Should Greenhouses Have Ventilation? Garden Tips 2023

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We make greenhouses from many different types of material, from wood and metal to bricks and timber. Regardless of the type you opt for, the basic principle of greenhouses is that they provide growers with a controlled environment for successful plant production. Essential elements the plants need to grow are balanced temperature, humidity, and light.

There is no doubt that greenhouse ventilation is a vital component that will ensure your plants are going to thrive. The basic function of a ventilation system is to remove air from inside the greenhouse and replace it with clean air, usually from outside. But there are different ways of achieving this, some mechanical and others natural.

Do you need ventilation in a greenhouse?

Greenhouses have been used for growing plants for centuries. In essence, they are enclosed spaces, with glass or transparent plastic roofs and sides, that are heated naturally by the sun. 

There are many different types, but their basic nature makes it possible for us to control important variables so important for plant growth. These are primarily light, humidity, and temperature, and this is where ventilation comes in.

It’s vital to ensure that your greenhouse has proper ventilation. Apart from anything else, whatever material your greenhouse is made of, plastic or glass, it can overheat in hot, sunny weather. 

Passive ventilation

Every greenhouse will have natural ventilation in the form of single or double doors. They may also have windows that open and close allowing fresh air to enter the structure.

Also known as passive ventilation, natural ventilation relies on natural forces to circulate and move air in and out of the indoor space. These forces include wind and thermal buoyancy, which lets dense, cold airlift the warm air up and out of the greenhouse.

Commonly, in addition to doors and possibly windows, we can control ventilation by installing greenhouse vents. There are two types that are found on the sides of the greenhouse or on the roof.

Roof vents often run the full length of the ridge and they operate manually or with openers that are automated. Sidewall vents are often louvered.

If yours is a large greenhouse, a good rule of thumb is to ensure that you have at least 10 square feet of ridge (roof) ventilation for every 50 square feet of the floor area. This equates to 10 square feet of vents for every 20% of your floor space. 

The glass-to-floor area ratio in small greenhouses, with a floor area of less than 300 square feet, tends to be higher. This means that you’re going to need a much higher percentage of ridge ventilation. 

Heating Cooling Ventilation

Remember that a greenhouse ventilation system goes hand-in-hand with heating and cooling, not just the circulation of air. 

Greenhouse heating can be useful in very cold weather when heat escapes because of inadequate insulation. 

The most common form of greenhouse heating is via electric hot-air heaters that are used with greenhouse fans. Hydronic systems that work by circulating hot water are also an option. 

Importance of ventilation in greenhouses

The U.S. Botanic Gardens has produced a Greenhouse Manual for educators. It focuses on school greenhouses but has invaluable information for home use. 

Additionally, it discusses the importance of greenhouses as well as the importance of ventilating them. It also discusses different types, including the types of glazing materials we can use. 

Because greenhouses are designed to capture heat from the sun, you need to take steps to ensure that the greenhouse doesn’t overheat. Opening the door of a small greenhouse is sometimes enough, but it also allows pests to infiltrate the space. 

Shadecloth can be helpful on the inside of a greenhouse. It’s a simple way to ensure cool air but isn’t as effective as airflow fans. 

What you need for greenhouse ventilation

There are various types of ventilation systems that you can use for greenhouses. These include various vents as well as fans. 

Vents

We use vents for ventilation cooling and air circulation in greenhouse ventilation systems. They operate on the principle that heat is removed when wind and temperature create a difference in pressure. 

There are different vents that you can use for cooling greenhouse environments. These include both roof and sidewall vents.

As the wind blows over the roof it creates a vacuum that sucks the hot air from the vent. When it blows across open sidewall vents, cool air will enter the greenhouse and drop to the ground.

If the sidewall vents are closed, the cool air that comes in from the roof vent will fall and the hot air will escape through the roof vent. 

Roof vents should be located on both sides of the roof ridge. The American Society of Agricultural Engineers recommends that they should be about the same size as 15-20% of the floor area

Side vents are useful, but they don’t work as well as roof ventilation. Also, louvered vents are good for regulating airflow, but they are difficult to draught-proof when it gets cold in winter. 

Other vent options include automatic devices that open and close when greenhouse temperatures change. But generally, they respond slowly and need to be supplemented with other ventilation types.

These may be manual and as basic as opening windows and doors. Alternatively, you could opt for motorized vents, some of which are activated by heat sensors. 

Fans

Fans are a type of mechanical ventilation. They may be installed on walls or in windows or air ducts.  

Electric greenhouse fans are commonly used to circulate air inside greenhouses. Horizontal airflow fans (HAFs) also distribute the heat evenly and they reduce condensation that can result in disease on foliage. 

End-wall fans are quite large and can effectively exchange all the air in a small greenhouse with fresh, clean, outside air. They work best if the distance from the fan to the other side of the greenhouse is less than 150 feet. 

Evaporation cooling

This type of ventilation is ideal in areas that get extremely hot. Evaporation cooling combines a fan ventilation system with wet pads that are installed along an end or side wall. 

As the fan circulates, it draws air through the wet pads. One potential problem is the possibility of algae growth that reduces the effectiveness of this cooling greenhouse system. 

Misting the air also helps with evaporation cooling.

How ventilation is done in a greenhouse?

Ultimately, greenhouses rely on the air distribution that natural passive ventilation or mechanical ventilation provides. When using vents, the greenhouse ventilation system is reliant on wind pressure and thermal gradients. 

Mechanical ventilation is more reliable but obviously more costly. It also needs to be regularly maintained. 

However, for any greenhouse ventilation to be effective, you will need to monitor temperatures. You will also need to look out for signs that ventilation or shading is inadequate, and do something about it. 

Conclusion

Ventilation is essential when growing plants in greenhouses. You can rely on natural ventilation methods or install a mechanical system.

Essentially, the aim of ventilation is to circulate and replace stale air inside the greenhouse with fresh air from outside. Budget and physical constraints will determine what kind of ventilation you use.

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