How to Cool a Greenhouse in Summer

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Keeping your greenhouse cool in the summer isn’t easy, but it can be done. And it doesn’t have to cost a lot of money either.

In this article, we’ll walk you through 11 methods for how to cool a greenhouse. We’ve included simple, cost-effective greenhouse cooling solutions such as passive ventilation and shade curtains, as well as more advanced options to beat the heat.

Understanding Heat Build-Up in Greenhouses

photo showing how heats build-up in a greenhouse

How a greenhouse works is through a physical principle appropriately called the “greenhouse effect.” The interaction of sunlight and the clear greenhouse walls create an internal environment that warms up much more quickly than the environment outside.

When sunlight hits a greenhouse, only visible light passes through. That light is then absorbed by the materials inside the greenhouse, including the plants, soil, water, and flooring.

Once absorbed, this light is converted into heat and released by the materials as infrared radiation. Since infrared heat cannot pass through the clear outer walls of the greenhouse, it becomes trapped. This is what causes greenhouses to heat up faster than the surrounding environment.

Natural Greenhouse Ventilation Techniques

photo of an automatic window with temperature sensor

Your first line of defense against an overheating greenhouse is passive ventilation.

This natural greenhouse ventilation technique allows hot air to escape the building while pulling cool air in to replace it. This is accomplished by manipulating your vent openings to take advantage of wind currents.

When wind passes over the roof of your greenhouse, it pulls air out of the upper vents. This creates a vacuum inside the greenhouse, which, in turn, pulls air in from the lower vents. Since hot air rises and cool air sinks, this air tends to be cooler than the air lost through the roof vent.

If you want to achieve adequate ventilation using this method, your greenhouse must have lower and upper vent openings. Most greenhouse kits come with these already built in. But there’s more you can do to maximize the effect of this process, including:

  • Install temperature-dependent auto-openers on your vents.
  • Assure the area of your roof vent openings represents at least 15 to 20% of your total floor area.
  • Ensure your sidewall vent openings are equal to the area of your roof vent openings.
  • Orient the greenhouse, so the summer wind blows over the roof ridge to create the largest vacuum effect.
  • If possible, use a removable roof setup to eliminate the greenhouse effect on warm days.

Note: Passive ventilation is the most cost-effective option for keeping a greenhouse cool. But it requires wind speeds of at least 2 to 3 miles per hour. Without wind, even well-ventilated greenhouses may overheat on sunny days.

Artificial Greenhouse Cooling Methods

You can use many active ventilation and artificial methods to for cooling a greenhouse. These range from relatively inexpensive options, like fans, to more extreme measures, like an AC unit.

Exhaust Fans

greenhouse with an exhaust fan

On calm days when there’s not enough wind for proper ventilation, an exhaust fan can help push hot air out while pulling cooler air in.

These ventilation fans are available in various sizes to accommodate small to large greenhouses. You can even purchase smaller models along with a solar panel, eliminating the costs of running the fan.

Circulatory Fans

While exhaust fans help draw cooler outside air in, circulatory fans help de-stratify stagnant pockets of air.

In a poorly ventilated greenhouse, hot, dry air rises to the top while cooler, humid air hangs near the floor. These stagnant pockets need to be broken up and mixed so plants can access the carbon dioxide they need to breathe.

Circulatory fans won’t lower the temperature in your greenhouse on their own, but they will ensure proper air circulation is achieved. And using circulatory fans together with open vents will help lower the temperature by pushing hot air out of the structure. This form of active ventilation is beneficial for large greenhouses.

Evaporative Cooling

gardener watering the greenhouse flooring

Evaporative cooling is another way to cool a greenhouse.

The most common form of evaporative cooling is a swamp cooler. But there are additional, less expensive options you can use. In addition to helping achieve an ideal temperature, the excess water can help increase humidity, which is excellent for dry climates but problematic in very humid climates.

Swamp Coolers

Evaporative coolers, also known as swamp coolers, force the hot air inside the greenhouse over water-saturated pads using fans. As the hot air passes through the wet pad, some of the heat is used to evaporate the water. The result is cool air, which is then forced back into the greenhouse to decrease the overall temperature.

These appliances are fairly expensive and require a good deal of power to operate. Not to mention they need a constant supply of water. But, for large greenhouses suffering in the summer heat, swamp coolers are beneficial.

Damping Down

Damping down uses the same concept as a swamp cooler but in a more rudimentary form. By spraying water over the floor of your greenhouse, you can bring the temperature of the air down through evaporation. This is especially true if you have a high thermal mass flooring, such as stone, concrete, or gravel.

Heat absorbed by the floor is used to evaporate the water on its surface. The floor will absorb more heat from the air to compensate for this lost heat, thereby cooling the greenhouse.

This is an easy and fairly cost-effective solution. However, it is not recommended on days with high temperatures because raising the humidity levels too much can actually make things hotter. So this technique is best used in moderation or in combination with exhaust fans.

Natural Evaporation

Another even simpler method for lowering your greenhouse temperature is to fill up a kiddy pool or another large vestibule with water.

Water has a high thermal mass, which means it also absorbs heat from the air. When there is high surface-to-air contact, the heat absorbed is used to evaporate the water. As the heat is used, more heat is pulled from the air, lowering the overall temperature.

Just as with damping down, this method is best paired with plenty of ventilation to avoid driving moisture levels up too much and creating an overly humid environment.

Cool Misting Systems

greenhouse with a hanging misting system

Misting systems are a doubly effective way to keep your greenhouse cooler and safer for plants. By hitting the plants’ leaves with fine water droplets, you help keep them hydrated, preventing much of the damage caused by high temps.

Because misters put out water in fine droplets prone to evaporation, they also help pull heat from the air and turn it into evaporative energy.

Misters are a great option for cooling your greenhouse during peak temperatures in the early afternoon, but they shouldn’t be overused. Not only can they create too humid an environment, but they can oversaturate soil and suffocate root systems.

Air Conditioning

When all else fails, an air conditioning system may be warranted for your greenhouse. Just as your home AC puts out cool air, a greenhouse AC pumps cold air to help regulate temperatures.

However, the big difference between your home and a greenhouse is that the latter is not well insulated. Having the vents open for airflow is important. But even with them closed, the thin walls of a greenhouse mean cool air will be readily lost to the outside.

This all makes running an AC to cool a greenhouse fairly inefficient. However, there are some cases that might warrant it, such as a closed grow house or a highly controlled greenhouse environment. In these cases, a ductless mini-split air conditioner is your best option.

Keep A Greenhouse Cool With Shading Techniques

greenhouse with exterior sun shade on the roof

One way to prevent your greenhouse from filling up with temperature-raising infrared radiation is to block out some of that light.

Shading your greenhouse can be highly effective in keeping the temperature lower. However, you’ll need to balance this with the light requirements of your plants. Many plants, especially vegetables, require a full day’s worth of direct sunlight to thrive.

Curtain Systems

Curtain shade systems are an easy way to block sunlight only when needed. Ideally, you should use curtains made of durable polyolefin film. These come in different opacities and are rated by how much sun they block, from 30% to 90%.

The best curtain systems are retractable. You can pull them out and shade plants during the height of the day and retract them as the sun grows less intense. This helps you balance heat control with light needs.

Exterior Shade Cloth

By draping the exterior of your greenhouse with PVC composite fabric, you can reduce temperatures inside fairly quickly.

Of course, shade screens can have drawbacks, especially for larger greenhouses. When permanently attached to the roof, you can’t easily remove them on cloudy days or during less intense sun periods.

With that said, this solution is best for plants that prefer less sun, such as ferns, tropical plants, and certain flowers. Remember that you still need to choose a shade grade based on what will be best for your plants while maintaining some heat control.

Natural Shading For Your Greenhouse

Another option for using shade to cool your greenhouses interior is to plant well-positioned deciduous trees.

In the summer, tall trees can help keep the sunlight off your greenhouse. These trees lose their leaves in the winter, which allows sunlight through during the colder months, so your winter greenhouse stays warm.

You can also use shade plants inside the greenhouse to reduce how much sunlight makes it to the soil and greenhouse floor. Fig trees, grape vines, and other broad-leafed plants are perfect for casting shade on the ground. And they produce delicious produce that you can add to your harvest.

What Temperature Should Your Greenhouse Be?

In general, plants cannot tolerate temperatures above 90 degrees for extended periods.

Obviously, there are some exceptions to this rule, such as cactuses and other plants that have evolved for extreme climates. But most vegetables, flowers, and exotic plants prefer temperatures below 85 degrees.

Working to keep your greenhouse below this mark will not only keep your plants healthier, but it will help them produce better for a higher yield at the end of the season.

Related Article: How To Heat a Greenhouse

Frequently Asked Questions

What temperature is too hot for a greenhouse?

The optimal temperature range for a greenhouse depends on the types of plants one is growing. Generally speaking, temperatures between 65-85°F are considered ideal in greenhouses as this range offers warmth and humidity that many plants need to thrive. However, temperatures higher than 90°F can become too hot for some plants.

What happens if a greenhouse gets too hot?

If temperatures become too high due to inadequate cooling, it can cause leaf burn, wilting, and stunted growth.

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Aaron Green
Aaron is the founder of and Essential Home and Garden. He likes to spend his spare time with his family, and doing DIY projects in the home and garden.

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