I never thought greenhouse ventilation would be important. I’ve always thought a greenhouse would be helpful when it comes to growing my own vegetables, but there are so many questions when it comes to greenhouses, including whether they need ventilation. I decided to do a little research to find out.
Do greenhouses need ventilation? Yes, greenhouses need ventilation. Ventilation is actually one of the most important parts of a greenhouse. Here are the main things a ventilation system does to keep plants healthy.
- Regulates the temperature
- Regulates humidity
- Provides carbon dioxide
Now that you know greenhouse ventilation is important for healthy plant growth. Let’s take a closer look at how they do this and help you figure out the best ventilation system for your greenhouse.
Reasons Why Greenhouse Ventilation is Important
Greenhouse ventilation is important for more than just the obvious reasons I mentioned at the beginning of this article. Here’s a list of all the ways ventilation works to keep your plants healthy.
- Controls Temperature
- Controls Humidity
- Provides Carbon Dioxide
- Prevents Pests
- Helps with Pollination
Controls temperature: While everyone thinks of sunlight being super important for greenhouses, too much heat without an escape route can cause problems for your plants.
This can happen even in cooler weather. If heat is trapped in the greenhouse, it raises the temperature, which is bad for plant growth and can cause plants to wilt or even die. A ventilation system helps remove any extra heat and allows your plants to do well.
Controls Humidity: Humidity can also build up in your greenhouse. Water comes off plants and if the water isn’t removed, it builds up in your greenhouse and causes high humidity.
While some plants do well with humidity, most can quickly fall victim to all the nasty things that like a humid environment, things like mold, fungus, and mildew. Ventilation helps control humidity by pushing the moist greenhouse air out and bringing in drier, outdoor air.
Keeping weeds off the floor of your greenhouse also helps reduce humidity because weeds hold a lot of moisture.
It might be a good idea to buy a device that measures humidity. I found this thermometer on Amazon that measures both temperature and humidity.
I could go all sciency on you, but basically, it’s important to understand that temperature and humidity are directly related so it’s important to monitor both.
Here’s a look at what percentage humidity you should keep your greenhouse at based on temperature.
| Degrees Fahrenheit | Humidity |
| 50° | 83% |
| 61° | 89% |
| 68° | 91% |
| 86° | 95% |
Provides Carbon Dioxide: If you’re not supplying carbon dioxide artificially, you need ventilation in your greenhouse to provide enough for your plants to grow. A ventilation system does this.
Plants need carbon dioxide to complete photosynthesis. This is the process where they turn carbon dioxide, water, and sunlight into glucose, which they need to survive.
If the amount of carbon dioxide goes down, the rate of photosynthesis also decreases and your plants don’t grow as well.
Prevents Pests: A ventilation system also helps prevents common greenhouse pests from invading your plants. Pests multiply in warm, humid areas so a ventilation system that keeps temperature and humidity down is key.
Another way to manage pests is to know the common greenhouse ones, so you can keep your eye out. Here are some of the most common greenhouse invaders.
- Aphids
- Gnats
- Shore flies
- Bloodworms
- Thrips
- Whiteflies
- Leafminers
- Mealybugs
- Two-spotted spider mites
- Cyclamen mites
- Slugs and snails
Helps With Pollination: The environment takes care of pollination outside of a greenhouse – a wind blows or a buzzing bee hops from flower to flower doing its job to pollinate. Inside a greenhouse, though, there is no wind.
A ventilation system with good fans helps shake the plants and allows pollination to happen. A sign of a pollination problem is if you notice the plants in your greenhouse aren’t producing fruit.
If you want to grow the best, most fruitful plants possible, a good ventilation system is key to control all the environmental factors I just talked about.
Types of Ventilation Systems for Greenhouses
Now that you know you need a greenhouse ventilation system, you have to figure out what type of ventilation system you want to set up. There are two basic types of ventilation systems: natural or mechanical.
Here’s a quick look at the basics of both systems.
| Natural Ventilation | Mechanical Ventilation |
| No powered fans | Uses fans |
| Relies on wind | Can be automated |
| Relies on thermal buoyancy | Sealed environment = no pests |
| Less control | More control |
Natural Ventilation
As you can see from the table above, natural ventilation in a greenhouse is just as it says – natural. It doesn’t use any fans, but instead uses wind and thermal buoyancy to move the air. Thermal buoyancy is based on the fact that when air is heated, it naturally rises.
Most people who use natural buoyancy have a series of roof vents as well as vents on the sides of your greenhouse. When the temperature inside the greenhouse rises, the hot air also rises and goes out the vents in the roof.
As the hot air goes out of the roof vents, a vacuum is created, which brings in cooler air through the side vents. Wind blowing over the top of your greenhouse can also help pull cooler air in through the side vents.
If you go the natural ventilation route, you might consider buying and installing a solar powered vent opener. These automatically open the vent when the paraffin wax inside the mechanism expands due to the rise in temperature.
Solar powered vent openers don’t require any electricity or batteries, so it saves you time. Here’s a look at one solar powered vent opener I found on Amazon.
Mechanical Ventilation
Mechanical ventilation uses fans or other devices to keep air circulating through your greenhouse.
With a mechanical ventilation system, you have much more control over the circulation in your greenhouse. You can decide when to run the system and at what power. A mechanical ventilation system also keeps pests away from your plants because the greenhouse is sealed.
Pros and Cons
You’ll have to weigh the pros and cons of each type of ventilation system to determine which is best for your greenhouse.
Obviously, a natural ventilation system doesn’t cost anything except the labor involved in making the vents. A mechanical ventilation system, on the other hand, costs any time you use it on top of the original cost of the purchase.
With a natural ventilation system, you’re at the mercy of the weather. Little wind means less ventilation. With a mechanical system, you control when it runs.
Choosing the Right Spot and Size for Your Ventilation System
The size and location of your ventilation system depends on whether you pick a natural ventilation system or a mechanical ventilation system.
Natural Ventilation: If your greenhouse is made of a more flexible material, just rolling up a portion of the wall may generate enough airflow. If it’s built with more rigid materials, though, you’ll have to put in a little more work to ensure you have good ventilation.
Ridge or roof vents and sidewall vents are critical to a successful natural ventilation system. The total combined roof vent area should be equivalent to the total combined sidewall vent area. Each area should equal around 20-percent of the floor area. Take a look at this table to see some examples.
| Greenhouse size | 20% of square footage | Roof Vents Area | Side Vent Area |
| 10 ft X 30 ft= 300 ft | 20% of 300 = 60 ft | 60 sq ft | 60 sq ft |
| 20 ft X 40 ft= 800 ft | 20% of 800 = 160 ft | 160 sq ft | 160 sq ft |
| 25 ft X 35 ft= 875 ft | 20% of 875 = 175 ft | 175 sq ft | 175 sq ft |
If you choose natural ventilation, it’s a good idea to have a circulation fan in your greenhouse to ensure the air reaches all your plants. Any fan similar to this aluminum one I found on Amazon should work.
Mechanical Ventilation: Before you decide what size fan you need for mechanical ventilation, you need to decide whether you’ll use your greenhouse all year or just seasonally. If you’ll use it all year, base the fan size on the temperature of the hottest months.
If you go this route, though, make sure the equipment is controlled either by a thermostat or that you have more than one option for fan speed. This way you can use less intense ventilation during cooler months.
Ventilation fans are rated based on the air volume they move related to cubic feet per minute. You want to choose a fan for your greenhouse ventilation system that is rated for one air change per minute. This means you need to figure out the volume of your greenhouse in order to know which fan to buy.
Time to put your math cap on and work your brain a little. Volume is figured out by multiplying the length of your greenhouse by the width by the height. Take a look at this table for some examples.
| Greenhouse Dimensions | Volume | Fan Cubic Rating |
| 30 ft X 10 ft X 10 ft | 3,000 cubic feet | 3000 cubic ft per minute |
| 20 ft X 17 ft X 10 ft | 3,400 cubic feet | 3,400 cubic ft per minute |
| 40 ft X 20 ft X 10 ft | 8,000 cubic feet | 8,000 cubic ft per minute |
Once you know the volume of your greenhouse, you can choose a fan with that number as the minimum. For example, if the volume of your greenhouse is 3,000 cubic feet, choose a fan with a rating of 3,000 cubic feet per minute or greater.
You can see that the rating on this exhaust fan from Amazon is 3,000 CFM, which stands for 3,000 cubic feet per minute.
Since heat rises, it’s best to place your exhaust fan toward the top of your greenhouse. Even with the manual ventilation system, placing a circulating fan across the greenhouse on the ground helps circulate air in and out.
Using Your Ventilation System During Other Seasons
If you’re using your greenhouse all year, it’s important to modify your ventilation system to account for the changes.
In the winter, you might even need an outside heat source to moderate temperatures in your greenhouse. Humidity becomes a problem because of the difference between the temperatures outside and the temperatures inside your greenhouse.
The difference in temperatures creates extra humidity. You don’t want to run fans too much because you don’t want to lower the temperatures too far, so a good guideline to follow is three air changes per hour rather than one per minute.
This means you take the volume of your greenhouse and multiply it by .05. Here’s our table from the example above with new numbers to reflect three air changes per hour.
| Greenhouse Dimensions | Volume | Fan Cubic Rating |
| 30 ft X 10 ft X 10 ft | 3,000 cubic feet | 150 cubic ft per minute |
| 20 ft X 17 ft X 10 ft | 3,400 cubic feet | 170 cubic ft per minute |
| 40 ft X 20 ft X 10 ft | 8,000 cubic feet | 400 cubic ft per minute |
You can achieve these rates either by buying a smaller fan or turning the dial down on your current fan if it has multiple speeds.
In the fall and spring, your rate of ventilation will fall somewhere between the amount you use it in the summer to remove extra heat and the amount you use it in the winter to control humidity. This is where the thermometer that measures temperature and humidity we mentioned above comes in handy.
Heating Options
As I mentioned above, you might need to consider heating your greenhouse in the winter to maintain a good temperature for your plants.
The ideal temperature for photosynthesis to happen is 77 degrees Fahrenheit, but many plants do well with a daytime temperature anywhere between 70 and 80 degrees Fahrenheit.
Ideal nighttime temperatures can vary a bit depending on which plants you’re growing in your greenhouse. Here’s a look at some plants and the best nighttime temps for them.
| 50° Fahrenheit | 65° Fahrenheit | 70° Fahrenheit |
| AzaleasDaisiesCarnationsBeetsCarrotsLettucePrimroseSweet Pea | RosesTomatoesPoinsettiasLiliesGeraniumsGardeniasDaffodilsAfrican Violets | Tropical plants |
Here’s a look at some of the types of heaters you can choose to use to heat your greenhouse.
- Conventional fuel-based heaters
- Electric space heaters
- Portable space heaters
- Oil-filled radiant heaters
- Ceramic space heaters
- Infrared space heaters
Conventional fuel-based heater: A conventional fuel-based heater may use kerosene, natural gas or wood to operate. These are best for larger greenhouses because they emit higher levels of carbon dioxide. You also need to make sure you have plenty of ventilation.
Electric space heater: An electric space heater is safe and easy to use. They’re best for smaller greenhouses because you would probably need a few of them to heat a larger greenhouse.
Portable space heater: Portable space heaters are convenient because they are lightweight and easy to move wherever you might want them.
Oil-filled radiant heaters: An oil-filled radiant heater uses oil and electricity, which cuts down a bit on your electricity cost. They’re also usually on wheels, which makes them easy to move.
Ceramic space heater: Ceramic space heaters produce their heat in a tiny box. These heaters are cheap and fairly energy efficient. The unit also doesn’t get hot to the touch. They’re best for smaller greenhouses.
Infrared space heater: Infrared space heaters directly heat up objects in a room, in this case, your plants. The air in the greenhouse then becomes warmer from the heated plants.
After looking at the different available heat sources, it’s best to consider the space you need to heat and your budget to make the best decision on your heat source.
Cooling Options to Help With Temperature Control
A ventilation system combined with some other cooling options helps maintain the temperature of your greenhouse in the warmer months.
Here are a few options for cooling your greenhouse.
- Misting System
- Fogger
- Evaporative Cooler
- Shade Covers
Misting system: A misting system does exactly what it says. It puts out a fine mist that lowers the temperature in the greenhouse as it evaporates.
This system does have a downside, as the misted water may not always evaporate and could cause higher humidity. A misting system like this one on Amazon would work.
Fogger: A fogger operates in the same way as a misting system. It puts fog particles out into the environment that remain there until they evaporate. The particles it puts out are usually slightly smaller than the water from a misting system. Here’s one on Amazon. (https://www.amazon.com/dp/B07KFFVCGB?tag=farmandgard01-20)
Evaporative cooler: An evaporative cooler is a closed system that stores water inside a cooler and then puts the cooled air out into the greenhouse through an exhaust system. Here’s an evaporative cooler I found on Amazon.
Shade covers: Shade covers are frequently used to keep greenhouses cooler. A shade cover blocks part of the sun’s rays from entering the greenhouse. This can reduce the temperature by 20- percent. This is a fairly inexpensive option as you can see from these shade covers on Amazon.
Keep in mind; even with these cooling systems, you’ll still need a good ventilation system to really control the temperature and humidity in your greenhouse. Good ventilation equals healthy, productive plants.