The majority of greenhouse structures have built-in windows for natural air supply and ventilation. In glasshouse greenhouses, these are 3-pane vents that are mostly located on the roof tops, alternating with fixed panes. In polyethylene greenhouses and polytunnels, the venting window is throughout the ridge, in a continuous ventilation – either on one side of the ridge or both. Natural airing lets fresh air come in and releases humidity and heat from the top of the greenhouse – and this natural airing is good enough ventilation for most geographies. However, in cases where it is not enough, you can add forced ventilation using exhaust vents and fans.