High grade your high tunnel

Bob Rimol discussed high tunnel construction and other recommendations at the 2025 Ohio Ecological Food & Farm Association’s annual conference. Rimol is the owner and founder of Rimol Greenhouse Systems based in Hooksett, NH. Rimol manufactures and distributes greenhouses across the U.S.

Framing Materials

“I know that at times when you start out and you buy your first tunnel, the path of least resistance is to use wood. It’s just cheaper than metal but the one thing is that it doesn’t last as long,” said Rimol. Over time, wood also attracts pathogens.

Rimol’s ideal framing material is steel because it lasts forever and contains no chemicals. Steel is preferable for certified organic growers who are prohibited from using pressure treated wood.

If a grower prefers wooden framing Rimol suggested using cedar.

Doors

Rimol likes to build large doors into the end walls because they are important for airflow. He recommended using double sliding doors on the end walls rather than hinge doors. In his experience, hinge doors tend to whip around in the wind causing the lock sets to eventually break.

For growers who need to accommodate a tractor or other large equipment, an overhead door that rolls up into a cylinder works well.

Over the years, Rimol has watched growers transition from having one door in each end wall to having one large overhead door and one for human traffic on each end. The small doors are especially important for growers producing winter crops who need to minimize the cold air entering the high tunnel.

Sides

The most basic way to roll up the sides of the high tunnel is with a T-handle kit that must be operated manually. Rimol said many growers have transitioned from this labor-intensive system to gear boxes. Some of these gear box systems can even be operated with a cordless drill. Regardless of the mechanics, these systems still require a person to complete the task of rolling and unrolling the sides.

“Probably one of the biggest investments you could make in your tunnel is to motorize your roll-up sides. It is probably one of the best returns on investments that you can do,” Rimol said.

Most gear box systems run on 24-volt motors and some use solar power. Motorized sides save on labor. Another benefit is that because the systems can include thermostats, they allow the grower to dial in the temperature of the high tunnel. For example, in early spring when the weather is changing constantly, the motorized sides can roll up and down in response to temperature changes.

Gable Shutters

Rimol said that gable shutters are gaining more traction amongst growers. These can also be motorized and are most effective when they work on a thermostat. With gable shutters, a grower can ventilate without having to roll up the sides. They also allow hot air that is caught up near the peak to escape.

Gable shutters are a huge benefit for winter growers because they help reduce humidity. “I know some people that literally leave the shutters open all winter long,” Rimol said.

Ridge Vents

The ultimate in high tunnel ventilation, according to Rimol, are ridge vents. With mechanized sides and a ridge vent, a tunnel will never get too hot. They are, however, expensive, and installing the plastic covering is more complicated. Rimol noted that ridge vents are always installed on the leeward side of the prevailing winds.

Ridge vents are especially helpful when growing crops with a thick canopy such as tomatoes and cucumbers because they provide a consistent supply of fresh air and help reduce humidity that can promote foliar diseases.

Ridge vents are beneficial for winter growing if left cracked open about two inches. “The dehumidification is an unbelievable benefit,” Rimol said.

Horizontal Airflow Fans

Horizontal airflow fans (HAFs) are an affordable option that mix the air in the high tunnel by generating gentle but high volume air circulation. This action helps to eliminate hot and cold spots.

The combination of roll-up sides, large end wall doors and HAFs really “churns up the air,” Rimol said. In fact, research shows that the use of HAFs can increase carbon dioxide levels in the high tunnel which is beneficial for plant growth. HAFs can also reduce pathogens.

Rimol suggested one fan every 50 feet. A 12-inch variable speed fan is recommended for greenhouses that need more control with airflow, and a 20-inch fan is recommended for larger greenhouses that require maximum air circulation.

Polyethylene Covers

Six millimeter polyethylene (poly) is used to cover most high tunnels and comes with a four-year warranty. Rimol cautioned against keeping poly longer than that because as it ages, light transmission decreases, resulting in lower yields.

Most growers use a double layer of poly. For the inner layer, Rimol suggested using infrared/anti-condensate (IRAC) 6 mm poly. This poly is treated with additives that provide several benefits. The additives allow heat to enter during the day, but it doesn’t escape at night. (This is most useful during the colder months.) The additives also prevent large droplets of condensation from forming on the inside of the high tunnel.

Cost Sharing

Rimol encouraged growers to reach out to their local USDA-NRCS office to find out what cost sharing options are available. NRCS cost shares not just high tunnels but also upgrades, like ridge vents and environmental controllers.

by Sonja Heyck-Merlin