Ventilation in Greenhouses and High Tunnels

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Growing plants in a safe place is possible with greenhouses and high tunnels. In fact, high tunnels can make the growing season last longer. But climate control needs to be thought about in these partially enclosed areas. Adequate temperature, ventilation, and airflow are essential aspects that help keep the conditions healthy for your plants.

Understanding Air Flow in Greenhouses: Ventilation and Circulation

Greenhouses and high tunnels need two types of airflow: ventilation and circulation. Here's what you need to know about these two:


Ventilation works on the principle of swapping the indoor air with the outdoor air. This kicks out the warm and moist air from the inside and brings in cooler and drier air from the outside. Plants gobble up carbon dioxide for photosynthesis, so ventilation ensures they have a fresh supply. There are many ways to achieve this:
  • Using exhaust fans and inlet louvers
  • Having roll-up sides
  • Using ridge vents
  • Big doors on both ends of the structure


Circulation involves stirring the air inside the greenhouse to make sure it's even everywhere. The goal of proper circulation is to make sure every nook and corner of your growing space gets the same conditions. This is mainly done using horizontal air flow (HAF) fans. These fans swirl the air around, ensuring that even those tricky corners get some air movement.

Balancing the Two

Just circulating air without ventilating can trap plants in hot and moist conditions. This can stress them out and even cause disease. On the other hand, if you only ventilate without circulating, some parts of your greenhouse, like the corners, might miss out on the benefits of fresh air. You should aim for the perfect combo: adequate ventilation plus proper circulation.

Greenhouse Air Flow: Active vs. Passive Ventilation

If you're setting up a greenhouse or high tunnel, it's essential to think about how you'll ventilate it. You have two main choices: active or passive systems.

Active Systems

This is the "powered-up" version. It uses electric fans that need electricity to run. A typical setup might have two exhaust fans at one end, two inlet louvers at the opposite end. It will also have a thermostat that springs into action, opening louvers and powering fans when temperatures soar above 85°F. But remember, these systems need electricity. Hence, where you place your greenhouse or tunnel matters.

Passive Systems

These are more laid-back. They let nature do the work. By having openings in the structure, air flows in and out naturally. They're often favored for their lower initial costs and energy efficiency, not to mention that they don't require an electrical setup. As a result, with passive systems, you'll have more flexibility in terms of location. However, they rely on gentle winds to work best. So, avoid placing them near big buildings, thick trees, or anything that might block the natural breeze.
In short, choose active if you have electricity and want automated airflow. Go passive if you want a natural, energy-saving solution, but be smart about where you set it up!

Airflow Basics

If you're diving into greenhouse gardening, understanding airflow is essential.

Ventilation Flow Rates

During warmer months, your target is eight cubic feet per minute (CFM) per square foot of your growing space. Meanwhile, in a cooler season, aim for 2 CFM/ft². So, if you have a 30 ft x 96 ft tunnel (around 2,900 ft² of growing space), you'd need 23,200 CFM in hot times and 5,800 CFM when it's cooler. And if you grow in both seasons, consider using a two-stage thermostat to control two fans or adjust the rate with variable speed control.

Circulation Flow Rates

For circulation, it's a bit different. You'll want your flow rate to be a quarter of your greenhouse's total volume each minute. Using our same 30 ft x 96 ft tunnel that's 15 ft tall, it's roughly 34,000 cubic feet in volume. So, you'd need an airflow of about 8,500 CFM for circulation.

The Right Equipment for Your Greenhouse Systems

Ventilation Fans and Their Buddies, Inlet Louvers:

Have you seen those big fans in greenhouses? They're called exhaust ventilation. You'll find exhaust ventilation fans in sizes from 12" to 48" in diameter. Depending on the size and type, they can cost anywhere from $150 to $1,000. These fans move air at a rate of 1,000 to 22,000 CFM.
You can find these fans in a few styles:
  • Direct drive or belt-driven
  • Single, dual, or even variable speed
Some fancy setups have multiple fans with different temperature settings for various growing seasons.
Here's a fun fact: these fans usually sit on one end of the greenhouse, and right across from them on the other end are inlet louvers. Louvers help create a sweeping airflow across your greenhouse. The golden rule is to size your louvers with an area equal to the exhaust fans to maintain balance. And for a smooth operation, motorized louvers, preferably with a flange for solid sealing, are recommended. They seal better, which means fewer drafts and more energy savings. Expect to shell out between $50 and $250 for each louver.
By the way, motorized louvers can automatically open up when you switch on those fans, letting fresh air rush in. It's all connected in one smooth system!

Adjustable Sides for Greenhouses: Roll-up or Fold-down Sides

Have you ever thought about how cool it would be if your greenhouse walls could move? Roll-up sides make that possible!
Roll-Up Sides: Roll-up sides can be used as both entry and exit points for air flow depending on where they are placed, the use of other vents, and the direction of the wind. Usually, sheets of plastic that run the length of the tunnel and are rolled up around a pipe are used. You can roll the steel by hand with a simple pipe and tee or use a gear hand crank. There are also motorized roll-up motors with controls that can do this job automatically.
Fold-Down Sides: Are you not into rolling up? No worries, there's another nifty option. You can also get the same side vents with a system that folds down or rolls down. Fans that "blow up" the side curtains close them, and ropes and pulleys are used to raise or lower the side walls on some of these.

Easy Roof Ventilation: Ridge and Peak Vents

Hot, damp air can leave through ridge vents and peak vents. Ridge vents are different because one side ends higher than the other. Peak vents, on the other hand, run the length of a house and have a spooky look.
Instead of exhaust fans that make noise, these roof vents are quiet and save energy. They let in just the right amount of air when paired with roll-up sides. And here's a bonus tip: Putting these vents in when the tunnel or greenhouse while it's being built is usually the easiest and least expensive way to do it. What's cooler is that these vents can be adjusted help of motors and a fantastic system called a rack and pinion. You can change them based on the weather and your wants!

Gable Vents

Have you ever seen those holes near the top of some house walls? They are what we call gable vents. When paired with roll-up sides, these vents make it easy and quick for fresh air to flow through a house. It is a simple and natural way to keep the inside cool!

Horizontal Air Flow (HAF) Fans

Horizontal Air Flow (HAF) fans are a new kind of fan usually 12" to 24" wide. Based on the type, they can range in price from $80 to $300 each. Additionally, they are strong and can move air at speeds of 1,000 to 5,250 CFM.
Smart growers have found a way around the problem. Box fans like the ones you might have at home are used. They might have to buy more of these box fans and change them more often, but they may be less expensive than regular HAF fans.
However, there is a catch: fans made by HAF move air a certain distance, which is known as the "throw." If you want to ensure the air mixes well everywhere, you should put these fans about every 50 feet. However, it's better to place them every 20-30 feet in places with lots of plants like tomatoes and cucumbers.
Most of the time, these fans are arranged in a loop or "race track" style to ensure air moves everywhere. However, this method is not foolproof. This setup can leave a couple of corners with not much air movement. Before setting everything in stone, it's a good idea to test the fan placement.

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