Tag Archives: air movement

What is and isn’t “proper ventilation”?

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Proper ventilation, in a nutshell, equals adequate air changes and movement while maintaining adequate light levels and humidity in a given area. Proper ventilation is about proper balance.

The challenging part of achieving this balance is that there is such a wide range of requirements for different plants. It is important to identify what is ideal for each variety that you are growing. It will not be feasible to have a perfect climate for each variety of plant, but you really should avoid grouping together plants with a wide difference in requirements. That would simply ensure that you are not doing very little properly.

Putting shade cloth over a greenhouse may lower the temperature to an acceptable level for your plants but that does not equal good ventilation. You may be depriving the plants of their critical light requirement plus you are not doing anything to enhance air movement.

Turning on an exhaust fan may create the required amount of air changes or movement, but with a temperature far below the plant optimum, your plants still will not achieve their potential. Fans also draw air in a straight line from intake to exhaust. This does not address “stagnant corners”.

Many times, light equals heat. They go up together and go down together. It is important to understand how and why they are controlled differently and where the overlap on that control is.

Generally speaking, proper air changes equals one air change per minute. Some plants require more and some can get by with less. For this, you will need to know what the air volume is for your building. It is a little trickier to calculate this on a gothic or quonset shaped structure. Using the length x width x 80% of peak height will give you a close enough number in either cubic feet or cubic metres.

With the next 4 posts we hope to create a more thorough understanding of how to achieve a good environment for your plants. This information will be helpful as to research options so that you can balance costs with benefits and return. Especially in the area of ventilation, many times expenses associated with proper ventilation will be offset by exponentially higher returns.

What are the considerations for growing over the summer?

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Ventilation, excess heat, making sure roll up sides are critter proof

One of the critically important things that each gardener/grower needs to know is the temperature threshold of each plant.

This is the temperature at which a plant will stop growing and an even higher temperature at which a plant will die.

When a plant has passed the threshold where it stops growing, it will take some time to recover and start growing again.

This is why proper ventilation is so vitally important. On a sunny day, proper ventilation is defined as one air change per minute. You must know the volume of air in your greenhouse and have your ventilation method capable of moving that much air.

A forced ventilation system is the simplest to set up and control since it is thermostatically controlled. The up front and operating cost are significant. Each exhaust fan has a cubic feet per minute (cfm) rating which must be equal or greater than the air volume of the building. A forced ventilation system also has a motorized louvre which will open each time the fan turns on.

On a shorter structure that is facing into the wind, there is the option of ventilating through the doors and windows. This is an economical system but the hardest to control.

Adding roll up sides to a structure has the capacity of moving a lot of air as long as there is air movement around the greenhouse.

This is why it is a good idea to install a window as high on each end as possible. Since warm air rises, this will create a “chimney” effect on a calm day. The down side of roll up sides is the potential for draft across the floor and the fact that you have to be there to open and close the system. You may need to put up mesh to deter uninvited guests.

In addition to changing the air in the greenhouse, it is also important to circulate the air horizontally. Stagnant air pockets invite disease problems. There is not a prescribed rate to move the air, it is just important to keep it moving.

A proper watering system would not be considered as a part of making a greenhouse more efficient but it must be remembered that a greenhouse will increase the water needs of all of the plants.

A final consideration for growing over the summer is the potential need for shade. Most vegetables are good for full sun, all the time, but it is important to remember that this does not apply to all plants.

Wishing you a productive growing season!

What are the considerations for growing over winter?

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Heating, interior tunnels, air circulation, humidity control

When someone is intending to grow in a plastic covered greenhouse over winter, the first order of business is to put a double layer of plastic with air between.

This cushion of air acts as insulation and will reduce heat loss by about 30%. A secondary benefit from this cushion of air is that it reduces condensation in the greenhouse dramatically.

Even if you are not going to heat the greenhouse, it makes sense to put the extra layer of cover with air between since it will provide a significant extra margin for cold.

The small fan which puts the air between the layers is made to work against pressure. It can not “over fill” the cavity.

When installing the second layer of plastic, it is not pulled tight. This allows the second layer to puff up to about 10 cm which is the optimum space.

It is the dead air space that creates the insulation value. If the air is moving, you will not have optimum efficiency. This is the reason it is important to be vigilant about patching any holes in the plastic when they occur.

Especially if your intention is to grow without heating, you could benefit from installing and using small interior tunnels. These tunnels would only be covered during severe weather. Since the air volume is small, even the heat from a few light bulbs will make a difference.

Interior air circulation is even more important in the winter time than it is in summer. In the winter time when things are more likely to be closed up, the probability of stagnant air is high.

It is important to monitor the humidity level in your greenhouse as well. Getting rid of excess humidity will cost you some heat, but you are creating a healthier environment for your plants.

If you are using a forced ventilation system for either humidity or heat control in the winter, it is important to have the thermostat close to the incoming air so that things will shut down quickly if the incoming air is extremely cold.

Any heating system which is used in a greenhouse should have a stainless steel heat exchanger and burner. The standard aluminized units simply will not stand up long term in the moist environment of a greenhouse.

We hope you have found value in this series of posts, helping you prepare for your new growing adventure! We look forward to working with you for your projects!

Roll Up Side Wall Considerations

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Ventilation can either be done passively through openings or mechanically with fans. Roll up sides are an economical way of getting lots of air movement since no electricity is required. Roll up sides become even more effective when used on longer buildings. Roll up sidewalls are often used in combination with a small exhaust fan for early and late season ventilation when opening the sides is not practical.

If a structure is very exposed, it is best not to open more then 3’ in height due to potential of wind damage. If a structure is extremely sheltered, it is best to go even up to 6’ to create maximum opening. Roll up sidewalls are most effective when used in combination with peak end wall vents to create a “chimney effect” to draw warm air out of the building, especially when it is very calm. These vents are effective for air movement when outside temperature does not allow opening of the sides.

When a structure has a low profile, you will need to be careful during rainy periods due to moisture getting into the structure area. High profile structures (with straighter walls) usually work better with roll up sidewalls. When preventing a floor draft is an issue, the roll up mechanism can be raised and then the structure is lined with a skirt for the bottom 2’-3’. When using this method of ventilation, it should always be done on both sides. An effective use of this method includes opening the “downwind side totally and the opposite side on marginally.

Download our Roll up Side Wall Installation information sheet for more information

27'wide with Roll Ups roll up sides

The benefits of going to a bigger structure

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What are some of the benefits of going with a bigger structure over a smaller structure?

Budget often dictates that someone needs to start small, especially when a person is just starting out as a grower with a greenhouse.
There is also the perceived notion that staying smaller means less heating cost. In itself, that is true, since heat loss is in direct proportion to surface area exposed to the outside.

As I have mentioned before, it is very important to weigh expense
against return.

A smaller air volume has less natural circulation.
Proper air circulation for plants is critical regardless of the season.
Think of a deep pond versus a shallow pond. The deep pond never has algae on the surface because of the increased movement.

Going with a taller greenhouse will automatically increase the circulation the same way.

Another point to consider with a taller greenhouse, is that there is more open space above the plants.

This open space is where moisture can go, away from the plants even before the greenhouse ventilation system does its job.

With a lower greenhouse, moisture is always in close proximity to the plants. In a taller structure, the plants will be dry much sooner.

Moist plants in a stagnant air mass are prone to disease. These plants will, as a result, produce less.

As always, be aware of the bigger picture.

Greenhouses 101: Climate and Air Effects on your Structure

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You might be wondering, How does air volume affect the climate in a greenhouse? There also is the matter of air changes. What is the connection between shape and aerodynamics? You need to make sure it stays put.

There are 3 forces on every building which need to be considered if you want it to stay where you put it and in the shape you built it. There is down force (usually from snow build up), up lift (by wind) and lateral shift (both wind and snow). Mother nature is not restricted by lack of patience and will work away at any vulnerabilities in the structure, often without notice until it’s too late. There must not be any wiggle room that the weather can work on these vulnerabilities or things will come loose, unstable, and wear out faster..

There simply is no such thing as too many anchors. There are also various tips we can give you for ensuring that the plastic is tight enough to avoid wrinkling and flapping about, while balancing the issues of it being too tight. This can easily extend the life of your roof cover.

We also advise, especially this time of year, to cover the structure as soon as you can as the more snow you have on that spot which has to melt, the more moisture you will have in the building. The more moisture you have in the building the more condensation issues you will have. You want to give that ground the most time possible to dry up before you need to start using the building.

Contact us for more tips and considerations for building a structure that will stay put and have a longer life, we would be happy to help you.

Check out the presentation video and the rest of the series Norm spoke on Greenhouses 101 here. Stay tuned for the end of  January 2016 when he presents Greenhouses 202!