Proper ventilation is the difference between a thriving greenhouse and a disease-ridden hot box. Plants need fresh air for CO₂ exchange, consistent temperatures for healthy growth, and moderate humidity to avoid fungal problems. This guide covers the fundamentals you need to get it right.
Three problems arise without adequate airflow. First, temperature spikes: an unventilated greenhouse can exceed 120°F on a sunny day in minutes, cooking your plants. Second, humidity buildup: warm, still, moist air is the perfect incubator for botrytis (gray mold), powdery mildew, and damping-off disease. Third, CO₂ depletion: plants in a sealed space can consume the available CO₂ within hours, stalling photosynthesis even under bright light.
Roof vents exploit natural convection: hot air rises and exits through the peak, drawing cooler air in through lower openings. They’re the most important single ventilation feature. Automatic wax-cylinder vent openers (brands like Univent and Bayliss) open and close based on temperature with no electricity required—a must-have accessory.
Sidewall louvers at or below bench height serve as air intakes. The combination of low intake and high exhaust creates natural stack-effect ventilation that works even on calm days.
Door positioning matters: on breezy days, opening a door creates cross-ventilation. If your greenhouse has doors on opposite ends, the natural draft can move significant air volume.
Exhaust fans provide reliable, controllable air exchange regardless of wind conditions. Mount the exhaust fan at one end and a louvered intake at the opposite end. Size the fan for one complete air exchange per minute in summer (greenhouse volume in cubic feet = required CFM). In winter, throttle to 2–3 air exchanges per hour using a variable-speed controller or two-stage thermostat.
Circulation fans (HAF) don’t exchange inside/outside air—they circulate what’s already inside. This eliminates temperature stratification (hot at the ceiling, cold at the floor), distributes CO₂ evenly, reduces leaf moisture, and strengthens stems through gentle air movement. Mount above crop height in an oval or circular pattern. Plan one HAF fan per 500–800 sq ft.
| Season | Goal | Approach |
|---|---|---|
| Spring | Prevent overheating on sunny days, maintain cool-season temps | Auto vent openers + door cracking on warm days |
| Summer | Maximum cooling, prevent heat stress | Exhaust fan at full speed + shade cloth + all vents open |
| Fall | Reduce ventilation gradually as temps drop | Exhaust fan on thermostat, close lower vents at night |
| Winter | Minimize heat loss, control humidity | Brief midday venting (10–15 min) for humidity relief; HAF fan only |
Undersizing the exhaust fan. A fan that’s too small can’t keep up on hot days. Always size for one air exchange per minute and choose a variable-speed model so you can throttle down in cooler weather.
No intake vent. An exhaust fan without a matching intake opening just strains the motor against negative pressure. Provide at least 1.25× the exhaust fan area in intake vent opening.
Skipping winter ventilation. It’s tempting to seal everything tight in winter, but humidity builds fast in a heated greenhouse. A brief daily venting cycle (even 10–15 minutes midday) flushes excess moisture and prevents condensation problems.
For specific fan and vent opener recommendations, see our Best Greenhouse Fans buying guide.