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.
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.