The bathroom fan needs to be incorporated into the interior of the home by maintaining the air barrier on the outside of the fan unit. This is most easily accomplished by using a poly pan box around the fan unit.

Ductwork coming from exhaust fans (also found in range hoods, and ventilation systems) that penetrate the envelope need to be sealed to the air barrier. In most cases this air barrier is provided by the polyethylene sheathing. Penetrations made by ducts through ceilings or walls, are to be sealed with appropriate sealant materials and techniques to prevent air leakage. Securing the duct at the penetration will help to maintain the seal long term.