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My bathroom exhaust fan recently broke. When I went to replace it, I noticed that it wasn't being exhausted from the attic.

I was about to make a separate hole in the nearest gable to exhaust the fan, but my wife asked if I could butt the exhaust pipe up to the existing gable vent in order to avoid making another hole.

I have seen conflicting answers in my research online. (I've also viewed this question: Can I vent a bathroom fan through an attic window?, but there's not enough support for that answer for me to be confident.)

The existing vent is louvered with a screen behind it. The bathroom exhaust pipe would be pushed up to the screen (I wouldn't be cutting the existing vent), secured by a box (with a 4-in hole cut in it to hold the pipe exactly) that will be attached to one of the supports holding the existing gable vent. (I'll also caulk around the pipe where it enters the box.)

Is this acceptable, or does the bathroom exhaust need to be vented directly outside?

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I wouldn't exhaust the bathroom through the gable vent.

First off, you'll be reducing the size of the gable vent. Whatever area you block with the exhaust duct, is a reduction in the area of the gable vent.

Secondly. Depending on how the attic ventilation is designed, the hot moist air exhausted from the duct, could be drawn back into the attic through the gable vent.

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I wouldn't hesitate to attach the hose to your gable vent, assuming that it's made of a material that won't be affected by moisture. Secure the duct in such a way that the airflow isn't directed at lumber.

Drawbacks include lack of a secondary backdraft flap, visibility from the exterior, and lint accumulation.

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International Mechanical Code (IMC 501.3) does not allow this. It must be direct vented, typically through the soffit with a terminating cap.

  • I don't see language in that section indicating that this wouldn't qualify as "direct venting". – isherwood Oct 4 '18 at 13:29

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