = 5 K, E, Cd and H are assumptions or recalled values, not measurements of your site.
- Overstated load: treating all absorbed sun as sensible heat ignores plant evaporation, so real needs are lower; the opposite risk is a dense, tall crop that blocks the airflow.
- Not a controller: the sum gives a size, not a temperature forecast. Read the thermometer.
Common Mistakes
- Measuring vent area against glazing — the rule is a fraction of floor area.
- Opening only one side in still air — no flow develops without a second opening at a different height.
- Closing late — vents opened at noon should close in the late afternoon, not the evening.
Safety
Heat illness
A closed greenhouse at 50 °C or more can overcome a person working inside within minutes. Open the vents and let the air exchange before you enter, and do not work inside in the hottest hours.
What’s Next
- Learn next: Shading Strategies — cutting the load rather than only removing it
- Combine with: Stack Effect and Thermal Chimneys — the buoyancy physics for buildings