I know that a greenhouse is not air tight so I imagine it breathes somewhat and exchanges gas with the outside. However, I also assume that it’s going to use up the co2 in it for plant growth. How does it not run out? I think you could vent it, but then wouldn’t that cool it off and thus defeat the purpose of a greenhouse? All my web searches either give climate change facts or co2 supplementation by farmers.
It’s called diffusion. Unless the greenhouse is hermetically sealed, and the rate of consumption does not exceed the rate of diffusion, the ratio of CO2 will be similar to that outside.
A simple vent will be enough if not just small gaps in the structure.
Also, most green houses have fans that can help circulate air in from the exterior.
I’m inclined to think the fans are more for fanning the plants so they can perspire and breathe more readily. Many plants need a breeze to grow well.
Usually both, in my experience.
You’ll have fans for plants that firm up stems and stuff, in addition to helping with transpiration.
Then you’ll have fans or passive vents near the highest point that move air outside, usually for the sake of venting excess heat and humidity. If you live somewhere cold, these probably stay closed through colder times and are active/open only during the height of summer (these are often the passive vents). If you live somewhere warm, it’ll probably always be active fan-driven, and usually running.
A lot of people have their ventilation system hooked to climate control sensors, so it only engages when it crosses a certain threshold.