Natural ventilation guidelines

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August 18, 2015

John W. Bartok Jr.

Natural ventilation is more common today in new greenhouse construction and in retrofits of existing greenhouses. This is due to increasing electricity cost for fan operation and the desire for more uniform cooling in the crop area.

Natural ventilation systems operate on the principle that heat is removed by a pressure difference created by wind and temperature gradients. Wind plays the major role. For a well-designed greenhouse, wind speeds of 1 mph are adequate to keep the inside temperature within two degrees of outdoor ambient. Weather records show that there are very few days that the wind is less than 1 mph, especially if the outdoor temperature is above 80 degrees Fahrenheit.

Buoyancy, the effect of air getting lighter as it is heated, also aids ventilation. The greater the temperature difference between inside and outside, the greater the air movement.

Natural ventilation systems do not work well where evaporative cooling is needed, as the heat and high-humidity air created is easier to remove by positive air movement. Also, insect screening cannot be used with natural ventilation as not enough air can get through the screen material.

The following are things to consider when remodeling or building a greenhouse with natural ventilation:

Size of the vents – American Society of Agricultural and Biological Engineers standards recommend that that the combined roof vent area should equal the combined sidewall vent area and each should be at least 15 to 20 percent of the floor area. For northern climates 15 percent may be enough but greater amounts are needed in warmer climates. Research by G.P.A. Bot in the Netherlands showed that having sidewall vents in a single or double span house quadrupled the ventilation rate as compared to just roof vents. In large, gutter-connected greenhouses where sidewall vent area per floor area decreases as the width increases, larger roof vents are needed. Open roof designs may eliminate the need for sidewall/endwall vents where more than 50 percent of the roof is open.

Vent location – The typical location of roof vents is hinged on both sides of the ridge. This allows operation so that the leeward vents can be opened to create a vacuum at the top of the ridge. Gutter vents have not worked well because the vacuum is not at strong and they can be inoperable when snow fills the gutter. Sidewall vents are usually located at bench height. This works well to cool plant foliage but is affected by how plant vegetation is orientated adjacent to the wall. It can also have an effect on the plants if the vents are opened during cool weather. Some growers install the vents higher up the wall and allow mixing of the outside and inside air before it reaches the plants. This intercepts wind having a higher velocity. The installation of guillotine vents has eliminated the problem of the rack and pinion system interfering with the plant growing area. With gutter-connected greenhouses, roof vents are more important than side vents.

Greenhouse height – The trend toward taller greenhouses has helped ventilation because it increases the buoyancy effect and gets the hot air higher above the plants. It also provides buffering of the air and reduces quick changes in temperature. The standard gutter height is now about 14 feet and taller greenhouses are used for some crops. This also allows space for energy/shade screens, tall crops such as tomatoes and cucumbers and multiple layers of hanging baskets.

Orientation of the greenhouse – Where possible, the greenhouse should be orientated to intercept the normal summer wind along the sidewall. A check with the local weather station can provide this. Also trees, greenhouses and buildings should not obstruct the natural air flow.

Operation – Horizontal Air Flow (HAF) fans should not operate when natural ventilation is being used as it may counteract the ventilation air flow. Controls should be installed to give high wind and rain protection. Seals around vents should be checked before the winter heating season to reduce infiltration.

 

Bartok is a regular contributor to Greenhouse Management and an agricultural engineer and emeritus extension professor at the University of Connecticut. He is an author, consultant and a certified technical service provider doing greenhouse energy audits for USDA grant programs in New England.

Have a question? You can write John at jbartok@rcn.com.