Best fan configuration for enclosed growing?

[theloyalplum]

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!