Heating & Ventilation in Your Greenhouse

Heat in the Greenhouse

What type of heat are you considering for your greenhouse? There are plants that love tropic humid environments, while others thrive in hot, dry conditions.

Whichever types of plants you decide to raise you will be able to experiment and control your greenhouse temperature through a heat control unit.

Gardeners tend to use electric or gas heaters. With a gas heater it is extremely important to remember to properly ventilate, not only for plants, as it can give off deadly fumes, but for you, as you work.

Electric heaters also have their drawbacks. One major drawback is cost. Another drawback is if the power goes out. The loss of heat in a greenhouse could mean certain doom for your plants as they freeze overnight in winter. If you are going to use electric heat, have a back up heat source so as not to loose your entire collection of plants in the hours following an outage.

The newest heat systems, combined with polycarbonate panels,’ work together to successfully control the environment within a greenhouse. Automated systems do not rely on concrete or brick to absorb heat for overnight cold spells.

A freestanding greenhouse will need to have its own heating controls. A lean to greenhouse could function off of heat controls from your home. If your heating source has unused capacity, and you have a lean-to greenhouse, you could use a heat source with a zoned heat control to maximize environment controls.



Heating Tips

There are a number of ways to cut down on greenhouse energy costs during winter. Wrapping greenhouse walls with bubble wrap cuts down on heating costs. The bubble wrap works as an insulator. Placing large containers of water in the greenhouse or using 4-6 inches of stone or pea gravel as flooring are both methods of absorbing heat during the day that is naturally released at night. If you live in a colder climate, or if you are considering buying a commercial sized greenhouse, consider building the foundation below frost level.

A greenhouse overrides the natural cycle of weather. It does so by utilizing the energy of the sun. In the coldest times of winter you will want your greenhouse to remain above 50 degrees. Using the sun's power you should only need to add additional heat on rare occasions. You can grow any type of plant you want in a greenhouse, or in a cold house, without the addition of too much heat if you simply watch the seasons; know when the sun is strongest and which plants thrive in what type of sunlight. If you are planting tomatoes from December through February you will find that you need to add supplemental lighting for your plants to thrive unlike March to May when your crop will thrive on sun alone. From October until February is the cold season, and from March until September is the warm season. Adjust for heat intensity as the weather goes.



Cooling your Greenhouse

Harnessing the sun’s energy in the summer months creates the perfect environment for tropical plants, squash, melons and long season squash and melons.

With proper ventilation you can continue to grow plants, vegetables and fruit through the summer while you also work in your outdoor garden area.

Cooling your greenhouse, at times, may seem a little more difficult than heating the greenhouse. Once you have survived the first year and are more familiar with how your greenhouse reacts to your climate, knowing how and when to increase heat vs. ventilation will become second nature.

No matter which type of greenhouse you buy, all greenhouses will need some type of ventilation. A minimum of two vents, one on the roof and one on a side will stabilize the environment in the greenhouse. If you find that you need additional venting you should keep in mind that you can cut glass/polycarbonate or plastic panels, you can also drill holes into the framing of your greenhouse but you should NEVER cut the framing of a greenhouse to insert a fan. This will compromise the stability and strength of your greenhouse with devastating effects. If you find that you have no other alternative, consult with your dealer for a more appropriate venting system.

Cross ventilation helps prevent the greenhouse from overheating. An exhaust fan or any shutters should be situated so that they can take advantage of prevailing winds. Generally an 8 x 12 greenhouse should have a combination of roof and shutter vents that equal in size to approximately 20% of the total greenhouse square footage ... remember your open door can count as part of that percentage.

In strong winds and on cold winter days, be sure to close the vents and latch them securely for optimal heating efficiency and to protect your roof vents from damage. Carbon dioxide (CO2) and light are essential for plant growth. As the sun rises in the morning to provide light, the plants begin to produce food energy (photosynthesis). The level of CO2 drops in the greenhouse as it is used by the plants. Ventilation replenishes the CO2 in the greenhouse.

Because CO2 and light complement each other, electric lighting combined with CO2 injection is used to increase yields of vegetable and flowering crops.

Bottled CO2, dry ice, and combustion of sulfur-free fuels can be used as CO2 sources. Commercial greenhouses use such methods.

Cooling of greenhouses can easily be accomplished using evaporative coolers, exhaust fans, misting systems and shade cloths. Evaporative coolers are typically recommended for extremely warm areas. Typically misting systems, shade cloths and exhaust fans are adequate for cooling in moderately warm areas.

There are rainproof paints available that you can put on the ‘windows’ of your greenhouse. When you need a little shade, add some paint. This method is an inexpensive solution for cooling greenhouses.

You do have choices in ventilation for your greenhouse, including electric or solar powered vents. Ventilation systems are controlled so that if you are away from home when the temperature goes down the vents will close automatically. A thermostat within the greenhouse triggers an automated ventilation system. In every type and style of greenhouse it is important to have at least one (two is better) -roof vent. Using roof vents combined with automatic openers will ensure that you don’t forget to open the vents, or if you go away for a period of time your ventilation system will still be activated. A choice productive for cooling is a thermostat-cooling fan which evenly circulates air in a greenhouse. You should have a battery back up in case of power outages.

You also have the choice of using circulating or cooling fans. You should be careful when using fans in a greenhouse though because a particularly fragile plant could break from its force. Another option for gardeners is to install a misting system or to manually spray mist to keep plants cool. This is an ideal solution for those living in hot, dry climates.

In the hottest regions try painting glaze over the ‘windows’ in the greenhouse, this will lessen the suns rays as they penetrate the glass, cooling the interior of the greenhouse. There are two types of ventilation systems that are generally used in greenhouses; a window that opens in the roof, or a basic cooling fan.

If you are going to install an exhaust fan, you should keep in mind that you will also need an intake fan on the other side of the greenhouse. This will keep the air continually fresh stabilizing the environment for your plants. For determining the correct exhaust fan size for your greenhouse, first you'll need to calculate the volume of the greenhouse (Volume = Length x Width x Height) and then multiply that volume by 0.75 for the needed ventilation rate in

CFMs (cubic feet per minute). Please note that for an exhaust fan to operate properly, the air intake fan or intake shutter should allow cross ventilation with a steady exchange of air about every minute.

Ventilation is most essential for plant growth in your greenhouse because plants need a continuous supply of carbon dioxide to continue growth. Without carbon dioxide your plants will surely die. As you keep the airflow in the greenhouse fresh and flowing, you also are keeping diseases and insects at bay – as they tend to thrive in a stagnate environment. There are a number of solar-powered vent openers that are filled with a waxy substance that expands when it heats up and contacts when it cools. The ones that I use begin opening at approximately 65 degrees and are fully open by the time that its 80 or so degrees.

Automatic vent openers are one of the greatest greenhouse inventions. They are generally solar-powered (designed to open as the greenhouse air heats up) and require nothing more than simple assembly. Just remember to take the wax-filled cylinders inside during the winter when it freezes - the cylinders can explode if frozen so by bringing them inside you can extend their life and get more for what you pay for.

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