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Lurking for years so thanks for all the great info!

In our master bathroom there is a small water closet that just has the toilet (about 6' x 3'). The WC and the tub area both have exhaust fans that are connected in the attic and then vent out the roof. The venting is done poorly with 3" flex duct that is buried in fiberglass and blown in insulation and has probably been banged around during other repairs (the entrance to the attic is above the door to the bathroom so any work done involves walking near there). The connection through the roof is also loose and needs to be fixed.

I want to redo the ducting, make sure the fans are sealed around the ceiling, and I will likely just replace both fans since they are 12+ years old and I'll be up there crawling around anyway.

Do I need a fan in the WC that vents out of the roof? This fan isn't expected to move moisture, just odors. We don't run this fan during showers, only when using the WC for maybe 5 minutes a day. If I do need a fan, is there any version that doesn't need to vent to the outside? Like one with charcoal filters or anything? I do like having the fan for odors and for masking sounds...

If I can remove that fan or the ducting between the two, it will be a much easier install and then I won't be worried about it being damaged when working up there. Thanks!

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Inspected ducts in attic

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  • What does "Inspected ducts in attic" mean? Did you manage to cut yourself off there?
    – FreeMan
    Commented Sep 9, 2022 at 15:01
  • Any authorities in Virginia you could contact? Might be more direct.
    – Solar Mike
    Commented Sep 9, 2022 at 16:29
  • Code applies to new construction and significant remodeling. This ain't that. Do what you like, following common sense and the best available options.
    – isherwood
    Commented Sep 9, 2022 at 18:40
  • @FreeMan haha I think I was cutting and pasting and something went wrong
    – Bcemail
    Commented Sep 12, 2022 at 13:36
  • 1
    @isherwood good point! I try and do things "to code" but fixing what was done wrong sometimes doesn't lend itself to that
    – Bcemail
    Commented Sep 12, 2022 at 13:37

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I'd be surprised if a fan was required by code in a room with a window. Local specific rules cannot be looked up for free, so the only people who can help answer that for sure are ones with access to and experience with your local building code. But I'd be very surprised.

I would take the opposite approach ... this is maintenance of an existing installation. I would replace the toilet fan with a good, high volume, not too quiet fan. I'd run good insulated ducting to the roof or gable or soffit or wherever seems best, and put smart controls in the bathroom and toilet to turn it on whenever there is humidity or occupancy, and run for 20+ minutes each time. You'd be surprised how little this costs.

The fan in the toilet will be pulling air from the bathroom. I'd experiment with that first ... I don't think you need two, and putting it in the toilet will satisfy your privacy-by-noise objective and also keep the noise further away from the master bedroom.

Assumption: You don't have a steam shower or other reason to need a second fan in the bathroom.

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  • That's a good point, I'll be up there fixing the other duct anyway. I was planning on using rigid ducting, but should I use insulated flex duct instead? Since the ducts connect together just before exiting through the roof, I was wondering if there is any concern with the bath fan letting humid air into the toilet area? Or maybe the air will just follow the path of least resistance and go up and out. Not sure if the fans that are installed now have a flapper, or if that was standard 10-15 years ago. Thanks
    – Bcemail
    Commented Sep 12, 2022 at 13:50
  • Each fan should have a flapper and the roof/soffit vent should have one too. When ducts are shared, it's best to have an inline fan downstream of both. Otherwise you definitely will have flow between rooms if only one fan is on. The flapper will not completely eliminate this. You could perform an easy sniff-test that I won't describe in detail :). As for rigid vs flex, I think either is fine but insulated is good (helps with noise reduction and prevents condensation in the attic) and insulated flex duct is cheaper and easier to work with.
    – jay613
    Commented Sep 12, 2022 at 15:06

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