I'd be careful about venting into an attic. It's going to depend on the building construction but a lot of older houses have poor venting in attics, meaning just an opening without a fan to push the air out, and they can get clogged easily (birds, leaves, etc...)
When you vent a grow room, unless you have a dehumidifier you are venting a lot of humidity in said attic and that creates mold and/or weakening it to where The wood will rot and need serious repairs to its support structure.
But then again, if the bathroom fans exhaust there too a grow tent won't change much regardless of how well vented the attic is...