Don't quote me on this...but on building my enclosure for my ackies, I tried something where I had a 1 inch (about) piece of pvc run from the top of the cage to the bottom. In this way, heat generated in the cage will not escape out the top. The cooler air at the bottom of the cage will circulate with the air outside the cage (at the top). I don't know how much of a difference this made, but I believe it kept ventilation good along with keeping the ambient temp up (it gets cold in the northeast)
Seemed to make sense to me, figured i'd throw it out there.