Best fan configuration for enclosed growing?
 

I have setup an indoor greenhouse on a southern windowsill which is nearly done. So not necessarily a terrarium, but not really greenhouse per se... but anyways...

Is it more efficient/better to use fans to exhaust air from somewhere on top to ideally create an updraft for air movement? Or is it better to pump in the air? Or even, is it better to recirculate if there's a humidity issue? I'm able to achieve ~70-90% RH with a small ultrasound humidifier, but stagnant air at that humidity can be just as devastating as dry air! Which works best, keeping in mind that reserving heat would be necessary in the winter months as my LEDs don't produce too much in the way of heat. Much thanks!

Best fan configuration for enclosed growing?
 

I always use fans to circulate fresh air around the plants. As you said, high humidity plus unmoving air does not make for a healthy environment, particularly as the temperature and light begin to diminish as winter rolls in.

I grow orchids in Exo Terra terrariums which have small ventilation holes across the front. I replaced the wire screen top with glass, so it is totally enclosed except for the ventilation holes in the front. I run a PC fan mounted inside the box near the top and get good air movement inside the box. I maintain humidity around 75% and in two years have never had mold or fungus problems.

How do you connect the fan to an electrical supply?

estacion, you can use an AC/DC converter (wall brick) like a cell phone charger (output voltage around 5-12V DC works). You can cut the connectors, and there are 2 wires, then check which wire is positive and negative. The fan may have 3 or 4 wires, but you only need to connect 2 wires (+ and -). Note that some of the older AC/DC converters are not efficient (and waste energy). I collect these from trash transfer stations. I use a Kill-a-Watt to see if they waste energy (plug it in without anything connected), and I only use the ones which don't waste energy idling.

This is my current favorite for a fan:
Amazon.com: Noctua SSO2 Bearing Fan Retail Cooling NF-A14 iPPC-3000 PWM: Computers & Accessories
It is not cheap, but efficient, quiet (if you drive it at a lower voltage), durable, and waterproof. If you use the full 12VDC, it moves quite a bit of air (a bit noisy, but it is not bad for the amount of air movement).

For the original question, I do same thing as Matt recirculating. If you exhaust or intake, it basically defeat the purpose of terrarium. I have an exhaust fan, but it is thermostatically controlled to prevent overheating. When it kicks in, the humidity goes down to the ambient quickly.

The goal of fan use is to create a buoyant, irregularly tumbling of the air, not necessarily a breeze (or "hurricane" as I have seen in some greenhouses).

It keeps the environment from having stagnant pockets, without the plant-drying effect of too much constant breeze.

I use a Silenx four plug variable speed controller. It connects to standard house power outlet and can control up to four 3-pin PC fans. The controller sits outside of the box, so it runs the fans in my three Exo Terra terrariums. I like it because I don't have to mess with connecting the fan to a phone adapter, which does work well, but it also allows me to vary the speed of the fans.

Amazon.com: Silenx IXAFCEX Four Channel External Fan Controlle: Computers & Accessories

It doesn't have great reviews on Amazon, so I bought a second one as a backup in case the first one died. But I have been using it for nearly three years 24/7 and it is still working great for me.

Thanks guys for the recs! I'll be using a couple fans to circulate fresh air into the greenhouse. I tested a external power supply for fans and it works really well with my 120mm fan that's really gentle on the leaves. I have a smaller, more powerful fan that's gonna draw in fresh air all connected to a timer! Crossing my fingers!

Hmm, interesting about the irregular tumbling of the air. I have my fans hung on wires from the top of the boxes, and the spin of the blades causes the fan to oscillate back and forth, so the air does not blow in a constant direction. I've thought of using something more rigid to stop the oscillation, but I think I will just leave it alone and keep the irregular movement.