Eco-Friendly Cooling Tips For High Tunnels and Greenhouses

A garden designer explains how she keeps her under-cover growing spaces cool all summer.

Entrance to greenhouse
Anthony Lee / Getty Images

As the weather begins to warm up, many growers with greenhouses or high tunnel structures may struggle to keep the space from overheating. So how do you keep the space cool enough for you and your plants without excessive energy outlay or high costs?

My own high tunnel can heat up quite quickly in the summer. But by using some of the techniques described below, I still manage to keep the space at manageable and more even temperatures throughout the year. I have also, through my garden design work, learned a lot about what works well even in places where summers are much warmer. So here are my top tips for eco-friendly cooling for under-cover growing spaces.

Keep Growing Spaces Cool Through Passive Solar Design

The first and most important thing to understand is that keeping growing spaces cool in summer depends on good initial design. If you are starting from scratch in creating a new greenhouse or high tunnel, passive solar design is one of the most important things to bear in mind.

Passive solar design is all about making the most of the sun's energy while keeping temperatures as stable as possible all year round. It involves thinking about the direction and angle of the sun throughout each day and throughout the year. And taking steps to catch and store the energy from the sun in materials with high thermal mass. Materials with high thermal mass absorb heat energy during the day and release it slowly at night when temperatures fall.

Materials like earth, stone, clay, ceramic, brick, and water have good thermal mass. Including such materials in a greenhouse or high tunnel helps keep it cooler in summer, and warmer in winter. So choosing materials with good thermal mass is key for year-round growing. Adding pathways, bed edging, or containers filled with water inside the structure can also be useful strategies. Even a structure that has not been built with passive solar design in mind can often be improved through the addition of such features.

Consider Ventilation When Designing

Another important thing to consider from the outset in greenhouse or high tunnel design is ventilation. Ideally, a structure should have doors at both ends, so that there is a through-flow of air. Additional side ventilation can also be useful. In my area, keeping the doors at both ends open on hot summer days is usually sufficient—alongside the use of thermal mass in the design—to keep the space cool. However, in warmer summer climate zones, side vents which can be opened on particularly hot days can be beneficial. How much through-flow of air can be achieved will depend on where exactly the structure has been positioned—so the orientation of a new structure is something else to bear in mind.

Use Fans for Eco-Friendly Cooling

In warmer climate zones, and for larger under-cover growing structures, fans may sometimes be required to increase airflow and keep temperatures down. Solar-powered fans, which can be purchased for this purpose, are an eco-friendly solution. There are a number of options on the market to explore.

Try DIY Evaporative Cooling

 Temperature in an under-cover growing area is strongly linked to humidity. And adding a DIY, rustic "swamp cooler" could be beneficial in some areas. At their simplest, evaporative coolers can simply consist of permeable clay vessels and/or saturated cloth placed in a breeze to cool the air through the evaporation of water. I sometimes dry laundry in my high tunnel—so this is one low-cost option to consider.

There are also far more sophisticated evaporative coolers which are a lower-energy alternative to other air conditioning/cooling systems. These can be particularly beneficial in hotter, arid climates.

Damp Down For Humidity and Cooling

One final strategy you can employ to cool your under-cover growing spaces in summer is to damp down (pour water onto the floor of the structure), hose, or spray your paths or other hard surfaces. Again, as the water evaporates, this can help to cool down the space. Of course, for this to be an eco-friendly solution, water availability has to be taken into account. Do not be prolific with water where this is in short supply. And always use rainwater you have harvested on your property whenever possible.

These are just a few simple tips to help you keep your greenhouse or high tunnel cool over the summer months, so you can continue to grow crops as successfully (and comfortably) as possible.