I'm not an expert, but all greenhouses I've personally seen have natural ventilation with vents high up (such as at the peak of the roof) and near the ground.

I'm not an expert, but all greenhouses I've personally seen have natural ventilation with vents high up (such as at the peak of the roof) and near the ground. The greenhouse effect makes the air inside a lot warmer than outside. This reduces its density, and it becomes buoyant and spills out the roof vent. There may also be fans, but you can get some ventilation for free (energy-wise).