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Community and Q&A

Bathroom venting concerns

A. Bradford | Posted in General Questions on

I have a 2 story/2-unit balloon-frame building from 1929, plus a hip roof. The 1st and 2nd floor bathrooms are on top of each other on the back center of the building. Both have fans, but neither vent to the outside!! We have mold/moisture concerns in the back rooms of the house. No wonder.

Venting each fan has a separate issue.
For the 1st floor fan, the joists run the wrong way to the outside wall of the bathroom. To run with the joists, it would be at least a 15′ run over other rooms to exhaust to an outside wall. My instinct is not to cut holes in the joists to make a short exhaust run, but isn’t 15’+ just too far for effective venting?

For the 2nd floor, there is no wall space available, as the hip roof hangs at the level of the ceiling. So we have the choice of going through the roof, or through the soffit. I’ve only read “NEVER VENT THROUGH THE SOFFIT” but the only reasoning is that soffits are part of the roof venting assembly. We are currently insulating our attic to be an Unvented assembly. Would a soffit vent be safe in this case? We live in the cold north and I do not want another hole in the roof.

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  1. Expert Member
    Malcolm Taylor | | #1

    Fifteen feet is not a bad run, especially if there aren't too many changes in direction to induce friction losses. Check the fan manufacturers specs. Some may want you to increase the size of the duct a bit.

    The concern with exhausts in soffits is solely with vented roof assemblies. You can buy directional vent outlets to make sure the moist air isn't put straight onto your siding.

  2. GBA Editor
    Martin Holladay | | #2

    A. Bradford,
    I agree with Malcolm that there is nothing wrong with a 15-foot bath exhaust fan duct, especially if there are few elbows. Use smooth-walled duct (galvanized duct or thin-wall PVC pipe), and upgrade from 4 inch duct to 6 inch duct if you are worried about the static pressure.

    For the second-floor bath, I would install a roof vent if I were you. In cold weather, soffit terminations create icicles.

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