8 Questions on Greenhouse Ventilation

Greenhouses are natural hot boxes, designed to trap in heat and create ideal climates for plants that need regulated conditions in which to grow. They can get especially hot during summer months when temperatures soar, so it’s important to include a good ventilation system in your greenhouse to exchange the stale air with fresh air. Whether you own a greenhouse or are thinking about buying or building one, it is essential to know what is happening outside and inside of the growing environment. In this article we will address some frequently asked questions about ventilation for greenhouses and help you identify the solution that’s right for you.

1. What are weather stations and why do I need one?

No matter what automation solution you choose for your grow system, make sure that you have a weather station. Weather stations collect information on the environment outside of your greenhouse. They help you monitor key metrics like outside temperature, humidity and solar levels, while measuring rain and wind direction.

A weather station will be in conjunction with any sensors you have operating INSIDE of the greenhouse that will measure the microclimate.

Points to consider:

• Install your external weather station in an accessible location. You’ll need to regularly clean it to ensure your readings are accurate, especially if you’re in an area prone to bird life, leaf fall or other external factors.
• Your data is only as good as your sensor so purchase a robust system that will last and give you accurate readings.

2. What are greenhouse roof vents?

Roof vents are a passive cooling method, utilizing the natural convection of hot air. Roof vents are one of the most efficient ways of cooling a structure, especially if you are in a temperate climate. Hot air rises to the top of the greenhouse and goes out the vent, drawing in the cool air underneath. An open roof structure is adjustable; opening the vent will allow you to keep the greenhouse cool whereas closing the vent will help you contain heat. Open roof vents include open-panel, retractable-film, flat-roof and low-profile systems.

Points to consider:

• Bringing in the cooler outside air into the greenhouse during warm weather conditions is cost-effective. Many growers have found this method helpful as it has shortened production time and produced better quality plants.
• Open roof vents will help you maintain desired temperature and humidity set points.
• An open structure roof will allow more natural light into the greenhouse.
• To resolve the issue of unwanted pests, place mesh screen covers around open areas. However, be aware that this will reduce the amount of air movement by up to 45% depending on the type of screen.

3. What are side vents?

Side vents protect your plants and increase the airflow within the greenhouse. If you have the plants in your greenhouse placed on a bench, you can simply roll up the side walls to the height of the bench and let the cool air flow underneath, creating an ideal environment for the greenhouse plants. The use of side vents reduces the necessity for fans and electricity, thus reducing overall cost and making it a budget-friendly solution.

Research (Santorini 2005, Wang 1999) has shown that the most effective vent configuration is a combination of roof and side vents, especially in warmer climates at low altitude.

4. What is buoyancy driven ventilation?

Buoyancy driven ventilation is a system in which a buoyancy force is created by the difference between interior and exterior temperature which drives airflow. Warm air has a lower density and will rise above cold air – creating an upward air stream like a traditional fireplace.

The advantages of buoyancy driven ventilation include:

• It does not rely on wind and can take place on hot summer days.
• In comparison to wind, the buoyancy ventilation has a more stable airflow.
• Greater control in choosing areas of the air intake.

The limitations of buoyancy driven ventilation include:

• Buoyancy ventilation relies on the difference in temperature outside and inside.
• Depending on the location of your greenhouse, air quality will be influenced by surrounding pollution.
• Buoyancy ventilation is influenced by the design of the greenhouse.

5. What is wind driven ventilation?

Wind blowing outside the greenhouse creates a small pressure difference between the windward and sheltered side of the greenhouse, allowing air to move towards the sheltered side. Wind-driven ventilation is most resourceful when the vents in the greenhouse are oriented to take advantage of the prevailing winds. However, the wind's effect is small if vents on the roof are not complemented with the side walls.

Points to consider:

• The length and size of your greenhouse will impact the wind effect.

6. What size should the roof vents and side vents be?

American Society of Agricultural and Biological Engineers standards suggest that the area of the roof vent should equal the combined sidewall vent area, and each should be at least 15 to 20 percent of the floor area.

7. Where should the vents be placed around the greenhouse?

Roof vents should be hinged on both sides of the ridge, while side vents are usually located at bench height.

8. Why is the height of the greenhouse important?

The taller the greenhouse, the easier it is to ventilate because of the increased buoyancy effect and ability for hot air to rise higher above the plants.

It’s important to note that if your greenhouse is located in an area with heat and humidity, you may also need to consider additional ventilation equipment. If you’re ready to start discussing ventilation options for your greenhouse, contact Bluelab today.