The exchange of indoor air for outside air is referred to as ventilation. This leads to the exhaustion of warm, humid air and replacing it with cooler, drier air. Exhaust fans, ridge vents, or roll-up sides are used to remove humidity and heat from the room.

The active exchange of indoor air for outside air is ventilation. This exchange is critical for removing heat and humidity from the structure and replacing it with drier, cooler outside air. This also aids in the replenishment of carbon dioxide, which is consumed during the photosynthesis process.
End-wall exhaust fans combined with input louvers, passive peak vents or ridge vents combined with roll-up sides, roll-up sides alone in a location with a reliable crosswind, or even big doors on each end wall are all options for ventilation.

The air inside the structure is mixed through circulation. Horizontal airflow (HAF) fans are commonly used for this. Circulation is essential for ensuring that each square foot of growth space receives nearly similar growing conditions. HAFs circulate air across the growth area, eliminating no air flow zones. These fans help to compensate for ventilation restrictions, which frequently result in areas of no airflow, typically in the corners.

If you ventilate without circulation, you risk having corners or other zones that do not benefit from air exchange.
If you circulate without ventilation, you risk the building being extremely hot and humid, which can lead to plant stress and illness.

Passive systems utilize openings in the structure's envelope to allow natural airflow. Passive systems are frequently favored over active systems due to their lower initial cost and energy efficiency, as well as greater flexibility in installation (e.g., electric utilities are not needed). Passively ventilated tunnels, on the other hand, rely on consistent, gentle prevailing winds to help with ventilation. Avoid placing these tunnels too close to major structures, dense tree lines, or other impediments to ventilation.

Use 8 CFM/ft2 of growth space for hot season growing and 2 CFM/ft2 for cooler season growing. The overall growing area for a 30 ft x 96 ft tunnel is roughly 2,900 ft2. The hot season ventilation need is thus 23,200 CFM, while the cool season ventilation requirement is 5,800 CFM. If a structure is used throughout the cool and hot seasons, two stages of ventilation with two fans controlled by a two-stage thermostat can be used. The ventilation flow rate can also be adjusted using variable speed control.

The recommended circulation flow rate is 25% of the total growth volume per minute (taken from Greenhouse Engineering by Bartok and Aldrich). The overall volume of a 30 ft x 96 ft tunnel with a 15 ft peak is roughly 34,000 ft3. Because one-quarter of such volume is about 8,500 ft3, the total HAF flow rate required is 8,500 CFM (ft3/min).

Exhaust ventilation fans range from 12" to 48" in diameter, costing $200 to $1,000 apiece and providing between 1,000 and 22,000 CFM. Exhaust fans can be direct or belt driven and can be designed to operate at a single, dual, or variable speed. Multiple fans with staged temperature controls can also be used to create at least two levels of ventilation for various growing seasons. 

To generate sweeping flow, exhaust fans are often positioned on a common endwall, with input louvers installed on the opposite end wall. The inlet louvers should be equivalent to the area of the exhaust fans. To improve energy efficiency and plant health by preventing drafts, motorized louvers with a flange are advised. Louvers range in price from $50 to $250.

Motorized louvers can open and close air inlet locations. These are frequently linked to the same control circuit that runs exhaust fans, so the air inlet is opened when the fans are switched on.

Depending on the location, use of additional vents, and prevailing wind, roll-up sides can act as both an input and an outflow for ventilation. This ventilation technique is commonly performed by running sheets of plastic the length of the tunnel and rolling them up around a pipe to the desired degree of opening. The rolling mechanism might be manual (hand crank) or  motorized roll-up motors and  controllers for greenhouse automation  .

Ridge vents and peak vents are roof sections that open to allow warm, humid air to escape through the top. Their designs differ slightly, with ridge vents arising from one side of the tunnel finishing above the other at the ridge, leaving a vertical segment that can be opened or closed. Peak vents are usually a portion of roof that runs the length of a Gothic-style structure and can be opened or closed. Both are viable alternatives to exhaust fans. These vents, when paired with roll-up sides, can provide exceptionally effective, silent, and energy-efficient ventilation. Both are often the simplest and least expensive to install when the tunnel or greenhouse is originally constructed. They are powered by a motor and a rack and pinion system and may be adjusted to various levels of opening depending on the temperature and the control system.

Gable vents are ventilation apertures located at the top of the end wall. As a passive ventilation alternative, these can be beneficial when paired with roll-up sides.

Horizontal air flow (HAF) fans are typically 12" to 24" in diameter, cost between $80 and $300, and deliver between 1,000 and 5,250 CFM. Some growers have had success with inexpensive box fans, noting it takes a larger quantity and need replaced on a regular basis, they are less expensive than commercially available HAFs.
It is vital to note that HAF fans offer the rated flow for a specific length of "throw." It is normally recommended to deploy HAF fans every 50 feet along the length of flow to provide evenly dispersed mixing flow throughout the growing area.
In practice, a maximum spacing between HAF fans of 20-30 feet is more appropriate, especially with dense vegetation in specific crops (e.g., tomato and cucumbers). HAF fans are often positioned in a "racing track" configuration to increase flow throughout the growth region. In actuality, this plan does occasionally leave at least two corners with somewhat low flow, and growers should test the proposed layout before finalizing it.

Although this article is primarily on ventilation, heaters can help improve the impact of ventilation when humidity control is required. Heated air can transport more moisture, allowing ventilation airflow to remove humidity more efficiently. The size, intended inside temperature, outside temperature, and tunnel envelope are all aspects to consider when sizing a greenhouse or high tunnel heater.

The most basic controls are switches that turn fans on and off. These are inexpensive and simple to install, but require the intervention of an operator to make modifications. It is typical to have ventilation fans and HAF fans on different switches to allow greater operational flexibility.

A thermostat, which is a switch that automatically opens and closes based on a temperature that can be adjusted. Thermostats can be configured to control heating (close on falling temperature) or cooling (open on rising temperature) (close on rising temperature). The thermostat's sensing probe should ideally be aspirated (air flowing over it), covered from radiation gain (heat gain caused by incident sunlight), and situated to precisely monitor the air temperature the plants are experiencing

These systems monitor and regulate various outputs by monitoring multiple inputs such as temperature and humidity sensors. Previously, these were thought to be prohibitively expensive for tiny farms or individual tunnels. However, the prices have been drastically decreased. These controls are useful for automating more complicated high tunnels or greenhouses with heating systems, ventilation fans, circulation fans, roll-up sides, and even irrigation. These systems are typically accessible through computer or smartphone to assess conditions and change setpoints.

This is some text inside of a div block.Excessive heat and humidity can result from inadequate ventilation. Excessive temperatures can cause plant stress and yield loss, while high humidity can cause fungal infections. 
You most likely have insufficient ventilation if you are experiencing high temperatures and have exhaust fans running consistently or have all passive ventilation options completely open. 

This is some text inside of a div block.If you have widespread fungal disease, you most certainly have insufficient ventilation. The efficacy of your ventilation system can alter throughout the growing season as additional vegetation restricts air movement. Standing in the middle of the tunnel or greenhouse should provide a pleasant breeze.

Inadequate circulation might cause inconsistencies in the tunnel or greenhouse. This can manifest as hot spots with high temperatures, cool places with low temperatures, humid areas, and condensation. Look for leaf movement in all areas of the tunnel or greenhouse to indicate the presence of airflow. Survey tape strips cut to identical lengths and affixed to the tunnel framing to hang down can also indicate where air is flowing and where it is not.

Keeping an eye on a high tunnel or greenhouse may be a chore that requires a lot of time and concentration, leading to anxiety. Certain controls can be automated to significantly reduce the amount of labor necessary. Greenhouse environmental controls  often run  $1,500-$3,000 for a controller, sensors, and roll-up motors, in addition to any existing fans. An automated greenhouse ventilation controller will pay for itself in around 250 growth days with a labor cost of $20 per hour and 15 minutes of labor required per house twice day.