Quick start
Fill the greenhouse size and vent details, choose a method, then calculate. When results appear, you can export them as CSV or PDF.
Calculator
Example data table
| Scenario | Inputs (metric) | Typical outcome |
|---|---|---|
| Warm tunnel | 20 m × 8 m × 3 m, 2 sides, 1.2 m opening, wind 2.5 m/s, ΔT 6°C, Cd 0.60 | Opening needed often 30–80% depending on screens. |
| Hot greenhouse | 30 m × 10 m × 4 m, 2 sides, 1.5 m opening, wind 1.5 m/s, ΔT 8°C, losses 35% | May require longer vents or larger opening height. |
| Mild shed | 12 m × 6 m × 2.6 m, 1 side, 0.9 m opening, wind 3.0 m/s, ΔT 4°C | Often meets needs with smaller vent ratios. |
Tip: run both methods to sanity-check results for your climate.
Formula used
1) Target airflow
If you choose air changes per hour (ACH), the target flow is:
Q = (ACH × Volume) / 3600.
If you choose flow per area:
Q = (FlowPerArea × FloorArea) / 3600.
2) Driving pressure
Wind component:
ΔPw = 0.5 × ρ × V² × Cp.
Stack component:
ΔPs = ρ × g × H × (ΔT / T).
Total:
ΔP = ΔPw + ΔPs.
3) Vent area from orifice flow
Q = Cd × Afree × √(2ΔP/ρ), so
Afree = Q / (Cd × √(2ΔP/ρ)).
Screen and obstruction losses reduce free area, and a safety factor increases the required opening.
How to use this calculator
- Pick your unit system and method.
- Enter greenhouse length, width, and mean height.
- Enter vent length per side, number of sides, and opening height.
- Set your ventilation target using ACH or flow per area.
- Add wind speed and temperature difference for better realism.
- Adjust Cd, losses, and safety factor if you use screens.
- Click Calculate, then export CSV or PDF if needed.
Practical guidance
Vent area planning for plant health
Side vents support temperature control, humidity removal, and CO₂ refresh. This calculator converts greenhouse size into floor area and volume, then estimates how much opening you need to reach a ventilation target. The “opening needed” percentage lets you see whether your design meets demand before investing in motors or framing changes.
Choosing a realistic airflow target
Two targets are offered. Air changes per hour (ACH) is useful when crop density or internal volume matters; higher values suit hot, sunny periods. Flow per floor area is helpful when you manage multiple houses with similar crops. Compare results from both targets to spot assumptions and avoid oversizing that increases drafts.
Interpreting wind and stack inputs
Wind creates pressure on the windward side and suction on the leeward side, pushing air through openings. The model uses wind speed and a pressure coefficient to estimate this effect. Stack pressure increases when inside air is warmer than outside; the effective stack height rises when a roof outlet exists, improving natural exchange. On calm afternoons, stack effect may dominate, so use a realistic ΔT rather than a best-case guess.
Accounting for screens and crop blockage
Insect netting, shade cloth, benches, and tall crops reduce free flow area. The calculator applies screen and obstruction loss percentages, then adds a safety factor to handle variability. Fine mesh can cut effective flow by 20–50% or more, especially when dusty or wet. If you routinely close vents partially for pest control, treat that as additional loss and rerun the numbers.
Turning results into build decisions
If the status shows insufficient opening, increase opening height, extend vent length, add the second side, or pair side vents with roof vents to boost stack effect. Record your chosen assumptions and export the summary as CSV or PDF for contractor quotes, maintenance logs, and seasonal tuning. When you automate vents, keep a manual override plan and inspect cables, gears, and end stops regularly to sustain airflow.
FAQs
1) What opening height should I design for?
Use the maximum safe opening your structure and roll-up system can maintain in wind. Larger height reduces required vent length. If you often run partially open, enter realistic losses and check the “opening needed” percentage.
2) Which method should I use: airflow-based or ratio guidance?
Use airflow-based sizing when you can estimate wind, temperature difference, and losses. Use ratio guidance for quick planning or early budgeting. Running both methods is a good sanity check before construction.
3) How should I model insect screens or netting?
Screens reduce effective free area, especially with fine mesh and dust buildup. Enter a screen loss percentage that reflects your material and maintenance. If you have double layers, increase the loss and rerun.
4) What wind speed value is most useful?
Use a typical on-site wind speed at vent height, not a storm peak. If your site is sheltered, choose a lower value and rely more on stack effect. For exposed fields, use a higher average.
5) Why does the roof vent option change results?
A roof outlet increases the vertical separation between inlet and outlet, strengthening stack-driven flow when inside air is warmer. That generally reduces the side opening needed on calm days and improves overall natural exchange.
6) What if the calculator shows more than 100% opening needed?
Your maximum opening cannot meet the target under the chosen conditions. Increase vent length, opening height, or number of sides, reduce losses, or combine side vents with roof vents or fans. Then recalculate and compare.