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I have 3 bathroom vents that all just vent into the attic. There is flexible pipe going toward the edge of the attic, as if it were going out a soffit vent, but there are no soffits here. So they are just venting the bathroom air to the edge of the attic.

What would be the ideal solution? My first thought was to get all 3 of them to go up to a single roof vent (the less holes I have to make in my roof the better) but I've heard that you can get the exhaust from one bathroom going into another instead of outside. Is there a good solution for this that would be somewhat simple?

Should they each have their own roof vent? If so, what is the ideal layout/spacing for these? do they have to be a certain distance apart? Is it better to have them be on the same level going across the roof or in a line going up and down the slope of the roof like in this picture? (See picture)

potentially going to use a roof vent like this one:

https://www.homedepot.com/p/Broan-NuTone-Roof-Cap-with-Built-In-Damper-for-3-in-or-4-in-Round-Duct-in-Black-636/202191718

Any insight or advice would be greatly appreciated! Really I'd just like to know the proper way to do this.

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  • There are three parallel flexible ducts venting into the attic? Or are they manifolded together into a single outlet?
    – popham
    Commented Mar 17 at 20:25
  • No soffit vents for the roof might be a bigger problem. I usually prefer bathroom vent though the wall if possible. Usually shorter and reduces holes/possible roof leaks, but depends.
    – crip659
    Commented Mar 17 at 20:32
  • @crip659 it looks like a gambrel roof, which wouldn't have soffit vents at that location. Can't see enough of the lower section to tell if there are soffits.
    – Huesmann
    Commented Mar 18 at 14:06
  • Venting out the wall, if you can, is a lot easier to weather-seal and maintain.
    – keshlam
    Commented Mar 18 at 16:16
  • @popham There are three separate ones venting into the attic. one from downstairs, 2 from upstairs.
    – chen
    Commented Mar 19 at 15:15

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The International Residential Code has commentary for M1505.2 that you should read. Technically that links to commentary from the ICC's International Tiny House Provisions, but it's a convenient paywall workaround for the same commentary. The commentary gives building officials the foundation to reject multiple fans that have been manifolded together. By my reading, the code advises building officials to accept manifolded fan venting if installed for a single family residence. For multifamily dwellings, again by my reading, the code advises building officials to reject manifolded fan venting.

On the adequacy of manifolded fans, here's a quick excerpt from that commentary:

In some cases, installers connect the outlets of bathroom exhaust fans together to a common duct to avoid multiple roof, wall or soffit penetrations. This will result in some exhaust air flowing backward through the fan that is not running because the backdraft dampers in such fans will allow some leakage.

Is this "some leakage" acceptable performance? For a single family dwelling, the answer depends on personal sensitivity and backdraft damper quality.

Doing the work myself (and not paying the mobilization cost for somebody else to show up multiple times), my instinct is to install the system manifolded. If I don't like the performance, then I can just add a couple more roof vents.

If the system has been manifolded together all of this time, but just without an outdoor vent for the single flexible duct, then I would be much more comfortable with the single vent (assuming that you've been satisfied with the system's air-commingling performance up to this point).

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  • Agree, and it's worth mentioning that modern bath extraction fans typically have a gravity and/or spring action damper that, more or less, eliminates reverse air flow.
    – Jimmy Fix-it
    Commented Mar 18 at 14:17
  • @Jimmy, the linked commentary talks about the dampers, but you're right that their existence should have been explicitly mentioned here.
    – popham
    Commented Mar 18 at 16:31
  • Thanks for the detailed reply. I'm leaning toward doing it this way. The manifold or "splitter" for 3 ducts going out 1 roof penetration. You made a good point that if I didn't like how it was working, I could always add in 2 more.
    – chen
    Commented Mar 19 at 15:28
  • Ad far as measuring whether I like the performance of the manifold solution, I'm not sure what I would do. maybe stand in one bathroom while a hot shower is being run in the other to see if there seems to be any humidity coming in? Maybe I should get some humidity sensors to quantify it. Could be a fun experiment too. Also considering additional backdraft dampers right before each of the 3 ducts enters the manifold.
    – chen
    Commented Mar 19 at 15:33
  • @chen, I wouldn't worry about what little bathroom moisture comes through the backdraft damper. It's nasty smell that worries me. You could burn incense below each fan while the other/others was/were running. The smoke would give a visual indication of any air circumventing the dampers. Putting incense at the active fan, you could also subjectively smell test in other bathrooms in search of the incense smell. A single instance of the latter test would make subsequent tests difficult because of lingering smell from the earlier test.
    – popham
    Commented Mar 19 at 16:07

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