Howdy Smokie, What is the temperature of the air space in the attic? One would no want to pull in air that is too hot or too cold, also a thought of concern, what is in the attic, insulation type, dusty, or signs of insects, etc.? In most building codes, the attics are vented to the outside air (either fan forced or the 'round' turbine type. The exhaust might be a better choice to vent into the attic (smell control) and just take in the air from the room into Your grow space. Some more information (or a diagram or photo) would help.
Good luck with your grow.