For greenhouse ventilation, ridge and sidewall vents should each be 15%–20% of floor area, with fans sized near one air change per minute.
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The targets are simple once you know them.
Two approaches cover nearly every greenhouse. Natural ventilation relies on ridge and sidewall vents driven by wind and rising heat. Mechanical ventilation uses exhaust fans and intake louvers. The rules below come from university and industry extension guidance and apply to hobby and commercial houses alike.
How Much Vent Area Does A Greenhouse Need?
Natural ventilation works when the ridge and sidewall vents are sized as a matched pair, per ASAE guidance and university extension recommendations. Combined sidewall vent area should equal combined ridge vent area, and each should be 15%–20% of the floor area. A 20 ft × 48 ft house with 960 sq ft of floor therefore needs roughly 145–190 sq ft of ridge vent and the same again in sidewalls.
Ridge vents should be top-hinged, run the full length of the ridge, and open to about a 60° angle. In summer, orient the house so prevailing wind blows over the ridge; the wind then creates a vacuum on the leeward ridge vent that pulls air through the windward side opening. Keep the windward vent opening about equal to the leeward ridge vent opening.
Natural ventilation costs nothing to run, but it depends on wind and on a temperature difference between inside and out. On calm, hot afternoons it stalls, which is when mechanical exhaust becomes the dependable option.
Mechanical Ventilation System Design: Fan Sizing And CFM
Mechanical systems are sized to exchange the greenhouse air about once per minute in summer. UF/IFAS greenhouse ventilation guidance targets one volume air change per minute to an 8-foot height, with a 25 ft × 96 ft house requiring about 19,200 cfm. As a check, extension guidance also treats roughly 7–10 cfm per square foot of floor as a summer design parameter.
Winter needs far less: about 1/4 volume air change per minute is enough, and bedding-plant houses not used in summer can be sized to 3/4 volume. Select fans for the required volume at the expected static pressure, using the minimum horsepower that meets it — static pressure and space constraints change what actually moves. Many houses run a mixed system, natural vents for mild days and fans for peak heat, with each side sized independently so both rules still apply.
For a small hobby house, a solar-powered exhaust fan can handle summer airflow without new wiring.
| Design Parameter | Natural Ventilation | Mechanical Ventilation |
|---|---|---|
| Sizing rule | Sidewall area equals ridge area; each 15%–20% of floor area | One air change per minute; about 7–10 cfm per sq ft |
| Summer target | Wind over the ridge pulls air through windward vents | One volume change per minute to an 8 ft height |
| Winter target | Ridge vent cracked to shed humidity | 1/4 volume change per minute |
| Inlet placement | Windward opening about equal to leeward ridge opening | Louvers opposite the fans; both ends in long houses |
| Ridge vent design | Top-hinged, full ridge length, about 60° open | — |