Natural Ventilation in Hoophouses
Roll-up sides and drop-down curtains are low-cost vent systems that can be installed in most hoophouses. These systems operate on the principle that heat is removed by a pressure difference created by wind gradients. A wind speed of 2-3 miles per hour is adequate to force cool air in the windward sidewall. The air traveling over the greenhouse creates a vacuum on the leeward side to pull the heated air out.
In both systems, a rail, either wood or metal extrusion, is attached to the greenhouse frame 3' to 5' above the base board. Rails are usually attached on both sides of the greenhouse to allow for cross ventilation. The plastic over the top of the greenhouse is attached to the rail to form a seal for air inflation. In roll-up systems, the remaining material becomes the vent. The bottom of the plastic is attached to a piece of steel tubing with clips or battens. A two-piece roll bar that clamps the plastic is available from Advancing Alternatives, Inc., Schuylkill Haven PA., www.advancingalternatives.com. To open the vent, the tubing is rolled up with a hand crank or vent motor. Opening the vent introduces cool air at the bottom which may or may not be desirable depending on the placement of the crop in the greenhouse.
In drop down systems, cool air is introduced at the top of the curtain allowing it to moderate before it reaches the plants. The bottom of the curtain wall material is attached to the baseboard with a batten or aluminum extrusion and the top is attached to the steel tubing. The curtain is lowered or raised by a system of cables and pulleys that are attached to either a manual or motorized winch. A separate sheet of plastic is frequently used for the curtain. This material can be a 4-year copolymer film or it can be a heavier material such as a reinforced polyethylene or polyvinyl.
Another drop down system utilizing inflated poly tubes (Poly-vent) is available from Poly-tex, Inc., Castle Rock MN – www.poly-tex.com. The poly tubes, held in place along the sidewall with retainers, are inflated in two stages by small blowers to provide two levels of ventilation. A back-up generator or 12 volt battery operated blowers are needed to handle power interruptions.
Natural ventilation is better
Roll-up and drop down systems have several advantages over fan systems.
• Eliminating the fans reduces the electric bill, a nice bonus in a time of increasing rates. Fan cooling for a typical hoophouse operated spring, summer and fall is 8¢ -10¢ per square foot based on 12¢ per kilowatt hour.
• During warm weather, the temperature inside the greenhouse can be maintained within a degree or two of outside. Unlike fan ventilation where the temperature at the fan end is 8ºF to 12ºF warmer than the intake end, natural ventilation will provide uniform temperature throughout the greenhouse. The best cooling is achieved where the greenhouse is not obstructed by other buildings and is orientated to receive the summer breeze.
• Opening the sidewall allows easy accessibility for moving plants into or out of the greenhouse.
• There is a reduction in noise if fans are not used.
A tight seal is important
Air leaks can be a problem, especially on windy days. There are systems that reduce infiltration including (a) installing plastic over the first frame on each end to form a seal, (b) attaching Velcro to the outside frames and an adhesive backed, felt strip to the plastic, (c) inflating a polytube attached to the end frames and (d) installing a curtain pocket.
To seal the bottom of a roll-up system, the bottom of the curtain can be lowered onto a shelf made of wood or metal that seals along the entire length. A continuous extrusion manufactured by Advancing Alternatives, Inc. can be installed to fully seal the bottom of the curtain. On drop-down systems the curtain is drawn up into a pocket or hood that sheds rain and snow and forms a tight seal.
To keep the curtains from billowing out in the wind, a retainer is needed along the side of the greenhouse. It needs to be firmly attached at the rail and baseboard as there is considerable pressure built up on the curtain in a heavy wind. Several methods are used.
1. Retainer clips attached at the rail and baseboard allow nylon or polypropylene cord to be laced into zig-zag pattern.
2. Curtain brackets made from 1" conduit or tubing placed at 4' to 5' intervals makes a permanent installation and allows easy access through the wall for materials handling.
3. Polypropylene strapping, 2" to 4" wide, attached vertically every 4' to 6' gives good support and reduces wear on the curtain.
Some growers have installed a double layer of poly curtain and inflated it to reduce heat loss. This also reduces the rippling effect of a single layer material and the associated wear.
Methods for raising and lowering the curtain
The simplest method for opening a roll-up curtain is a hand crank. Adding a universal joint allows the crank to be operated in any position.
To reduce the job of raising long, heavy curtains, a gearbox assembly that rides on tubing or a rail can be used. Usually the gearbox has a ratio of 10 to 1 or 15 to 1. Operation of the gearbox can be either manually with a crank or powered with a battery-operated drill.
Motorized gearboxes and tube motors are available that can be controlled by thermostats, humidistats, timers or a controller. Cost is around $1000. Limit switches that provide stops at the top and bottom are needed.
Drop down curtains are frequently operated with a winch. A series of vertical support cables are attached to the tubing in the top edge of the curtain. Usual spacing is at least one foot more than the height of the curtain. The other end of each cable is drawn through a pulley and then clamped to a main control cable. The main cable is supported by pulleys at each end. One end of the main cable is attached to the winch. A counterweight is attached to the far end to maintain tension and to lower the curtain evenly. The cables, clamps and pulleys should be stainless steel to have trouble free operation. The size of the winch is determined by the length of the curtain. One with two-way operation is desirable. Electric winches allow automatic ventilation. They are available in 120 volt and 12 volt models. Limit switches and a thermostat are needed.
Several manufacturers offer a ridge vent for their hoophouses. Vents operate based on wind pressure differences, air temperature buoyancy and the vacuum created by air passing over the ridge. The vent can be operated manually, motorized or fitted with the inflated tube system. Ridge vents make covering the hoophouse with film plastic more difficult as attachments are required on both sides of the vent.
Growers with hoophouses have found that roll-up and drop down curtain systems and ridge vents work well for warm season ventilation. A location with good summer breezes and plenty of space between houses will allow the temperature to remain within a degree or two of outside. The vent areas need to have a tight seal if the hoophouse will be heated during the winter.
John W. Bartok, Jr., Extension Professor Emeritus & Agricultural Engineer, Department of Natural Resources and the Environment, University of Connecticut, Storrs CT – 2013