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.

The Benefits & Drawbacks of Roll-up Sides

The main attraction of roll-up sides, as a method of ventilation, is that it is economical. There is not a significant investment and can be added to virtually any structure. Regardless of structure length, nowhere is very far from open air.

It is only by properly understanding the limitations and drawbacks that one can maximize the benefit. Just because something is inexpensive does not mean that it won’t end up being costly if it does not fulfil its purpose.

Roll-up sides should always be done in pairs (both sides). Even if you have a really high percentage of prevailing wind from one direction, there will be times when you need to ventilate into the prevailing wind.

Any time you have wind going in without opportunity of escaping,
there can be damage to the building.

The average vertical opening of roll-up sides is usually about 4’. If your greenhouse is in an extremely exposed area without any protection or wind breaks, quite often 2’ of vertical opening will be sufficient.

On the contrary, if the building is extremely sheltered, 6’ of vertical opening will be required.

One thing to remember on the height of opening is that if you have a low profile structure with a high opening, a sizeable area could get wet inside when it rains.

Regardless of the size of opening, if there is not a breeze, you will not have air movement.

It is based on this potential problem, that we recommend having end wall openings as high as possible. These openings will trigger a “chimney effect” air movement that will get rid of hot air that is trapped in the greenhouse. These openings will double as your first stage of ventilation since they would be opened when it is too cold outside to start rolling up the sides.

A drawback of roll-up sides is the amount of exposure they create for airborne weed seeds and pests. It is important to remember that when you use shade cloth to minimize this drawback, you are also significantly restricting incoming air.

The biggest drawback of roll-up sides is that there is no practical way to automate them. In other words, you have to be there to open and close them. This can be challenging on the cool, sunny days that have sporadic cloud cover. The sun goes behind a cloud and the temperature plummets and then soars when the sun reappears.

The best way to minimize this challenge is to add a small exhaust fan to the set up. This way you will have something to handle the borderline situations and then open the roll up sides once there is a more predictable need.

In the next article we will explore forced ventilation in greater detail. We also have many resources on our website, as well as videos if you want to explore the topic of roll-up sides further.

roll-up side videos

roll up side considerations

roll-up side Q&A

Storage on Blocks or Shipping Containers

Storage containers and over sized concrete blocks are the economical solution for needing to elevate structures to create more storage capacity.

One consideration is when the product being stored does not lend itself to stacking, such as salt, sand or soil, there is the added dimension of outward push on the wall. Not only does the pile push outwardly, but an operator scooping the product will create even greater push.

Storage containers and over sized concrete blocks are the economical solution for such a situation. The weight and the stability which both the containers and blocks give, eliminates the need to anchor into the ground.

Containers are often simply put on the ground.

When going more then two layers with the blocks, there should be a concrete pad or special preparation of the soil to provide stability.

When choosing this foundation method, it is important to consider how and where the shelter will shed water and snow.

The top of the wall or container must be sealed to the possibility of moisture going inside.

The other thing that must be given proper consideration is that the shelter is able to deal with the extra wind load created by elevating the shelter this much.

In most of these installations, the wind load on a building is at least double of what it would be if mounted on the ground.

sand storage on blocks Haystorage on blocksBrunelle Container pictures 001Brunelle Container pictures 012

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.

Important Notes About Hanleys & Warranties

Norm wants to caution anyone modifying their tunnel (or any structure) as this compromises the entire concept of the structure as well as voids the warranty and can start a whole host of other issues. Please call and speak directly with Norm, as well as read ALL the material on our website and installation manual before embarking on what can be economical and easy to use season extenders, or, very costly mistakes. Thank you for your interest

Hanley tunnels season extenders modified hanley


What are the considerations for growing over winter?

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! Reminder that the deadline is May 31st for Fall Delivery. We look forward to working with you for your projects!