I would not think a 1" gap will supply enough CO2 all the time. You could replace the solid door with a louvered door or cut out a 18" square hole and put an A/C louvered grill over it to make sure there is enough fresh air getting into the closet. It's not hard to make sure they get enough air even in a closed closet. Not enough fresh air and you will know very quickly. A grill might cost you $15 or 20 bucks and it's cheap for a grow of any size. Make sure, don't guess! Oh, and welcome to the forums and Good Luck!