Epiphytes do not need any more exposure to air than other plants. The idea is to have air movement as a way to keep the atmosphere buoyant to prevent stagnation, and the development of molds, as much as anything else. A small fan inside the tank can suffice, but you don't want it to be totally sealed, either..
I recently built a terrarium for jewel orchids and a few epiphytes. There is a 3" layer of PrimeAgra (LECA) on the bottom, covered in sphagnum, in which the plant are growing. The back is EcoWeb, which a bulbophyllum is starting to climb on, and I'm working on getting mosses, ferns, and other epiphytes on it, too.
4 times a day, for 15 minutes, a pump comes on (there is a small tank in the stand), running water down the EcoWeb to the bottom of the tank. That waters the plants on the EcoWeb, wets the LECA, which moistens the moss for the "terrestrials", then drains back into the tank below.
As far as ventilation is concerned, My tank top overhangs the sides to facilitate the three T5 fixtures, giving it a 1" x 11" opening on each side, and I have three 1/2" holes near the top to vent heat from the lights. (See attachment - crude drawing). I have no other air movement, but the upward flow - slight as it may be - seems to be sufficient, as all of the plants are doing very well, there is no sign of mold or anything, and the tank sees 65° nights and 75° days, 75%-85% RH with no further supplementation or controls.
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