Tips for Cooling a Polytunnel

In this article, we will discuss some tips for cooling a polytunnel in an eco-friendly and sustainable way. This is an important consideration for all undercover growing areas. Ideally, you should have considered this issue from the outset. But there are also strategies which can be employed to improve an existing setup.

As the weather begins to warm, you will soon notice that a polytunnel will quickly begin to heat up. On sunny days, the inside of a polytunnel will obviously be much warmer than outdoors. This is, of course, often an advantage. The heat will allow you to grow a wider range of crops.

But sometimes, excessive heat can become a problem. It can leave plants stressed and wilting, and dramatically increase the amount of water required for irrigation. Fortunately, there are a range of ways to keep things cool. Here are some tips for cooling a polytunnel which won’t break the bank, and which will take more thought than effort:

Ensure Adequate Ventilation From the Outset

First things first, if you do not yet have a polytunnel, you should think about how you will keep the structure cool from the very beginning. This begins by thinking careful about the structure of the polytunnel itself, and whether it will have adequate ventilation. Think about the number, size and position of the doors, and of any vents on the polytunnel.

The orientation and positioning of a new tunnel will also be important. Orientating a polytunnel to catch summer breezes may sometimes be beneficial. Make sure you understand your site, sunlight and shade, and where winds are coming from, before deciding on the final position.

Polytunnel owners often think of wind as the ‘enemy’. But while wind can sometimes be destructive, but can be beneficial too.

Cooling By Keeping Doors and Vents Open on Warm Days

This might sound very obvious. But before you consider any other interventions for cooling a polytunnel, make sure you are seeing to the basics. Make sure you get out early on warm days and get all those doors and vents open, and keep them open over the course of the day. Altering the doors and vents on your polytunnel means that you can react dynamically to the weather and conditions on a given day, and provide just the right level of ventilation.

Add Thermal Mass To Your Undercover Growing Space

Thinking about passive solar design is another key way to make sure you can keep your polytunnel cool. Whether you are starting from scratch, or seeking to improve an existing polytunnel, add thermal mass.

Materials with thermal mass are those effectively catch and store heat energy during the day. They are good at retaining that heat, and then releasing it slowly when the temperatures in the space start to fall. Adding these materials can keep the temperatures in your polytunnel cooler in summer, and also stabilise day time and night time temperatures during the coldest months. So adding thermal mass is a great strategy for solving a number of temperature-related problems in a polytunnel.

Some materials with good thermal mass that you should consider adding in your polytunnel are water containers (especially those which are black), stone, brick, and ceramics/ clay.

DIY Evaporative Cooling

If things are still too hot, you can consider a range of other options. One interesting option that you could consider is an evaporative cooler. When air passes over a damp surface, or water, the air is cooled. And you do not necessarily have to spend a fortune buying an evaporative cooler for your undercover growing area. You can make one yourself. At their simplest, evaporative coolers can simply be permeable clay vessels and/or saturated cloth placed in a breeze to cool the air through the evaporation of water.

Or Simply Dry Laundry in Your Polytunnel

You might not even have to make an evaporative cooler at all. Sometimes, combining functions can help you achieve similar results. One interesting use for a polytunnel, which could also help you keep your polytunnel cool, is to use it to dry laundry.

As the wet laundry is dried, humidity will be increased somewhat, and this can help the temperatures to fall. And since you can also get your laundry to dry more quickly, this can be a win-win situation.

Spray Down Your Polytunnel in Hot Weather

Where water is not in short supply, you could also cool down the polytunnel by spraying down all pathways and hard surfaces. As the water evaporates, this will also have a cooling effect.

Consider Cooling a Polytunnel With a Fan

If there is not adequate airflow in your polytunnel, then you can of course consider getting a fan. This will of course involve energy use. But if you choose a solar powered option, then this can be an eco-friendly and sustainable choice.

Reducing Soil Temperature

All of the solutions above have focussed on reducing the temperature of the air in your polytunnel. But when thinking about cooling a polytunnel, it is also important to remember that the temperature of the soil is also a key consideration.

You can maintain suitable soil temperatures by:

• Making sure you avoid bare soil. Vegetation or mulches on the soil have an insulating effect. Bare soil will heat up (and try out) much more quickly in summer (and cool down more rapidly in winter). Vegetation (ie crops in your beds) can shade the soil and reduce the amount of the sun’s energy which reaches it.

• Taking a no-dig/ no-till approach. Cultivated, tilled soil will generally be warmer.

• Watering correctly and making sure drainage is sufficient. Soil will be cooler if it drains more freely. Regulating soil moisture is key in keeping soil at the optimal temperatures.

• Making sure that there is sufficient organic matter in the soil.

Of course, keeping soil from reaching more extreme temperatures can reduce stress on your plants. So this can be just as important as reducing the air temperature in your undercover growing area.

Planting to keep the soil covered is perhaps the most important thing. Maintaining a healthy and diverse crop cover throughout the summer months is one of the best things you can do if you want to keep your plants and soil healthy throughout the year. Think about how you can layer the plants to shield the soil, increase humidity levels, and create microclimates. This will certainly be a good strategy to help you make the most of the space.

Follow the tips above and you and your plants should find it much more comfortable in your polytunnel over the summer months.

How do you keep your polytunnel cool? If you have any more tips or suggestions, please do share these with us in the comments below.