There is no ventilation in my bathroom now and I would like to put a fan in. There is no attic or crawl space in my house. Looking up at the ceiling, it is a layer of drywall, then insulation, then open space (6 inches) that allows air to pass through, and above that the underside of the roof. My concern with simply installing a fan is that it will let cold outside air in. Any ideas? Thx.