Re: How much passive ventilation is too much for plants?
I have six 12 x 12 x 18 Exos set up for dart frogs (so, pretty humid). I mist these by hand once in the morning, and sometimes again in the evening. I cover the screen with acrylic inserts on top of the stock screen top. Covering just the front screen allows the viv to dry out in a plant-healthy fashion between mistings; covering the front section entirely and covering the back except for a ~1 1/2 inch gap keeps things pretty moist -- algae grows on the back and sides that I clean off every few months.
I have three 18 x 18 (two 18 high, one 24 high) and five 8 x 8 x 12 set up for crested geckos. I mist once daily in the evening. These have stock screen tops, no additional covering. They usually dry by morning (down to ambient humidity ~50%RH, I assume but have never measured), and grow a range of hardier standard viv plants (Neoregelia, Fittonia, Pilea involucrata, Pellionia, Begonia).
I think I could turn and live with animals, they are so placid and self-contain'd.