The amount of ventilation will vary depending on the size of the spa, the size of the room, and the season. A good quality exhaust fan that is allowed to run during the entire time that you are in the spa plus for at least 1 hour after you exit the spa will help reduce the moisture level. A spa cover that is in good condition will help to seal in moisture. And make sure that you keep your spa covered when not in use to avoid adding to the moisture issue. Check with a qualified company dealing with heating and air conditioning for an analysis of the size of your room and the types of vent systems available.