I’ve been thinking lately that a greenhouse might be a productive adventure for my family, but I don’t really know how they work. I know the basic concept of harnessing the sun’s rays, but there must be more to it than that.
How Does a Greenhouse Work? A greenhouse works by harnessing the sun’s rays and setting up the best environment for growth. An ideal environment means maximum plant growth and production. A greenhouse does this by allowing you to control the following.
- Temperature
- Humidity
- Light
I’m sure you’ve figured out that a greenhouse doesn’t just control temperature, humidity, and light on its own, you have to have the right set up. I’m going to tell you what you need to make sure your greenhouse works exactly as you want it.
The Structure of the Greenhouse
The structure of a greenhouse is an important part of how and why a greenhouse works. Greenhouses need to be made of a material that allows the maximum amount of sunlight to pass through. For that reason, greenhouses are generally made from the following materials.
- Glass
- Fiberglass
- Single-wall plastic
- Double-wall plastic
- Plastic film
Here’s a breakdown of what you can expect from each type of material.
Glass
Glass windows and panels are a more permanent type of greenhouse compared to other choices. They’re also relatively easy to maintain. Glass panels keep your greenhouse sealed and are good at keeping in the heat and humidity in your greenhouse. Glass panels allow 90-percent of light to come through.
Fiberglass
Fiberglass is more affordable than glass, but still more permanent than other options. Another benefit of fiberglass over glass is that it doesn’t require as sturdy of a frame. You should only use clear, transparent or translucent fiberglass on your greenhouse. This allows the maximum amount of light to pass through.
Resin coated fiberglass is the best choice and lasts for 15 to 20 years. The amount of light that comes through is the same as glass initially but gets worse as the fiberglass becomes discolored.
Single-Wall Plastic
Plastic comes in a number of rigid materials including acrylic or polycarbonate. This plastic can be used on curved greenhouses, making it easier to use in those cases than glass or fiberglass.
Initially, plastic allows as much light through as glass but will start to turn yellow over time. Plastic can also be more expensive than glass.
Double-Wall Plastic
This plastic is similar to the single-wall, but is made up of two plastic walls with a layer of plastic webbing in the middle. This webbing helps with insulation, which helps your greenhouse hold in heat better.
The double-wall does make this plastic less flexible and reduces the amount of light that gets through to about 80-percent.
Plastic Film
Plastic film is normally made up of two layers with a layer of air in the middle, which serves as insulation. It’s lightweight and doesn’t cost much, but you will have to replace it every one to five years. It still allows about as much light through as glass.
No matter what material you use to cover your greenhouse, the greenhouse works the same. Here’s a look at how a greenhouse harnesses the sunlight and uses it to create the perfect growing environment.
- Sunlight shines into the greenhouse.
- Light and heat from the sun warm the plants and air inside.
- Glass or plastic walls trap the heat inside your greenhouse.
- Your greenhouse stays warm even during the night without the sun.
It’s also important to choose the right location for your greenhouse to maximize the amount of sun it gets. The best spot for your greenhouse is south or southeast of any structures and clear of any nearby trees.
Once you have the right structure and spot, you’ll need to add some other things to your greenhouse to make sure it works as it should.
Ventilation Options for Your Greenhouse
While a greenhouse’s ability to harness and use the sunlight is exactly what you want, it can also cause some other problems. The heat trapped inside your greenhouse can cause it to get too warm and also cause too much humidity.
Here are some problems you can run into when the temperature and humidity are too high.
- Plants wilt or die
- Leaves get sunburn, which results in lighter than normal leaves
- Mold and fungi can take over plants
- Pests invade greenhouse because of mold and fungi, which is the perfect environment for them
One of the best answers to controlling the heat and humidity is to make sure you have a good ventilation system in your greenhouse. There are two types of ventilation systems: natural or mechanical. Here’s a look at the basics of each type of system.
| Natural Ventilation | Mechanical Ventilation |
| No powered fans | Uses fans |
| Relies on wind | Can be automated |
| Relies on thermal buoyancy | Sealed environment = no pests |
| Less control | More control |
Natural Ventilation
A natural ventilation system involves vents on the top and sides of your greenhouse. As the heat rises, it flows out the top vents and pulls cooler, drier air in through the side vents. A high-powered fan can also help circulate air. One like this industrial fan on Amazon would work well.
When you have the vents installed on your greenhouse, make sure the total area of the roof vents equals 20-percent of the volume of your greenhouse. The total area of the side vents also needs to equal 20-percent of the volume.
Mechanical Ventilation
A mechanical ventilation system uses an exhaust fan to blow out the hot air and pulls in cooler air through special openings called louvers on the opposite side of the greenhouse.
Exhaust fans are rated on how many cubic feet of air they can move per minute. You want a fan that can move one cubic foot of air per minute during the summer months.
This means if the volume of your greenhouse is 3,000 cubic feet, you want a fan with a rating of 3,000 cubic feet per minute (CFM). This exhaust fan on Amazon is a good example.
The ventilation system you choose depends on your budget and the size of your greenhouse.
Using mechanical ventilation in a small greenhouse will create too much circulation, but using it in a very large greenhouse isn’t very economical either. You would have to have multiple systems to make sure the air circulates through the entire greenhouse. Therefore, mechanical ventilation is probably best for a medium-sized greenhouse.
A natural ventilation system costs as much as a mechanical system initially but probably costs less over time. It does require more of a time commitment, though, since you can’t just automate things.
A good ventilation system does more than just help control temperature and humidity. It also does the following:
- Provides Carbon Dioxide: Carbon dioxide is necessary for plants to complete photosynthesis. This is when they turn carbon dioxide, water, and sunlight into glucose or sugar. This is vital to a plant’s growth and survival. If the amount of carbon dioxide goes down, so does the rate of photosynthesis.
- Prevents Pests: With lower humidity, pests aren’t as tempted to invade your plants.
- Helps with Pollination: The air blowing through the greenhouse helps give plants a little shake and encourages pollination. This is something that happens naturally with wind or bees outside of the greenhouse, but you have to offer some assistance in a greenhouse.
Using Cooling and Heating to Control Temperature
Even if you have the right structure for your greenhouse, the right location, and a good ventilation system, you’ll probably still have to make some modifications during summer and winter.
Maybe you live somewhere with wonderfully moderate temperatures all year long, but if you’re like me, you deal with hot, humid summers and some days with below-freezing winter temperatures.
The ideal temperature for your greenhouse to work best is 80 degrees Fahrenheit, although somewhere in the range of 80 to 85 is usually acceptable. If it’s higher or lower, you need to fix it.
Cooling the greenhouse: You have quite a few options when it comes to cooling your greenhouse down. The cooling method you choose depends on how much you need to lower the temperature and your budget.
Here’s a look at some ways to cool down the greenhouse.
- Shade cloth
- Water barrels
- Evaporative coolers
- Misters
- Plant deciduous trees
Shade cloth: Shade cloth is a fairly easy, affordable option for lowering the temperature. You can drape it over areas on top of your greenhouse or hang it directly above plants.
Shade cloth comes in different color nylon or in a material called aluminet. Aluminet is a better choice than the nylon varieties because it actually reflects some heat away from the greenhouse rather than just offering shade. Here’s a look at some aluminet I found this aluminet on Amazon.
Shade cloth comes with different ratings depending on much shade it blocks. This table shows you what density you should buy for specific plants.
| Plant | Shade Cloth Density |
| Vegetables | 30 – 50% |
| Most plants | 40 – 60% |
| Shade-loving plants (orchids, ferns) | 70% |
Water barrels: Filling large tubs with water and placing them around your greenhouse can also help lower the temperature. This can also help if you live in a dry area and the humidity in your greenhouse is low.
Evaporative coolers: Evaporative coolers are a fairly energy-efficient way to cool down the greenhouse. After dry air passes through a compartment where water is stored, it is cooled and circulated back through the room. Here’s an evaporative cooler from Amazon.
Misters: Misters work to cool down a greenhouse by putting small water particles out into the air. When the water particles evaporate, they cool the air.
Plant deciduous trees: Planting deciduous trees on the west side of your greenhouse can help provide natural shade during the summer. Deciduous trees are ones that lose their leaves every fall.
They’ll shade the greenhouse in the spring and summer, but the sun can shine through their empty branches in the fall and winter.
Heating the Greenhouse
When temperatures dip in the winter, you might have to create more heat in your greenhouse. Here are some ideas to accomplish that.
- Water barrels
- Heater
- Compost trench
Heater: Adding an electric heater is an easy way to raise the temperature. Just make sure the heater is away from anything flammable and an automatic shut off is a good idea. Then, if it gets too warm or has an air supply problem, it will turn off. Here’s an electric heater from Amazon that has a safety feature to keep it from overheating.
Water barrels: Placing water barrels around the greenhouse isn’t only helpful in cooling down a greenhouse, but also in raising the temperature. Tubs should be black, which helps ensure they absorb the maximum heat possible. The barrels warm in the day and then give off the heat during the night.
Compost trench: Dig a trench down the center of your greenhouse or in a couple of different spots if the center isn’t open. Fill the trench with compost. The chemical breakdown that happens in compost releases heat energy.
There are quite a few different components when it comes to getting a greenhouse to work the way it’s designed. If you have all the factors we discussed in place, though, your greenhouse should work at its best potential and produce beautiful plants for you.