Ask the expert: greenhouse ventilation

We spoke with Bill Mathis, sales manager at Atlas Manufacturing, Inc., about the most frequently asked questions he receives from growers about greenhouse ventilation. From the basics to the design parameters, here's what you need to know about airflow in your structure.

What is natural/passive ventilation?
Natural ventilation operates on the principle that heat is removed from the greenhouse by wind and temperature differences between the inside and outside of the greenhouse. Wind is what we primarily depend on. Even light winds as low as 1-2 mph are sufficient to keep the inside temperature of the greenhouse within a few degrees of the outdoor temperature. When ridge vents are used, the hot air inside the greenhouse will rise and escape through the roof opening. The greater the difference in temperature between the inside and the outside of the greenhouse, the greater the results.

What are the advantages of natural ventilation?
There are a lot of crops that do well in greenhouses with natural ventilation. The lower initial cost sidewall rollup curtains and the high electrical cost of operating exhaust fans makes natural ventilation an attractive option. Greenhouse sidewall vents provide a low cost, low maintenance solution.

What are some different types of vent systems?
There are several options to choose from when designing your greenhouse with natural ventilation. The most economical design for sidewall ventilation uses a woven polyethylene material that rolls up on a crank rod, or opens from the top and drops down to open. The roll up system works best on taller sidewall structures and is often used in conjunction with a base wall to provide wind protection from plant material. The drop- down system works best on greenhouses with 4-6 feet tall. It operates by the winding and unwinding or a rope around a stationary drive shaft. The advantage to this type of system is that it can be partially opened to provide ventilation while providing protection for plants without the use of a base wall.
Also available are rigid wall vents that operate on a rack and pinion system. These vents are often covered with polycarbonate but can also be covered with poly film. This is a much more expensive system, often running 2-3 times the cost of the roll-up and drop-down vents. The advantage is that when closed they provide a much tighter seal, helping to hold in valuable heat during the winter months. When covered with polycarbonate, they also provide many years of service with minimal maintenance and upkeep.

How do these vents operate?
Greenhouse vents, whether located on the roof or walls, can be operated either manually or with motorized cranks. Manual cranks are more popular with retail locations or in other settings where someone is there to open and close as needed. Temperatures can change rapidly inside the greenhouse and if someone isn’t there to open and close the vents, plant loss can occur in a short amount of time.
A more reliable system is a motorized crank coupled with an environmental controller. The controller will monitor the air temperature and open and close the vent incrementally to maintain a consistent temperature. The result is a more uniform crop and significant reductions in heating costs during winter, spring and fall production.

What other things should be considered when designing with natural ventilation?
In addition to the things previously discussed, there are a few things to consider when designing a greenhouse with natural ventilation. Greenhouse height can play a big role in how well your greenhouse ventilates and how well you are able to maintain consistent temperatures. Sidewall heights of 10’ or more will provide more favorable results because it gives more room for hot air to rise and get away from the plant material. Additionally, the larger air mass provides a buffering effect creating a more stable mass of air that will not fluctuate as quickly. 
Orientation of the greenhouse is also very important. The greenhouse should be positioned so that the sidewall opening is pointed toward the prevailing summer winds. The ridge vents would need to be installed so that they are pointed away from the prevailing wind, reducing the likelihood of wind damage and increasing the natural airflow.

For more: www.atlasgreenhouse.com