A.Harrison McCampbell responds: The International Residential Code (Section R303), referring to mechanical ventilation in the bathroom, requires that "the minimum ventilation rates shall be 50 cfm for intermittent ventilation or 20 cfm for continuous ventilation. Ventilation air from the space shall be exhausted directly to the outside."
This seems pretty clear: Venting to the attic is out of the question, as that is not directly to the outside. Venting into the area behind the soffit panel would be the same thing, while venting through a hole in the soffit is only a step away. While using a vent cap mounted in the soffit panel might be considered venting to the "outside,&...
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