exhausting Into attic would be my best bet?
Attic fan that you could control in a similar way to all the other fans in the room or vent to a tube that goes out the roof.Exhausting into an attic is very problematic if you don't have very good ventilation up there. The humidity drawn from the grow area can condense in the attic in the cold seasons, and cause severe mold problems.
You're best to vent directly outside.