10 tips for keeping your greenhouse cool

Natural ventilation

1. Use side vents in addition to roof vents to quadruple the cooling rate. In freestanding greenhouses, vent openings should be provided on both sides of the ridge and on both sidewalls. Vent operation should be such that the leeward vents are opened to produce a vacuum at the top of the ridge. The National Greenhouse Manufacturers Association (NGMA) recommends that the combined sidewall vent area should equal the combined ridge vent area and each should be at least 20% of the floor area. Note that 80 to 90% of ventilation is due to wind currents.
2. Open doors to allow more air in. The more air that can get in will result in more cooling. Natural ventilation is more uniform than fans but will not cool the greenhouse to a lower temperature than ambient outside air.
3. Use open weave interior shade screens. These work best as they allow hot air to escape uniformly throughout the house. An alternative is to crack closed weave screens, but these may create uneven growth due to the sun shining on some parts of the plant canopy.
4. Turn off air circulation fans. HAF fans should not operate when natural ventilation is being used, as it may counteract the ventilation air flow. This also increases energy use. Each fan adds 2.5 to 5 kilowatt hours to your daily electric bill.
5. Add a skirt to rollup sidewalls. A plastic strip 12" to 24" high will keep cold drafts from ground-grown plants early in the season.

Fan ventilation

6.    Maintenance is important. Check fan belts for wear and tension. A squeaking fan is a good indication the belt needs attention. Clean fan blades to increase air flow. Lubricate shutters.

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Source: Greenhouse Management