Bathroom fan duct routing — vent outdoors through a roof cap, never into the attic

IRC M1501.1 requires exhaust to terminate outdoors. Dumping humid air into an attic is a top cause of attic mold and sheathing rot.

Two roof and attic sections. The left green one shows the correct routing: the fan duct runs up and out through a roof cap with a backdraft damper, exhausting outdoors. The right red one shows the mistake: the duct ends inside the attic, where a red moisture cloud represents warm humid air condensing on the cold sheathing and causing mold, rot, and ruined insulation. — Source: IRC M1501.1

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Two roof and attic sections. The left green one shows the correct routing: the fan duct runs up and out through a roof cap with a backdraft damper, exhausting outdoors. The right red one shows the mistake: the duct ends inside the attic, where a red moisture cloud represents warm humid air condensing on the cold sheathing and causing mold, rot, and ruined insulation.

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