Re: New build, lots of questions...
Yes a top vent of sorts should be maintained. It was once believed that in order to maintain a certain level of humidity the entire cage needed no ventilation...that was false. And the appearance of condensation or lack there of on the glass is not a measure of the humidity rather the temperature differential that is exists between the interior and exterior air.
I actually prefer exhaust fans to intake fans because the air inside the enclosure moves at a lower rate (aside from the air in the vicinity of the fan). Where as the air on an intake fan will continue to move through the enclosure at a higher rate. Some inhabitants don't do well in drafty environments particularly if the incoming air is drastically cooler.
In fact if your lights put off heat in anyway, the heated air at the enclosure ceiling will naturally move out of the vent thus creating a draft to draw in air at the bottom. Vivaria aren't fish tanks we don't need to change out the air as frequent as we change water. We just need change. Remember hot air rises so we can use physics to our advantage.
Dendrobates l Ranitomeya