Sidewall Savvy

To reduce the cost of fan ventilation during spring and summer, many growers have installed roll-up sidewalls. Either manually operated or motorized, they provide good cooling for poly-covered hoophouses and narrow, freestanding greenhouses. The savings in fan electricity can amount to several hundred dollars a year.

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.

Although roof vents are expensive (about $40/linear foot), they can enhance ventilation by creating a vacuum that draws the warm air out the roof. A less expensive alternative for houses that are 150 feet or shorter is to install large motorized shutters in the endwalls. These can be operated with a thermostat or controller.
 

Vertical sidewalls, wind ropes and a closed end-wall panel improve roll-up sidewall performance. photo courtesy of JOHN BARTOK

Sidewall composition
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 (posi-clasp roll-lock, manufactured by Advancing Alternatives) 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.

Roll-up sidewalls cost about $8 to $12 per linear foot. USDA EQIP funding is available in some states to help offset the cost.