More Scorching Hot Days Are Forecast So Here’s How To Cool Greenhouses & Polytunnels? My Suggestions Based On Practical Commercial Experience.

Plants need plenty of light, so shading greenhouses isn’t always the best plan. The problem is small greenhouses and polytunnels get far hotter than large ones, and most amateur greenhouses are relatively small compared with commercial ones.  And not only do they get hotter, they get hotter faster. And that means plants suffer unless we can cool our greenhouses and polytunnels.

How To Cool Greenhouses And Polytunnels With Automated Vents
How To Cool Greenhouses And Polytunnels With Automated Vents

Commercial greenhouses are built with 12.5% of the roof glass being hinged ventilation. With that amount of vent it’s possible to cool them adequately in most UK situations. Some, but not all amateur greenhouses have 12.5% vent. But each vent cost money to include so are often reduced below the ideal number. And in a small greenhouse the ideal amount of vent is actually higher than 12.5%.

How To Cool Greenhouses And Polytunnels

How To Cool Greenhouses And Polytunnels With Manual Vents
How To Cool Greenhouses And Polytunnels With Manual Vent his means you need doors art both ends. If you want to reduce heat, doors at both ends are a must.


Open as many windows as you can. And if your structure is on an allotment miles from home leave vents open over night. Being a bit cooler at night does less damage than being too hot.

Vents can be manual or automated. But in either case open them as much as possible.

A well built greenhouse will have side vents as well as roof vents. This means the air can flow through the house and cool it quicker and to slower temperature.

Louvered vents are installed in some greenhouse. If you have them, use them.

In my amateur greenhouse I have replaced a few half panes of glass with core board. Being small they are not so likely to blow out in a gale, but can be removed in about 20 seconds should it get excessively hot.

In polytunnels there are rarely vents. Though they can be installed. Building a tunnel on a slope is a good way to encourage a through draft. Where practical position the tunnel so that the natural slope allows the air to rise from the lower end to the top of the slope an out through a door or large vent. .


Big doors mean more ventilation provided you keep them open. If you are worried about flying pests, foxes, cats etc entering then fit a mesh door. More is made from a piece of plastic covered wire on a wooded frame and is used instead of the glass door. It allows pollinators in but keeps most other critters out!

Doors on each end of greenhouses and polytunnels make perfect sense. They allow much better control of the temperature and costs very little to build and install.


Damping the floor with water is a tried and tested way to reduce temperatures. The water evaporates and cools the air. Its simple, has no moving parts and works!

And if you can’t do this several times a day then you can try this trick. Lay a pice of absorbent material on the floor and place a container of water on the bench above. Now allow the water too slowly “drip” from the container onto the material. It will evaporate and cool the air. It’ll need some experimentation to get rates right but a steady drip can reduce temperatures.

Pot & Grow Bag Sizes

Your plants will do much better if they don’t dry out .. and they will dry out quickly in smaller pots. So the rule is to use as big a container or volume of compost and possible. Growing in the soil is even better and why I’m not keen on concert efloors which hold the heat in the structure. They act like a radiator for hours after the sun goes down.

Grow bags are one of my pet hates. One tomato in a full grow bag is going to struggle in the temperatures we are now getting. And in the past I’ve seen people crowd 3-4 toms to a bag. They are either going to die, get Blossom End Rot, or fail to pollinate unless you are every lucky.


There’s an argument for fans in greenhouses and tunnels where temperatures get very high. But there is a counter argument that says you need electric and its going to cost a lot of money to buy and run a big enough fan. Except in commercial polytunnels Im not convinced that fans make a lot of sense. And even in commercial polytunnels I’m reluctant to use them.


Because plats need sunlight I’ve always been against shading paints or materials. BUT, when temperatures get in the high 30s and over 40C I relent. A temporary shade in the heat of the day or on the every hottest days begins to make sense, especially if you can’t do all the other things I suggest.

How To Cool Greenhouses & Polytunnels Conclusions

With global warming this excessive temperatures are going to become more common. We all need to learn how to cool down and I’ll be adding more advice on How to Cool Greenhouses & Polytunnels in the future.

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