The Variations of Passive Venting

In a nutshell, passive venting is creating an opening and letting the warm / hot air escape. The simplest form of this is opening a door or window. Just because it is simple does not mean that this method will be effective in cooling your greenhouse.

If your ends face into the prevailing winds, if you can make your doors big enough and if your greenhouse is short enough. That is a lot of “if’s” and you will still be doing a lot of running back and forth to have the correct amount of opening for the amount of ventilating you need to do.

We have already reviewed the most common form of passive ventilation in the roll up sides. It was noted that roll up sides work much better when paired with a high opening to create a chimney effect. It is a fact that hot air rises so the higher you can create an opening, the more effective it will be.

If you are relying solely on gable end windows, either motorized or manual, they will need to be quite large. You will also need to have the benefit of a regularly strong prevailing wind.

The most effective form of passive venting is a continuous roof vent. This will provide a continuous opening in the precise area where the air is the hottest. A roof vent should always be mounted down wind of the prevailing wind. The benefit of being down wind is that the wind creates a vacuum as it goes over the greenhouse and sucks the warm air out.  The air intake for a roof vent is often a roll up side.

A significant down side of a roof vent is the up-front cost. The cost is the same for a narrow and a wider structure. This is the reason they are almost exclusively put on wider buildings. The cost is simply spread out over a bigger area. The effectiveness of a roof vent still makes it appealing in spite of the cost.

Roof vents can be controlled manually with a chain fall opener or with a motorized gearbox. A motorized system can be a simple open / close controller that you set the limit switches or it can have a proportional controller that allows for pre-set stages. It is with an automatic controller that the benefit of the roof vent will really become apparent. Every couple of minutes it will react and adjust to the inside condition of the greenhouse.

All of the expenditures involved with your greenhouse need to be weighed as a cost versus benefit or cost versus return. This process is especially challenging when considering the options of ventilation. Many of the expenses are subtle and hard to measure. The returns are equally hard to quantify since weather is an additional variable. This makes it even more important to keep accurate records and be aware of your options for improvements.

The benefits and drawbacks of forced ventilation

By definition, forced ventilation included an electrically operated exhaust fan at one end of a building and intake louvre at the opposite end.

The biggest advantage of this system is how simply it can be automated. Running your power supply through a thermostat will allow the system to come on and shut off at the set point.

There are numerous things which need to be considered for forced ventilation to be effective.

The prime consideration is proper sizing of the system. Most greenhouses require one air change per minute to keep the temperature within a tolerable range for the plants. Fans are rated for the cubic feet of air they can move per minute (cfm). By calculating the air volume in a greenhouse, you can then get a fan that is appropriately sized.

There also is a formula to match the air intake to the exhaust. When the intake is too small, the capacity of the fan will be restricted but if the intake is too large, it also is undesirable. A properly sized intake will create some turbulence so that incoming air is mixed with the existing air before it moves through.

Consideration must also be given if the intake has netting over it to control incoming weed seeds or insects. This will restrict more air flow than is usually expected.

The orientation of fan / louvre combination is important. If the intake is on a south facing wall, the incoming air will always be warmer than average. This means you will be moving warm air rather than cooling.

If a fan is blowing into a prevailing wind, the fan capacity can be significantly decreased as well. If you do not have a choice but point that way, it would be wise to consider a hood or other deflector over the fan.

When you are deciding on what size fan to get, it is often suggested to get two fans with that combined capacity. It will cost more up front, but the extra payback will be on extra flexibility or options for ventilating. This gives you the option of “half capacity”.

When planning for a forced ventilation system, it is important to understand the requirements and cost of getting the electrical service to that area and also what the on going operating costs will be. In both cases, these expenses can be significant.

One last thing which needs to be pointed out for consideration has to do with irrigation. In a forced ventilation structure, things dry out more quickly and they will also dry out unevenly. Just something to be aware of when you are thinking everything will get watered the same.

In our next article we will explore the opposite end of this spectrum … some of the options of passive venting.

What is and isn’t “proper ventilation”?

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?

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!

We hope you are finding value in this series of posts, helping you prepare for your new growing adventure! Reminder that the deadline is May 31st for Fall Delivery. We look forward to working with you for your projects!

How Can I make my greenhouse its best?

Different tips to make it more efficient, proper space for plants, proper light,
when to heat and not

There are numerous ways of making your greenhouse more efficient and a better return for your investment.

The first question which needs to be answered, however is, “What does it need to be efficient for?” The answer to that should be in line with your goals and priorities for your greenhouse.

As an example, if you are not going to be growing in the winter, you will not need to make the structure “heat efficient” but you will need to ensure that it is “snow efficient”. If you are growing in the greenhouse through the summer time, the efficiency of your ventilation system is critical.

Making your structure more heat efficient (summer perspective) will be expanded on in post #5 and more heat efficient (winter perspective) will be expand on in post #6.

When you intend to start plants in early spring and then move the greenhouse to start another crop, the moving process can be made more efficient by installing “skis” under the base.

Anchoring concerns are touched on in post #3. This is also a way of making more efficient use of your property with investing in more greenhouse space.

When you implement a trellising program, you will make more efficient use of the floor space in your greenhouse. It is important to consider the need for your plant load being balanced on your structure.

Another important thing to be mindful of is the shading that is created when you implement vertical growing. Under certain conditions you may need to consider supplemental lighting to make things grow.

One consideration that is important to be mindful of as well, is that in the confined space of a greenhouse, you can not grow everything. This is not just a space limitation, but a climate limitation.

Certain plants prefer it warm and some like it cool. Other plants prefer wet and others dry. It will be more efficient use of your greenhouse if you collaborate with a like minded person. You grow more of certain things and the friend does likewise and then you trade the extras.

Here is to working together and growing together!

We hope you are finding value in this series of posts, helping you prepare for your new growing adventure! Reminder that the deadline is May 31st for Fall Delivery. We look forward to working with you for your projects!

Coffee Time with Norm: Starting Soon!

Our weekly meet and greet chat is happening again this week, (STARTING in 15 min!) and we hope every week Wednesdays at 9am EST. We hope you can join us!

Today Norm will be covering Ventilation, and time permitting anchoring as well. You will be recorded if you join, but can choose to disable your camera if you are not comfortable having the recording posted on social media.

Join Zoom Meeting
https://us02web.zoom.us/j/83325248870?pwd=WkxKakRDakg4SzIyUVMzTlR2YXErQT09
Meeting ID: 833 2524 8870
Passcode: 5WBcJw

Join us online every Wednesday morning on Zoom to connect, chat, and find solutions to challenges you may have.
Since we aren’t able to get out to trade shows this year, we decided to host this quick weekly check in to connect and say hello!
Norm will be available to answer any questions you may have. You may also send them in advance to multisheltersales@gmail.com and we will answer them “live” on zoom.
We know you’re busy with your business too, so if you send in a question and can’t make it live, don’t worry, we will post a recording on Facebook and YouTube after the event.
Come out and connect, no matter which side of the country you’re on! this allows us to see everyone face to face that we may not normally get to because of distance. we look forward to having you join us in our new weekly special event.
Please note this is a time for connection, brainstorming solutions to your challenges, learning some information about structures and other questions. If you have a question about an order, or would like a quote, please contact the office separately and we would be happy to help you.
We are excited to have you join us for this new event and we look forward to your questions!
Join Zoom Meeting
https://us02web.zoom.us/j/83325248870?pwd=WkxKakRDakg4SzIyUVMzTlR2YXErQT09
Meeting ID: 833 2524 8870
Passcode: 5WBcJw
Coffee Time with Norm Videos