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hello, getting ready to start my first grow, hydro stuff is on the way. I have a perfect walk in closet in a bedroom upstairs, it is always cooler than the rest of the house with the door closed, so should be a perfect spot, now as far as venting the room, my home is a tri level, the attic is inaccessible on the upper level (where this room is), it does have a large vent to this attic space at the back of the house. Can I just run an inline duct fan into the attic? Im not really worried about heat as I will be using FFL, Can I just punch a hole in the ceiling and run my duct fan up through there/ I live in a dry climate, so humidity is never a problem....any idea appreciated!

I believe that should work out for you. Just be sure of two things.
1. That you properly seal the hole going into the attic to keep any creepy crawlies out of the GR
2. Ensure that you are not putting too moist of air into the attic or you may have mildew / mold issues later.