How to vent a Greenhouse


Greenhouses are an excellent addition to the homes or allotments of UK gardeners. They allow us to fight off the effects of our very temperamental climate, to get ahead start on our crops, and help us build a controlled environment where we can better fight off pests and disease that might otherwise kill off our plants.

One thing that is often overlooked by people new to greenhouse growing, is correct ventilation. Without it, whatever you are growing probably isn’t doing so to its maximum potential. 

Without proper greenhouse ventilation, conditions inside can get very humid in the warmer months and this can act as a breeding ground for some fungal diseases, so you really should be aware of how to vent a greenhouse.

Most experienced greenhouse owners will use a mix of passive and active ventilation methods, and this will give them better overall control of the internal conditions, and also save them a little money in the process.

In this article we will explain the difference between passive and active ventilation, and also give some tips about how to best utilise each one in order to keep your plants and crops as healthy as possible.

Why do I need good ventilation for my greenhouse?

If your pants don’t get the fresh air that they need, they can’t photosynthesize, and if they can’t photosynthesize, they’re not going to live too long.

Fresh air coming in from vents is really important, but so is the circulation of the air in the greenhouse, and we suggest using fans for this purpose, just make sure that they are positioned in a way that will blow the air around in a way that will benefit all the plants inside the greenhouse.

Obviously temperature control is a huge part of gardening, especially when using a greenhouse. Plants all thrive in different temperatures, and having a well designed ventilation system can give you precise control over the level of heat inside the structure.
As already mentioned, pests such as bugs can get very cozy in a greenhouse that doesn’t have proper ventilation and will soon infest your plants. Using blowers near susceptible plants can ward off those pesky aphids, and also lower the risk of disease for your botanical wonders by preventing too much humidity building up.

Inadequate air flow in the greenhouse can actually cause your plants to stop bearing fruit. This is due to their being no pollination like what they would usually get from the natural winds outside. Again, the use of fans can act as a good substitute, and as the air circulates in the greenhouse, it will promote pollination.

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What is passive ventilation for greenhouses?

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Passive ventilation is just a phrase that refers to the traditional, natural, way of getting air into the greenhouse. This is done through opened windows, air vents, and the like.

We’d advise using solar openers for your air vents as they will take care of themselves when it comes to opening and closing, saving you remembering to do so. These openers work really well in summer, and will automatically let more air in when it’s hot and sunny.

Many greenhouses come with good sized ceiling vents or high up windows as well as smaller vents closer to the ground. In summer, you should always keep these vents open for the following reason:

As the cooler air enters the greenhouse from lower placed vents, it causes the warmer air to rise and escape through the larger vents and windows you have set up in higher positions. This is called thermal buoyancy, and can work really well if the weather conditions are right.

However, there is also a downside to this. Namely, relying on the right weather conditions.

As we all know, the weather over here is rarely ideal for anything so it is not wise to only rely on passive ventilation for your greenhouse. If the winds are too strong, or too light, it could lead to problems, and this is why we recommend a mix of passive and active ventilation.

Active ventilation for your greenhouse.

So, what to do when conditions aren’t right for passive ventilation?

It’s pretty simple really. You’ll have to create the right conditions artificially, using things like cooling pumps, fans, and other devices designed for this purpose.

Fans are the simplest thing to set up to vent your greenhouse, and they work really well. When combined with a good cooling pump or cooling wall, they can be an excellent way of controlling the temperature and humidity in the structure, really allowing you to dial in the conditions as you want them.

The downside to using active ventilation systems and tools is that they run off electricity, and so there is the cost factor to consider. Those who are very environmentally conscious might not like the idea of using a lot of electricity to grow their crops either.

Having both passive and active ventilation systems set up in your greenhouse will give you options. When mother nature sends you the right weather, you can switch off the fans and other tools, when things aren’t as you would like with the climate, use your active systems.

So, to sum up the above: 

  • Proper ventilation is vital to the health of your plants and crops when growing in a greenhouse and can help regulate temperature, provide pollination, help with photosynthesis, and reduce the risk of problems from pests and disease.
  • You should use a combination of passive and active ventilation methods and systems.
  • For passive ventilation, you should ideally have vents in the base walls of your greenhouse to allow cooler air to enter and push the warmer air up towards the roof through thermal buoyancy. You also need a larger vent or window in the roof or high up to let the heat escape.

Good active ventilation tools include exhaust fans, cooling pumps and cooling walls. These use electricity to run on, so make sure you switch to passive ventilation when the weather conditions are right for it.

About Terry Smith

I’m Terry Smith from, a professional landscape designer, hobbyist gardener, and barbecue fanatic with 20 years experience building and restoring. So as you go through my site you'll watch me document some of the professional garden installs I make as well as the major projects I take on at home. While sharing those experiences and guiding you, I'll be recommending some great tools I use to enable this along the way so you can really buy in confidence. Always feel free to pop me a message:

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