Greenhouse Ventilation Calculator
Estimates the fan capacity (CFM) required to maintain a target temperature inside a greenhouse based on its footprint and the temperature differential with outside air. Use it when sizing exhaust fans or designing a new ventilation system.
About this calculator
Proper greenhouse ventilation removes excess heat generated by solar gain before it stresses plants. The airflow formula used here is: CFM = (length × width × ventilationType × max(1, (outsideTemp − insideTemp) / 10)) / 60. Length and width define the floor area in square feet. The ventilationType coefficient scales airflow requirements for different system designs (e.g., natural vs. mechanical). The temperature differential term, max(1, ΔT/10), boosts required airflow when outside air is significantly cooler than the target inside temperature, reflecting the greater cooling capacity of cold incoming air. Dividing by 60 converts the volume-per-minute basis to a practical CFM figure. The industry rule of thumb is to exchange greenhouse air once per minute during peak summer conditions, making CFM roughly equal to the greenhouse volume.
How to use
Example: A greenhouse is 100 ft long × 30 ft wide. The desired inside temperature is 80 °F and outside temperature is 95 °F, giving ΔT = 95 − 80 = 15 °F. VentilationType = 8 (mechanical exhaust). Step 1 — Compute the temp factor: max(1, 15/10) = max(1, 1.5) = 1.5. Step 2 — Multiply: 100 × 30 × 8 × 1.5 = 36,000. Step 3 — Divide by 60: 36,000 / 60 = 600 CFM. You would need fans rated for at least 600 CFM to maintain 80 °F under these conditions. Always round up and add 10–15% for safety margin.
Frequently asked questions
How many air exchanges per minute does a greenhouse need in summer?
The widely accepted standard is one complete air exchange per minute during peak summer heat. This means fan capacity in CFM should approximately equal the greenhouse volume in cubic feet. In very hot or humid climates, growers sometimes target 1.5–2 exchanges per minute. Insufficient exchange allows heat and humidity to accumulate, promoting fungal diseases and heat stress. The ventilationType coefficient in this calculator lets you adjust for system efficiency and greenhouse design.
What is the difference between natural and mechanical greenhouse ventilation?
Natural ventilation relies on ridge vents and side openings to create a convective chimney effect, moving warm air upward and out. It is low-cost and energy-free but depends on wind and temperature differentials, making it unreliable during calm, hot days. Mechanical ventilation uses motorized exhaust fans paired with inlet louvers to force a controlled air exchange regardless of outside conditions. Most commercial greenhouses use mechanical systems as primary cooling and reserve natural ventilation as a supplement, especially above 5,000 sq ft.
Why does the temperature difference between inside and outside air affect required fan size?
Cooler outside air holds more potential to absorb heat before reaching the target temperature, so each cubic foot of cool air removes more BTUs than warmer air would. When the differential is small (outside nearly as warm as inside), you must move more air to achieve the same cooling effect. The formula captures this with the max(1, ΔT/10) term, which scales up airflow requirements as the differential grows. In practice, this means that on mild days with small differentials, your fans must run closer to full capacity than on cool days.