If i may ask, what would be the benefit of extracting air from bottom, vs the light source.
If the ventilation air enters the chamber from the top and leaves via the bottom then the temperature gradient across the plants (from top to bottom) is smaller than it would be if the air entered from the bottom and left via the top. The same is true of the humidity gradient.
A smaller temperature gradient is preferable because it makes for more-even growth, and it stops the roots and base of the plants being so cold in relation to their tops.
Concerning humidity, in a top-down system the incoming dry air hits the top of the plants and then slowly increases in humidty as it passes downwards. This mimics natural conditions far better than bottom-up, and I imagine that it might well help to prevent or at least reduce the chances of bud rot.
This seems, to me, to be preferable to bringing dry air in and then passing it straight over the wet soil/medium where it immediately picks up moisture, and then having it wend its way up through the tangle of leaves, becoming even more damp along the way, before it finally hits the buds, by which point it's become laden with vapour and is unable to pick up very much more.
Yeah, I can see that being an issue, although with good runs of solid duct there would probably be little in the way of extra resistance. In any case, I suspect that a top-down system might still require more air flow overall than a bottom-up one does though.