Air exchange in tunnels is essential to avoid high temperature, high humidity, and low CO2 in tunnels, leading to plant stress or disease. Passive ventilation using roll-up sides is common in tunnels, though some tunnels use mechanical ventilation with fans to pull air through the tunnel. Generally, you must pick one or the other or use them at different times. Fans pull from the point of least resistance, so running an end-wall fan with the sides rolled up simply pulls air from around the corner, not from the other end of the tunnel. When sizing fans for ventilation the basic rules of thumb are 8 CFM/ft2 (of growing space) for summer cooling and 2 CFM/ft2 to remove humidity during cooler months. Note that this guidance is for peak ventilation needs.  Staged fans (e.g., one small, one large) or variable speed fan controls can help moderate the ventilation for various times of the year.
Passive ventilation is less than ideal in locations where tunnels are crowded together, there are lots with trees or other significant wind breaks, or in calm sites with little wind. A dense crop canopy later in the growing season also reduces passive ventilation.  

Installing a ridge-vent (along the top peak of the tunnel roof) will greatly increase the effectiveness of passive ventilation, though these can be costly and they make installation of plastic cover more complicated. Some growers who have ridge vents have installed “cat walks” to ease maintenance.  These can make plastic replacement and repairs easier. Gable vents high up on end walls can also improve ventilation by acting as outlets for warm humid air in warmer seasons and by allowing for low volume ventilation in colder weather. A 24″x24″ gable vent on each end wall is recommended for a 30′x96′ tunnel. These can be made of plywood and manually operated with hinges, ropes or cables and tie-downs. If using a louvered vent, be sure it has a flanged seal to close against. Thermostatic wax cylinder actuators may also be used which require no electricity, are relatively inexpensive and are passively controlled by the wax cylinder based on temperature.

HAF Fans. Horizontal air flow (HAF) fans are hung from the inside horizontal structural tubing to mix the air inside a tunnel to create consistent growing conditions, they don’t improve ventilation.  They are for circulating and mixing the air inside the tunnel. When installed and used properly, they ensure that plants and any control sensors are seeing the “average” conditions of the space. The first fan should be placed about 10′-15′ from one end wall to pick up the air that is coming around the corner from the other side. Subsequent fans should be located 40′-50′ apart to keep the air mass moving. In a 30′x100′ greenhouse, four fans are required, and the total fan capacity should be 6,000 CFM (2 CFM/ft2). The “empty” corner, where there is no fan can sometimes become a spot without air flow. Check to make sure you can feel air flow in all locations in the house.  You may have to add fans or reorient the ones on the end to promote adequate mixing flow. If a tall crop such as tomatoes is grown or if there are hanging baskets, a slightly greater capacity is needed to overcome the additional air flow resistance. Small, 1/10-1/15 horsepower circulating fans work well in providing the air movement needed. A permanent split capacitor motor can save as much as one-third the electricity of the more common shaded pole motor. Some growers have used inexpensive, simple box fans and just plan for frequent replacement. High efficiency vane-axial fans can increase the “throw” of each HAF fan meaning you need fewer fans to provide the same mixing flow.