What is Natural Greenhouse Ventilation?
Natural greenhouse ventilation happens when air moves along the width of the greenhouse from a sidewall intake through to either another sidewall on the other side or through ridge vents at the peak of the greenhouse. Hot air rises and exhausts through a ridge vent design creating a vacuum that pulls air in from the sidewall. Natural greenhouse ventilation is also called "passive greenhouse ventilation."
Simply by using roll-up sidewalls and ridge vents, you can achieve a good amount of ventilation if you live in a climate that does not get too hot. In warmer climates mechanical vs natural ventilation is more likely to be necessary. When it comes to managing temperature in a manual light dep greenhouse, natural ventilation can be a challenge. Crops need to be 100% light-proof while still getting sufficient airflow to prevent crop damage. Using mechanical ventilation in an auto light dep greenhouse may be more reliable for photoperiod sensitive crops.
Natural (Passive) Greenhouse Ventilation
Natural, also known as passive ventilation relies on wind and thermal buoyancy to create air movement within the greenhouse. Thermal buoyancy is simply the tendency for hot air to rise. On cool days, thermal buoyancy can be very effective at removing heat from a greenhouse through rooftop exhaust vents, but in warmer weather, it can have little effect. Taller greenhouses help keep warm air above plant height and tend to have a more efficient exhaust.
Natural Ventilation Features
- Tall structure height to draw hot air up
- Ridged roofs with vents to let out hot air
- Greenhouse shutters (louvers), exhaust fans, and vents for letting air in and out
- Wind patterns provide cooling and fresh air through roll-up sidewalls and shutters
- Optimized wind exposure in greenhouse design
- Shade cloth systems to aid cooling on hot and sunny days
The coolest a naturally ventilated greenhouse will get will be close to the outdoor temperature. If natural ventilation is not sufficient, adding fans and evaporative cooling helps force air movement into and out of the greenhouse.
Roll Up Side Curtains and Drop Down Curtains
Growers planning on using natural ventilation may consider building a taller greenhouse structure to increase space for warm air to rise up away from plants. Taller greenhouses also gain more airflow through roll-up side curtains because a stronger vacuum effect draws fresh cool air in near the ground as the warm air rises. Just 2-3 mph wind is enough to force cool air into the greenhouse, but it is important to make sure the greenhouse is well positioned to the prevailing wind to maximize wind exposure.
- Roll-up side curtains and drop-down curtains are very cost-efficient
- Best paired with insect screens to keep pests from entering the greenhouse
- Can be customized easily to fit any sidewall height
- Manual and motorized roll-up greenhouse side curtains are available
- Roll-up side curtains are a great option for hoop houses (gothic, quonset, or gable shaped roof) on sites with good wind during the summer
For both systems, a rail is placed 3-4 feet above the baseboard of the greenhouse on both sides. Plastic is attached to the rail and covers the top of the greenhouse, creating a seal. For roll-up sides, the excess fabric becomes the vent, when the fabric is rolled up it opens. Both sides can be opened to create cross-ventilation if desired. For drop-down systems, the bottom of the curtain attaches to the baseboard and rolls up to close and down to open.
Ridged Roof Vents
Sidewall vents or roll-up side curtains work especially well in ridged roof greenhouses with ridge vents. Ridge vents and sidewall vents in greenhouses create a pressure differential between wind and the greenhouse temperature. At a speed of 2-3 mph, wind and vents can provide over 80% ventilation. Combined ridge vent area and the total combined area of the sidewall vents should equal approximately 15-20% of the greenhouse floor area.
Example: A 10 ft x 30 ft greenhouse would have a floor area in square feet (ft2) of 300 ft2. 20% x 300 ft2 = 60, so a total area of 60 ft2 in ridge vents and 60 ft2 in sidewall vents is needed.
In passive ventilation systems wind pushes cool air in, and draws warm air out by creating a stronger vacuum effect. This effect is enhanced when the wind blows over ridge vents. Wind exposure can be optimized by constructing your greenhouse so the prevailing winds at the site blow over the ridge vents. Especially in summer, sidewall vents and roof ridge vents should be kept equally open. Closing sidewall vents, while roof ridge vents are open, will force cool air to enter under the warm air exiting the roof ridge vents.