Greenhouse Vent & Fan Sizing

Natural roof + sidewall vent area, exhaust fan CFM, and intake shutter sizing. Sized for your climate, glazing, and crop.

Calculate ventilation needs

Greenhouse

Length (ft)

Width (ft)

Average height (ft)

Climate & crop

Climate / peak summer Cool (Pacific NW; peak ~75°F) Moderate (NE/MW; peak ~85°F) Hot (South/SW; peak ~95-105°F) Desert (peak ~110°F+)

Crop type Leafy / cool (lettuce, herbs, transplants) Fruit-bearing (tomato, pepper, cucumber) Cannabis Nursery / propagation

Method preference

Ventilation strategy Natural / passive (roof + sidewall vents) Mechanical exhaust fans + intake shutters Hybrid (passive primary + supplemental fans) Evaporative cooling pad + fan wall

Air-changes-per-hour reference

Climate / scenario	Recommended ACH	Notes
Winter (heat retention)	0.5-1.0	Just enough to remove humidity; keep heat
Spring / fall	5-15	Moderate ventilation; passive sufficient
Summer cool climate	30-45	Passive often adequate
Summer moderate climate	45-60	Mechanical helps
Summer hot climate	60-90	Mechanical required
Summer desert	90+ + evap cooling	Cannot keep cool without evap or shade

Natural ventilation: the rules

Rule	Detail
Total vent area	15-25% of floor area for a balanced gable greenhouse
Roof vent : sidewall vent	1:1 ratio (equal area at top and side)
Continuous ridge vent	Most effective; runs full length of greenhouse
Sidewall vent height	3-4 ft tall along full length, 18" above bench/floor
Wind effect	5-15 mph wind drives natural ventilation effectively; calm days fail
Stack effect	Hot air rises; needs vertical separation between intake and exhaust (the higher the better)
Hoop houses	Need roll-up sidewalls + endwall doors; inadequate at peak summer

Mechanical fan sizing

Climate	CFM target / sq ft floor
Cool climate or supplemental	~6-8 CFM/sq ft
Moderate climate	~8-10 CFM/sq ft
Hot climate	~10-12 CFM/sq ft
Desert / cooling pads	~12-15 CFM/sq ft

Two-stage thermostat ideal: Stage 1 (small fan) at +5°F over setpoint; Stage 2 (full bank) at +10°F. Single-stage fans waste energy in shoulder season.

Evaporative cooling reality

An evap cooling pad + fan wall can drop greenhouse air temperature 15-25°F below ambient — but only when humidity is low. In humid climates (East Coast, Gulf), the cooling effect is small because incoming air is already humid.

Outside conditions	Realistic temperature drop
95°F / 30% RH (Phoenix typical)	~20°F drop → 75°F inside
95°F / 50% RH (typical Texas summer)	~12°F drop → 83°F inside
90°F / 70% RH (Florida / East Coast)	~5°F drop → 85°F inside
85°F / 90% RH (humid Gulf)	~2°F drop → 83°F inside (negligible)

Free under CC BY 4.0. Cite as "OAT Greenhouse Vent Sizing (openagriculturetechnology.com)". ACH and CFM standards from ASABE and university greenhouse extension publications.