Savings through the roof

Natural ventilation saves electricity, by eliminating the use of fans. Along with roll up sidewalls or large endwall doors, roof vents can provide adequate cooling. They can be motorized so temperature within the greenhouse is automatically controlled. They can save as much as 0.5 to 1 kilowatt hour per square foot per year.

Greenhouses with roof and sidewall vents operate on the principal that heat is removed by pressure difference created by wind and temperature gradients. In a well-designed greenhouse, wind speed of 2 to 3 mph provides 80 percent, or more, of the ventilation. Wind passing over the roof creates a vacuum, and draws the heated air out through the vent. If the sidewalls are rolled up or endwall doors open, cool replacement air enters and drops to floor level. If the sidewall vents are closed, cool air enters the bottom of the roof vent and heated air escapes out of the top of the vent.

Bouyancy, the effect of heated air rising, aids ventilation. Heavy cool air becomes hter as it is heated and rises toward the roof. On cool days, a large temperature difference creates excellent air exchange. On hot days, the buoyancy effect is almost nonexistent. When the vents are open, horizontal air flow fans (HAF) should be shut off to avoid destratifying warm air.
 

Vent design

Roof vents and side or endwall vents should each total 15 to 20 percent of the floor area to get adequate air exchange. The best orientation for the greenhouse is to have normal summer wind direction blow over the ridge creating a vacuum on the leeward ridge vent. In warm climates, vents on both sides of the ridge are an advantage.

Most vents are designed using standard rack and pinion hardware, with nylon bearings in the pinion yoke to support the drive shaft. Manual operation can be used but adding a 1/20 horsepower gearmotor and vent controller provides operation when no one is around. With motorized systems, limit switches need to be accurately located and adjusted. Roof vents are expensive costing from $30 - $50/linear foot.

Advantages to roof vents include more uniform cooling as cool air is distributed over the length of the greenhouse. With fan ventilation the air is drawn in through shutters at one end, heating up 5°F to 10°F as it travels the length of the greenhouse before being exhausted at the fan end. Energy costs are reduced as fan ventilation uses from 0.5 to 1 kilowatt hour per square foot per year.

Disadvantages include greater difficulty in covering the greenhouse with poly, as the vent divides cover into two sections and extra extrusions are needed to attach the poly. Some growers cover the vent with polycarbonate to make recovering the greenhouse easier. It is more difficult to install shade cloth as the vent area has to remain unobstructed. And there is more maintenance than with fans. Vents tend to get out of line and may not close tightly. The rack and pinion may slip, or wind may bend the vent frame resulting in a gap when closed.
 

Sidewall vents

Greenhouses with sidewall vents operate on the principle that heat is removed by a pressure difference created by wind and temperature gradients. Wind plays the major role. In a well-designed and -located greenhouse, a wind speed of 2 to 3 miles/hour provides 80 percent or more of the ventilation. It is best if the greenhouses are located so that the one sidewall intercepts the summer breeze. There also needs to be sufficient space between adjacent greenhouses so that the wind can reach the sidewalls and not be lifted over the greenhouse.

The basic roll-up sidewall consists of a rail 3 to 6 feet above the baseboard, a roller the length of the sidewall, guide bars or ropes, and a gear crank or motor to raise and lower the roller. The rail that separates the roof plastic from the sidewall plastic can be an aluminum extrusion (poly lock, wire lock, etc.), or it can be a 1-inch-by-6-inch board. This is attached to the hoops with tek screws or “U” bolts.

Steel tubing or a specially manufactured roll-bar is used to attach the lower end of the plastic. PVC pipe has been used by some growers, but it tends to sag when the temperature gets hot. To create a tight seal at the baseboard, a lip made out of wood or a special extrusion is used. This reduces air infiltration during cold weather.

Guy ropes are zigzagged every 4 feet from the rail to the baseboard to keep the roll-up from flapping in windy weather. This also guides the plastic when it is rolling up or down. The most common method of sealing the end of the roll-up is to fill in the space between the two end hoops at the two corners. This provides a fairly tight friction seal. Most growers close off the end of the roll-up with furring strips or wiggle-wire during the winter.

One of the easiest methods of rolling up the sidewall is to install a “Tee” at one end of the roller and insert a sliding rod to get leverage. Other mechanical operators include a hand-crank gear box with internal-brake, low-voltage and line-voltage gear motors and tube motors. A guide post is needed for all systems that have a non-stationary operator.
 

Manual or motorized?

Should you motorize your roll-up sidewall or leave it manually operated? If you have staff on duty during the day when ventilation may be needed, a manual system may be adequate. Motorized systems have caused problems for some growers, with the plastic becoming crooked on the roll or jamming up at one end. These systems need to be maintained on a regular basis. A controller is used to operate these systems.

Along with roll-up sidewalls, I usually recommend that a two-speed, thermostatically operated medium-size fan be installed with shutter in the opposite end-wall to handle early-season ventilation.

Roll-up systems work best with a greenhouse that has vertical sidewalls. With a quonset shape, the roll-up allows the rainwater to drip inside and can damage plants along the sidewall.

Bartok is a regular contributor to our sister publication, 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.