I recently built a terrarium of cedar and glass intended to hold orchids. It measures 14"w x 14"d x 20" high. It has a copper tray in the bottom. A 1" layer of river rock covers the tray bottom. I put a small amount of water in the tray and place the orchid pots on the rocks as with a humidity tray. Humidity, with the top open 2" ranges from 70-90%. On each of 3 sides, 3" up from the case bottom, I cut a slot 7/16" wide and 4" long to provide air inlets at the base. The top of the case is 14" x 14" and is hinged so that it can be opened for mantainance and to control ventilation and humidity.To test ventilation I placed a smoldering piece of paper on the bottom of the case with the top open about 2". In this configuration the smoke rose rapidly to the top and out of the case. My question is: do I have sufficient ventilation for orchids or do I need to install a fan? If a fan is deemed necessary how might it be located in such a small case so as not to blow directly on the orchids? Would placing it near the top to pull air up be best perhaps?