Greenhouse Fan Calculator
Work out the exhaust fan size your greenhouse needs to stay cool in summer. Enter your greenhouse and your climate to get the CFM, the intake opening and fans that fit.
How it works
How to size a greenhouse exhaust fan
A greenhouse heats up fast in the sun. The standard rule from university extension services is that an exhaust fan should replace all the air inside once every minute in summer. Mild climates can get by with less, and hot, sunny climates need more.
Fan CFM = Greenhouse volume (cubic feet) × Air changes per minute
Volume is the floor area times the average height. For a 10 x 12 ft greenhouse with 6 ft walls and an 8 ft peak, that is 10 × 12 × 7 = 840 cubic feet, so it needs a fan of about 840 CFM.
Intake opening: fresh air has to get in as fast as the fan pushes it out. Allow at least 1 sq ft of intake for every 250 CFM, using a louvered intake shutter or an open window on the far end.
No power? Roof and side vents that open to about 20% of the floor area can cool a hobby greenhouse on their own, especially with automatic vent openers and shade cloth.
Questions
Greenhouse ventilation FAQ
What size fan do I need for my greenhouse?
Multiply the floor area by the average height to get the volume in cubic feet. That number is the CFM you need to change the air once a minute. A 10 x 12 ft greenhouse with an average height of 7 ft needs about 840 CFM.
Where should the exhaust fan go in a greenhouse?
Mount the exhaust fan high on one end wall, on the side away from the prevailing wind. Put the intake shutter low on the opposite end so the air sweeps across all your plants.
Do I need an intake shutter with an exhaust fan?
Yes. Without an opening for fresh air, the fan cannot move its rated air. An intake shutter opens when the fan runs and closes to keep heat in when it stops.
Should a greenhouse fan run all the time?
No. Use a thermostat to run the exhaust fan when it gets too warm, usually above 75 to 85°F. A small circulation fan can run all the time to keep air moving and prevent mold.
More tools
Other greenhouse calculators
- Heater Size CalculatorHow many BTUs it takes to keep your greenhouse warm on the coldest night.
- Heating Cost CalculatorWhat a winter of heating will cost with electric, propane or natural gas.
- Snow Load CalculatorHow much the snow on your roof weighs and when to brush it off.
- Foundation CalculatorConcrete, gravel, pavers and lumber for your greenhouse base.
- Greenhouse Size CalculatorHow big a greenhouse you need for the plants you want to grow.
- Shade Cloth CalculatorThe right shade percentage and cloth size for your greenhouse.
- Grow Light CalculatorHow many grow lights it takes to give your plants enough light in winter.
- Rainwater CalculatorHow much water your greenhouse roof can collect for your plants.
Want a second opinion?
Tell us about your greenhouse and where you live, and we will help you get it right.
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