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I am installing a bathroom exhaust fan in a portion of the house which is a small addition to the original house. The attic above the addition is separated from the rest of the attic by the original roof (though portions were cut out for the A/C duct and wiring). I know that soffit vents are not typically recommended for bathroom exhaust fans, because roof vents cause air to be drawn into the soffit and exhaust from a soffit vent would get sucked into the attic. However, the attic above the addition does not have roof vents (thus air should not be sucked in through the soffits), so I am wondering whether it would be okay to use a soffit vent for the bathroom exhaust fan. I am in South Florida, where the temperature doesn't go below freezing in case that is relevant.

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    Normally you don't vent bathrooms into an attic or through attic ventilation portals. You install a roof vent designed for venting bathrooms, which has a weather flap. – isherwood Dec 20 '17 at 19:42
  • Related: diy.stackexchange.com/questions/8683/… – isherwood Dec 20 '17 at 19:43
  • I have edited my post to clarify; I am not trying to vent into the attic or through attic ventilation. I want to avoid cutting into the (new) roof, so I am exploring other solutions, one of which is to vent through the soffit. I know this is not usually recommended, but I believe this setup may be an exception. – Mel Dec 20 '17 at 20:25
  • You're asking if it's ok. The answer depends on your reasoning for not doing it correctly. The risks depend on the type of soffit and other nearby materials on which the moisture from the bathroom will act. – isherwood Dec 20 '17 at 20:31
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    International Mechanical Code simply requires it to be exhausted "outdoors" and 3 ft from operable openings, 10 ft from mechanical air inlets. IMC 2009: 501.2.1. Does your jurisdiction have some different code you need to follow? – Upnorth Dec 21 '17 at 6:12
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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. Keep the run as short as possible and as vertical as possible. Happy venting.

  • Thanks for your reply. One other issue is cost - if we vent to the soffit, we can install it ourselves. If we vent to the roof, we'd have to hire a roofer to do it. Any idea what the cost would be like to have someone install a vent in the roof? – Mel Dec 21 '17 at 23:50
  • What type of shingles do you have? Why do you say that you would need to hire a roofer? – Paul Logan Dec 22 '17 at 2:58
  • It's a regular shingle roof - we just don't know how to do that and would want to make sure it is done correctly so it doesn't result in leaks. – Mel Dec 22 '17 at 17:08
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    This does not have to be an intimidating job as long as the roof pitch is not too steep to work on. There are many DIY videos on You Tube detailing the procedure. Keep the vent pipe as short and as vertical as possible. A flexible metal line is a good choice for vent pipe. Install a hard pipe 90* elbow on the fan, pointing up. tape and clamp all your joints. Get the top of the roof jack flashing under the shingles. – Paul Logan Dec 23 '17 at 23:09
  • Ok, thank you! We will look up some videos. The roof is not steep at all and is rather low in that area, so the distance from the fan to the roof would only be about 2 feet. – Mel Dec 24 '17 at 14:06

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