How to Cool a Greenhouse in High Humidity – 8 Proven Methods

Let’s face it, humidity-related diseases are the worst. You never know when you’re going to get attacked by one and once you do, there’s no going back. Nasty diseases like botrytis can damage your plants faster than you can imagine.

However, there are ways you can prevent situations. That’s why I’ve gathered 8 different methods to reduce humidity in the greenhouse during summer.

Apart from that, I’ll also be explaining how humidity & temperature are related and how it affects plant growth.

So, without stretching it any further, let’s get into it-

What is the Relationship Between Temperature and Humidity

We measure humidity using a measurement called relative humidity. In layman’s terms, it determines how much moisture is in the air and how much moisture it can hold. But the temperature is what creates the problem.

Let’s explain how hot air plays the biggest role in high humidity-

Hot Air – More Humid Environment

The hottest summer days are always sticky and uncomfortable. The reason behind that is humidity. And if it affects you, chances are it affects your plants too.

As the heat rises, your plants perspire on an excessive level to cool down. However, the water they release evaporates into the air, increasing the humidity.

Hot or warm naturally has a higher humidity holding capability. Although there’s another problem. For every 20° F dry bulb temperature, the moisture-holding capacity gets doubled.

That means if the temperature rises to 70° F from 50° F, the water holding capacity is doubled up and the relative humidity gets sliced in one and a half.

Higher Humidity Isn’t Necessarily a Bad Thing

Don’t get the wrong idea. Higher humidity isn’t always bad. The right amount of high humidity can save your plants from overheating. For heat-sensitive plants such as lettuce and onions, excessive heat is bad news. That’s where dampening down(I’ll talk about it later on) comes to the rescue by absorbing the heat.

However, when the humidity gets too high, it creates a breeding ground for mold, botrytis, and fungi. Unless you want to grow large batches of mold, you need to keep humidity at an optimal level.

How Does Humidity Affect Plant Growth?

For understanding how humidity affects plant growth in your greenhouse, you need to know about vapor pressure deficit or VPD. In simpler terms, it’s the vapor difference inside the leaves and outside air. Simple, right?

Well, it’s not that simple for your plants when the humidity gets messed up. Plants are always adjusting their stomatal openings depending on the humidity and VPD.

When the humidity gets too high, the plants aren’t able to use water that much. But stomata are always in overdrive as the humidity is high. Likewise, the plants have to minimize water usage when the humidity falls down.

Long story short, if the humidity and temperature aren’t in the middle ground, the plant won’t grow properly.

Plant Transpiration

We all know plants suck in water through their roots and evaporate it with their leaves. This entire process is called transpiration. As the air gets drier, the transpiration rate gets faster too.

However, the plant can only speed it up to an extent. After that, the plant won’t be able to do anything even if there’s enough water in the growing medium. As a result, the plant will wilt at one point.

On the other hand, when the humidity is too high, the plant slows down the absorption rate. This also means the plant won’t get enough nutrients, leading to nutrient deficiency.


You already know how stomata respond to high humidity and excessive temperature. The same applies to the photosynthesis process. After all, it’s the process that makes food for plants. If somehow there’s a problem, the overall growth of the plant gets hindered.

This is how plants respond to humidity:

8 Tips For Keeping Your Greenhouse Cool in High Humidity

There are many different ways you can maintain a cool temperature in high humidity. From common affordable methods to expensive, state-of-the-art methods, I’ll be explaining them in a series.

First, try out the affordable methods. If these don’t work, consider spending some money on the other options-

  • Grow Drying and Timely Watering

Grow drying’ or keeping the greenhouse dry is the most affordable way to keep the temperature and humidity at an optimal rate. The method is simple.

    • Never overwater your plants. Give them only as little water as they need.
    • Avoid watering before sunset. You’d want to water at sunrise to boost photosynthesis and transpiration.
  • Sufficient Plant Spacing and Strategic Placing

Always remember that plants are nature’s own evaporative coolers. Chances are you’ll have a variety of plants in your greenhouse. If not, consider taking in plants like fig trees and grapevines.

The large leaves will provide enough shade for the smaller plants and soil. Needless to say, you shouldn’t only focus on grapevines or fig trees. Figure out the height of each plant type and strategically place them with enough space for air circulation.

Some people build greenhouses near big trees for extra shade. But there’s always the risk of damage in natural calamities. Shade cloth is far better in this matter.

  • Damping Down

Even though damping down essentially raises the humidity a little bit, it works in the long run. The process is simple. All you have to do is wet the hard surfaces in the greenhouse.

This process works better when there’s added ventilation. Apart from that, you should be careful about overdoing it.

  • Combination of Heating and Ventilation

An ideal combo of heating and ventilation is necessary to keep the humidity at the perfect level. The vents will bring in dry air from outside while the heating system will heat up the air, increasing its moisture carrying capacity.

Without the heat, there will be condensation. If you don’t know why condensation is bad for plants, don’t worry I’ll explain it in a bit.

On the other hand, without ventilation, the temperature would go haywire, leaving you with a hefty heating bill. When you turn on the heat, crack the vents a little bit (1-1.5-inch). That’ll ensure enough space for the heat to escape.

But if you have fans-

    • Turn on the fans for a few minutes without turning on the heater.
    • After a few minutes, turn off the fans and crank up the heat.
    • Remember that operating these two at the same time might draw in flue gases in your greenhouse.
  • Shading

Sometimes natural plant shading won’t work and you’d have to bring in the big guns. That’s where shade paints and shade cloths come into play.

Shade Paint (Shade Compound)

Whitewash or shade paint is a paint-like substance used for keeping the temperature down in greenhouses. The higher the concentration of the compound, the better shading you’ll get.

  • Shade paints are an inexpensive way to cool down greenhouses with high humidity.
  • Remember to apply the paint when it’s morning or afternoon.
  • Avoid applying if you feel like it’ll rain. The paint needs 12 to 24 hours of drying time.

Shade Cloth

If you ask me, shade cloths are much better as you have full control over how you want to use them. Although the effectiveness depends on the material you use.

Some people stick to aluminized polyester which reflects sunlight to the max. On the other hand, there are options like polycarbonate coverings. These aren’t as reflective but you do need a bit of sunlight, don’t you?

That said, figure out what you need and go for that material. Although polycarbonate coverings are more popular among greenhouse owners.

  • Evaporative Cooling Wall

Evaporative cooling walls are neither expensive nor cheap. But there’s no doubt about their effectiveness. The water they evaporate can bring down the temperature by 10 to 20 degrees. Although they work best when the relative humidity stays low.

There are different sizes available. So, figure out what size you need. Just remember that every square foot of pad will cool down 20 sq ft of foot area.

And you can also make your own evaporative cooling wall. The process might seem complicated at first but it’s rather easy. See for yourself how to make an evaporative cooling wall.

  • Geothermal Heating (Bottom Heat)

Geothermal heating or bottom heat is exactly like it sounds. You heat the bottom of the greenhouse to keep the humidity down. It also prevents any kind of condensation.

As the air temperature rises, it allows the air to move around inside the greenhouse. On top of that, the surface of the plants stays warm ensuring they don’t suffer from any diseases.

  • Misting System or Fogger

Foggers are essentially a type of evaporative cooler. But they work much faster. If you live in a hot region, evaporative cooling walls are enough for you. But let’s say, if there’s an overflow of snow in your region, this is what you should get.

Foggers and misters are almost the same things. But foggers emit even tinier water droplets. Although you should be careful about using them without any vents. That might increase the humidity even higher. As I said, these things work together.

What’s the Ideal Greenhouse Temperature and Humidity?

Despite having different requirements, the ideal greenhouse temperature and humidity is around 80°F (27°C). As you already know the higher the temperature the higher the humidity.

Here’s a quick chart showing different types of ideal humidity and temperature levels-

However, don’t focus too much on these so-called ‘ideal’ levels. The ideal level varies depending on the type of plant. For example, plants such as watermelons and cucumbers absolutely love that extra heat and humidity.

Still, you should never let the temperature go above 90°F or 32° C. As you’re planting in an artificial environment, controlling the temperature and humidity should be one of your first priorities.

Is Condensation in a Greenhouse Bad? Why Does it Happen?

Condensation can be a very bad thing for greenhouses. Condensation is when plants evaporate excess water and that same water droplets fall onto hard surfaces. When it’s on windows and anti-drip plastics, it’s not a big deal.

But the problem occurs when the water droplets fall down from the ceiling. You never know what disease might live there. Apart from that, an excessive amount of humidity is never good for your plants.

Here are a few pointers to prevent condensation-

  • Ensure enough air circulation
  • Keep adequate space between plants
  • Water plants timely and properly
  • Make sure the plants stay well-drained at all times

Will Condensation Kill the Plants in My Greenhouse?

Condensation is never good for your plants. But chances are it won’t kill your plants. When the air holds excessive water for far too long, that’s when the problem occurs.

While condensation alone won’t kill the plants, it’ll bring in mold and fungi. These things can cause plants to die faster than you can imagine.


How to keep a greenhouse cool in the desert?

The most effective way to cool down greenhouses in the desert is to use an evaporative cooling wall. Vents and shade do help in this situation but these aren’t enough for the low humidity environment of deserts. That’s where evaporative cooling walls come into play. You can even make one if you don’t want to spend the money.

How to cool a small greenhouse?

You should make the most use of natural ventilation for small greenhouses. As the area is small, combine the vents with a heating system or fans and the greenhouse will cool down in no time. Moreover, there are options like strategic plant placing and shades.

Is humidity bad in a greenhouse?

Humidity isn’t a bad thing for greenhouses. All plants need an optimal level of temperature and humidity to thrive. The problem occurs when the humidity gets too low or too high. That’s when the plant suffers from diseases like botrytis, powdery mildew, or other fungal diseases.

Here are the details of- What humidifiers are best for indoor plants

Does your location affect humidity and temperature?

Humidity determines how much moisture or water vapor is in the air. Needless to say, the weather conditions are not the same in different parts of the world. Regardless of where you live, you have to control the environment inside the greenhouse to ensure optimal growing conditions for plants.

Final Verdict

So, that’s a wrap. If you’re hiring someone to do it for you, keep a few things in check. If the person isn’t asking for geographical location, current airflow, weather condition, and greenhouse dimensions, that’s a big RED signal.

All I did was explain the methods of how to cool a greenhouse in high humidity. But there’s a lot more to each of the methods. Whatever you do, make sure you do enough research and learn what’s right and what’s wrong.

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