Chris Callahan and Becky Maden of University of Vermont Extension discussed high tunnel ventilation during a Vermont Vegetable & Berry Growers Association webinar. Callahan is an agriculture engineer and Maden is a vegetable nutrient management specialist.

According to Callahan, excessive temperature and humidity are often responsible for plant molds and mildews in high tunnels, including botrytis, gray mold, tomato leaf mold, powdery mildew and downy mildew. Growers should be shooting for less than 85% humidity in their high tunnels to help prevent these issues from occurring.

It’s important to differentiate between circulation and ventilation. In high tunnels, circulation is usually generated by horizontal airflow (HAF) fans. HAF fans mix the air in the space, helping to provide consistent conditions in high tunnels.

“If we don’t mix that space well, oftentimes what happens is the corners will have dead spots. You might see increased disease there because the air is stagnant and has more of an opportunity to condense and promote molds and mildews,” Callahan said.

Ventilation is the actual exchange of air – bringing fresh air into a space and exhausting the humidified air or excessively warm air from inside the tunnel to the outside. Callahan said, “HAF fans circulate the air. They mix, they stir, they distribute, so adding more HAF fans is not going to ventilate the tunnel.”

Passive systems – ones not requiring electricity – are the simplest ways to improve high tunnel ventilation. Rolling up the sides and opening the doors are examples of passive ventilation. If these are the only methods a grower uses, Callahan feels that it’s critical the high tunnel is built in a spot with reasonable crosswinds. Sometimes, he sees situations where passive ventilation is inhibited by hedgerows or woods.

Another form of passive ventilation is to add gable vents on the endwalls of the tunnel or a ridge vent. “This leans on the fact that essentially any vertical structure is a chimney. If you have hot air inside something and you give it an opening up high, if that air is hotter than what is outside, that’s going to drive ventilation,” Callahan said.

Gable vents can include wax cylinders which open and close louvers without electricity. These sealed cylinders are filled with wax that expands and contracts as the temperature changes.

On Maden’s vegetable farm they’ve taken a unique passive approach to ventilation by moving toward using higher ground posts with a slightly narrower overall width. She feels that the extra height has made a huge difference on ventilation.

She said, “The airflow just feels drier. They grow a nice tall crop. For tomatoes, it feels like a game changer.”

Callahan said that with this design, growers get more roll-up side height and also more height to drive the “chimney effect.” The moisture that is generated inside the tunnel has more space to occupy.

Unlike passive systems, active systems use electric motors to spin fans and move air. An example of an active system includes two endwall exhaust fans, two inlet louvers on the opposite endwall and a thermostat that opens the louvers and turns the fans on when the temperature is above 85° F.

An alternative to exhaust fans are ridge vents operated by a motor and a rack and pinion. These are sections of the roof designed to open and close, allowing warm, humid air to escape out the top. The drawback to ridge vents is the upfront cost; exhaust fans cost between $2,000 and $3,000 in a 90-foot tunnel, whereas a ridge vent is closer to $5,000. The electricity usage for the ridge vent, however, is much lower than exhaust fans. And ridge vents are quieter.

Another type of active ventilation is using HAF fans inside of the tunnel in conjunction with other ventilation methods. According to Callahan, HAF fans should be placed at least every 50 feet. If the crop in the tunnel has a lot of vegetation, like tomatoes and cucumbers, he recommended placing a HAF fan every 20 to 30 feet. HAFs are usually placed in a “racetrack” pattern. Unfortunately, in this system, the air in the corners tends to stagnate.

Vertical airflow fans are another option. They pull the air up vertically and send it down the side, where it hits a curved shroud and exits out the side. They are commonly found in commercial chicken houses.

“We really like vertical airflow fans,” said Maden. “With the HAF fans, it seems like when you have a really thick canopy of tomatoes, it might not be moving air the way we want. Vertical fans seem to be doing the trick.” She uses three in a 30-by-90-foot tunnel.

Another ventilation option is positive pressure ventilation – instead of pulling air out of the tunnel, a fan blows air in. Sometimes the air passes through a heater, and it is delivered through a sock with little holes distributed throughout it. They’re typically located up high, but some growers have found benefit from them lower, especially for early season or winter production.

Tom Akins of USDA-NRCS added that there is cost sharing available for existing tunnels through EQIP. For example, EQIP may cover retrofitting tunnels with ridge vents, adding HAFs and installing end ventilation units. The first step is to participate in an energy audit, which NRCS can also cost share.

Finally, Callahan believes that growers need more quantitative data to help inform their decision making about what ventilation practices are working and where they can make improvements. He thinks growers should measure humidity in their tunnels in a few different locations.

“I would encourage everybody to consider increased measurement and monitoring in both temperature and humidity. Remote monitoring, particularly for tunnel and greenhouse conditions, has become much more accessible,” he said.

For more information about high tunnel ventilation, go to

by Sonja Heyck-Merlin