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If you're running a hoop house or high tunnel, you already know: summer heat can turn your greenhouse into an oven. Even in spring and fall, a few sunny days can push temperatures past the point where your plants stop growing. A good ventilation system—specifically an exhaust fan and intake shutter—is the difference between a thriving crop and a wilted one.
But figuring out how big a fan you need, and how much intake area, feels like a puzzle. Most guides online throw around formulas without explaining them. So let's break it down: how to size your exhaust fan and intake shutters for your specific greenhouse, in plain English.
Plants need fresh air. They also need a stable temperature range—usually between 60–80°F for most crops. Without enough ventilation, the air inside a greenhouse can get stale, humid, and way too hot, which invites mold, pests, and stunted growth.
An exhaust fan pulls hot air out of the greenhouse, and that creates a negative pressure that draws fresh, cooler air in through the intake shutters. It's a simple system, but only works if both sides are sized appropriately.
The standard recommendation from greenhouse experts (like the University of Vermont Extension) is to ensure your greenhouse achieves at least 10 air changes per hour. For hot climates or dense crops, you might want up to 20 air changes per hour.
To calculate the CFM (cubic feet per minute) you need, follow these steps:
That's your minimum exhaust fan capacity. But remember, lower-powered fans lose efficiency against static pressure (like the resistance from ducting or louvers). So it's wise to choose a fan rated at least 20% above your calculation.
Your exhaust fan needs to pull air in from somewhere. Intake shutters are usually placed on the opposite wall, lower to the ground, so the incoming air sweeps across the plants. If your intake is too small, the fan will struggle to pull air and you'll lose efficiency. The rule of thumb: for every 1,000 CFM of exhaust fan capacity, you need about 1 square foot of intake opening area. However, most shutters have a certain net open area (the actual space air can pass through after accounting for louvers and frame). A typical aluminum shutter might have a 70-80% open area.
Let's say you choose a fan that does 1,200 CFM. You'll need approximately 1.2 square feet of theoretical intake. But to account for the shutter's net area, you'll want to size up: divide 1.2 by 0.7 (if that's the open area percentage) = about 1.7 square feet. So your shutter should be at least 1.7 square feet. In practice, it's often easier to go bigger, because you can always block off part of the shutter if needed, but you can't easily increase size later.
Let's say you have a 20' × 30' hoop house, 8' high at the peak, like the Bootstrap Farmer Round All-Metal High Tunnel Kit. Using our formula, you'd need around 600 CFM for 15 air changes per hour. That means a fan around 1,200 CFM would give you a comfortable buffer (about 30 air changes per hour, which is great for summer). For a fan of that capacity, you'd want an intake shutter of at least 1.7 square feet of clear opening.
Bootstrap Farmer's Greenhouse Aluminum Intake Shutter is a workhorse passive vent. It's designed to fit perfectly on standard greenhouse framing, with a weather-resistant aluminum build that will last as long as your hoop house. You can install it on the end wall opposite the exhaust fan, low to the ground, to create a cross-draft.
For the exhaust side, a quality inline duct fan like the AC Infinity CLOUDLINE PRO T6 can be adapted for greenhouse use, but you might also look at dedicated greenhouse exhaust fans. The key is to match the CFM rating to your calculated needs, and don't be afraid to overshoot a little for hot climates.
If you have a large hoop house, you might need multiple shutters. A good rule is to have at least 1 square foot of intake for every 5 feet of exhaust fan diameter, but again, calculate based on the CFM.
The best time to plan your ventilation is before you install your greenhouse or high tunnel. But retrofitting is possible, and these principles apply no matter when you add it.
Take it from growers who've been through it: a proper ventilation system pays for itself in healthier plants and higher yields. Use the formula above to calculate your CFM needs, choose an exhaust fan with a bit of headroom, and match your intake shutter accordingly. That's the secret to keeping your greenhouse comfortable even in the dog days of summer.
