A.The key words here are "large, well-ventilated." Most attics today are not as large as in the past, and many are not well-ventilated. Rather than trying to establish standards for those conditions, it is easier to require that all fans be vented to the outdoors.

I always recommend venting exhaust fans to the outside. If you must vent into the attic, however, fan discharge should never blow directly into the insulation, nor should it blow against the roof sheathing. If the discharge is brought a foot or two above the insulation and blown horizontally toward the middle of the attic so the moisture can diffuse into the air before it hits a cold surface, problems might be avoided. Most of the fans I know of have no ducting at all and blow directly into the insulation. While the amount of moisture from a bathroom fan may be considered small, it can be the "straw that breaks the camel’s back" in an attic with marginal ventilation and numerous vapor leaks.