Hi Adam,
Your comment about vent placement highlights a very complex subject.
Vent Positions
One thing I have noticed with some cages is that they have the lower vent on the cool side, and higher vent on the hot side, yet other cages have the opposite (high vent on the cool side, and low vent on the warm side). The basics of physics say that heat rises - however what I'm not sure on and may be an interesting study is what is the difference between these two designs.
My initial guess (and it's a guess) is that one would create more air flow than another and reduces humidity while the other lowers airflow a little, increases humidity and efficiency of the unit. Whether this is correct, and/or there are other benefits or flaws with each model I don't know. Which model you choose, and why you choose it may be worth adding into your project... (I'm guessing one would be better for humid environments and the other for colder environments. Think north qld vs inner land vic/nsw).
In essence, the placement of the vents within the thermal gradient has no overall bearing on the flow of air within the enclosure. The placement of the vents (one high and one low) will impact the efficiency of the enclosure though. If the outtake vent (hot air escaping) is close to the heat source, the hot air will escape before being able to heat the enclosure.
To help the following I will explain a little about thermodynamics. As air raises in temperature it becomes lighter and as air cools it becomes heavier. Hot air doesn't rise, cool air drops which forces hot air above.
The movement of cold or warm air causes fluctuations in air pressure. Warm air escaping will drop air pressure, this drop in air pressure causes cold air to be drawn in. The reverse is also true, cold air escaping will cause warm air to be drawn in. The air that is drawn in is equalising the pressure.
Therefore, the outtake vent on the opposite side of the heat source will make the heating of the enclosure more efficient. The hot air will fill up the top of the enclosure and upon reaching the outtake, be released. This will draw air from the intake vent (cold air entering) because of a drop in pressure.
Now, this cycle of decompression and re-compression can only work if the ambient air temperature outside of the enclosure is colder than the air temperature within the enclosure. The more of a difference in air temperature between the two will create a higher rate of decompression and re-compression (hot air out and cool air in). If the ambient air temperature around the enclosure is the same as the air temperature within the enclosure, there will be no air movement in and out. So, to get good air flow within an enclosure you need to be able to control the air temperature within and around the enclosure.
The humidity of the enclosure is not overly affected by the air flow. Humidity is directly affected by the abundance of water within the enclosure.
I hope this helps.
Regards,
Nick