In a greenhouse, ventilation is tied with light and irrigation as the most important aspects of growing plants. Strictly talking about plant requirements, ventilation drives gas exchanges in the greenhouse, plant respiration and temperatures. In its basic form, ventilation is the process of moving air from outside to inside and from inside to outside the greenhouse.
Example: Apex Greenhouses (made up) is building a new greenhouse. They will be growing lettuce and using NFT. Apex is located in Oklahoma City. The greenhouse they are buying is a freestanding, 8’ sidewall, 30’ x 96’ structure. At this point the only thing defined is the structure size, a double layer poly film roof and the layout of the growing system. The growing system is laid out with a single 4’ walk way down the middle, 12’ NFT channels going from the walkway on either side to the sidewall, setting a foot from the sidewall. At one end is the harvesting location, 12’ x 30’ and at the other end is the planting area, 12’ x 30’. The actual growing space is 30’ x 72’. To work out the ventilation we need input from Apex as to how they want to ventilate, climate information and the growing needs of the plants.Apex wants natural ventilation, due to its lower operating costs, consisting of a roof vent and sidewall roll-up curtains, each running the length of the greenhouse.
The idea is to open the roof vent and sidewalls to exchange cooler air for the air in the greenhouse. The warm air rises out of the roof vent and the cooler air enters in through the sidewalls as replacement. Does this work? Well, it does work, this process works. What are the drawbacks? Oklahoma is a 4 season location with shorter spring and fall seasons, a possibly mild to below freezing winters and a hot summer. This is in conjunction with growing a cooler type crop. Lettuce doesn’t do well in hot climates, in general.
Here is where the natural ventilation doesn’t line up with the crop and expectations. Natural ventilation is a way to ventilate a greenhouse and it does have a lower operating cost compared to fans and vents. There are some nuances to the nat. vent. It doesn’t seal up tight in the winter. In order to get it sealed up in the winter, the curtains would have to be locked in around the edges. This doesn’t give a double layer for insulation but it at least would stop drafts from coming in. Once locked in being able to ventilate on a warm or bright sunny day becomes difficult.
I don’t want to sound pessimistic about natural ventilation. It really does work well and is a great option but not for a location with great temperature swings and a sensitive crop. So where do we go from here? Tried and true, basic fan and shutter ventilation.
The ventilation system I would recommend is one that has ventilation fans at one end and a rigid vent at the other.
Air is pulled from the outside through the vent and across the greenhouse then exhausted out through the fans. Another addition to the system I would recommend, for the area, is an upper gable fan and shutter. These are mounted in the peak of the endwalls and used during the early hours of the day and in the fall, spring and winter when the outside temp. is low to mild but the sun is shining bright, warming up the greenhouse. The upper gable fan and shutter simply pull out the heat in the peak and replace it with cooler air. The third part of the system that would be recommended is an evaporative cooling system. This sits inside of the rigid vent and cools the air as it passes through. The sidewalls also need to be covered and it is recommended to use twin-wall polycarbonate, a rigid plastic clear covering.
Why did I recommend this option? It seals up in the winter but still can be used, it is variable in its uses, meaning between the evaporative cooling, upper gable fan and shutter and the base fan and shutter systems there are several ways to reduce temperature. Overall, there is more control over the greenhouse environment. You’ll find that the more you can control the better your crops will be.
The toughest part of this example is explaining how we went from natural ventilation, the lower operating cost, to a more expensive and higher operating cost system. The truth is that the natural ventilation wouldn’t have produced the highest quality crops. The crops, with a high degree of certainty, wouldn’t have been consistent. This is where planning a greenhouse needs to be done as a partnership. There are many options for each aspect of a greenhouse. What is right? Whoever you use to design your greenhouse, hopefully Stuppy, but if not make sure they have your best interests in mind and not just a quote to send out.