10 degrees to hot (f)

my grow closet it hovering right at 80 degrees(f) im shooting for the lower 70's would a passive intact system drawing cold air from underneath the floor lower my temps without raising the humidity of the my closet. i was thinking of using a jig saw to cut a 4" hole in the floor and inserting a 4" piece of pvc pipe through the hole with a screen to keep the unwanted's out like bugs.

Where is your passive intake now? Or are you so far not venting? If you are pulling air from outdoors, especially from under a home, I would want a higher quality filter because lots of stuff lives under there that would love to live with your babies. I personally wouldn't use anything less than a HEPA filter, but you could probably get away with someone more makeshift in a pinch. Plain screen only is definitely asking for trouble.

Why you gotta be so low? 80deg is cold compared to my room temps, which hover around 85-90. Roid is correct on that one. I'd be all about a HEPA if I were bringing in air from under the house.

ive got a 600 watt cool tube light so i have 25 feet of 6" ducting pulling air through the tube into the main room attached to the closet.

80 is fine, i wouldn't worry untill it gets above 85! but to answer your question yes your room will cool; down abit if you bring cooler air in.

So I am curious, what is directly outside the closet? Why is the duct so long? Are we talking a big closet that requires 25' of ducting to cover the span?

just haven't cut the ducting yet. and the room adjacent to the closet is hands off. tonight i cut two 4" holes in the floor one for the cool tube and one for a passive intake. i used some 3 1/2" pvc to channel the air from under the house into the closet. the temp is still to hot but it takes longer to get to the high temps than before. i need to order my inline fan and activated carbon filter so that the room will have negative pressure will pull air faster from underneath the house theoretically cooling the room more effectively..