I would think this is only a problem, or can even happen, if the attic soffit vents are right next to the bath
I’ve finish mounting and wiring the new Panasonic WhisperCeiling ventilation fan model FV-05VQ5 in the 2nd floor bathroom. Now I’ll install a new 4 inch soffit vent and insulated vent duct. This project is continued from Panasonic WhisperCeiling Bathroom Fan – Wiring and Mounting.
Avoiding penetrating the roof with a bath fan vent is the wrong disposition. Given current materials and techniques, a weather tight seal can be expected every time. And the trade-off is clearly on that side. Their just too many risks when dumping the vent fan anyplace but through the roof.
And the closer the termination is to the soffit vents that supply ventilation air to the attic, the more likely that moisture from the bathroom will be drawn into … The answer may have to do with the fact that cold air has an easier time entering these ducts when the fans aren’t on (because the soffit terminations
How to Install a Bath Vent Fan. Bathroom HeaterBathroom RepairBathroom PlumbingKitchen Exhaust FanBathroom Exhaust FanBasement VentilationResidential WiringDiy RoofingGarage Heater. Installing a Bath Vent Fan – How to Install a Fan or Heater – Home & Residential Wiring.
However, in order to make the turns necessary to penetrate the ceiling, enter the attic and then descend through the soffit, ductwork must make some tortuous bends. This reduces the efficiency of the vent and may trap heat or moisture in the elbows. When venting bathrooms or kitchen hoods the exhaust