Protect your plants from overheating! This easy to install greenhouse vent can be placed in the roof or end wall to provide supplemental ventilation for extremely hot conditions. During moderate weather, high tunnels are generally vented by rolling up the sides and opening the doors on each end. However, extremely hot and humid weather often requires additional ventilation, and the best vent position is at the roof or peak of the structure. The High Tunnel Vent is constructed of a rigid aluminum frame and shutters. Each vent comes with two types of vent openers: manual and auto-temperature. The manual opener is operated with a hand pull wire, while the auto-temperature opener operates automatically and can be adjusted to open at temperatures between 60-77° F. Vent acts as a great insurance policy in case you are unable to open your main vents or other venting fails. Vent installs in minutes with wiggle wire and channel locking. Vent opening and measurments: 12" Vent - 12 1/2" x 12 1/2", total vent measures 15" x 15" x 3". 18" Vent - 18 1/2" x 18 1/2", total vent measures 23" x 23" x 3" 24" Vent - 24 1/2" x 24 1/2", total vent measures 28" x 28" x 3" 30" Vent - 30 1/2" x 30 1/2", total vent measures 33" x 33" x 3"