I’m looking to add ventilation to several 2nd story bathrooms, and need to vent to the underside of the roof overhang (the house has a hip roof with no gable ends). Any suggestions as to external registers with spring-loaded flaps to keep out the cold and any interested insects? The local building supply stores only have products which rely on gravity to keep the flaps closed.
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The only type I've seen/used is the draft blockers, which are shutters kept closed by gravity. This isn't integral to the register, but is placed in a horizontal run of the ducting as close to the register as possible. They are marketed for dryer vent installation, but I've had good success with them in the bathroom vent installations I've done; they take a 4" duct. Granted, I use them in horizontal runs to gable end vents.
Do it right, vent 'em through the roof. Thru the soffit just lets the moisture back in through the soffit vents.
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Thanks for the feedback. I should have been more specific: this isn't for venting the plumbing stack, it's for exhaust ventilation. Would your advice change accordingly?
Don't vent your bathroom exhaust out the underside of the soffit. The moist air will travel right back up into the soffit and cause all kinds of problems. Also, your building inspector may not allow it.
Either go through the roof or a gable end.
Moisture being pulled back into the attic can be a long term concern. A solution is to run solid (non-perf) soffit 3'either side of the vent. I've done this alot and passed inspection. There is a exhaust flap made specifically for this application. I don't know the brand but our boxing guy got them from the supply house where he buys his vinly siding, soffit, etc.