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Community and Q&A

Bathroom venting through soffit?

dsmcn | Posted in GBA Pro Help on

Martin, you have several times stated your strong preference that bathrooms be vented through the roof, and not through the soffit. I ask again because I am not sure your earlier remarks would apply in this case.

My airtight and insulation layer is at the roof deck. I have a 15″ heel on the trusses, and a 4′ eave. Since my preference is to avoid roof penetrations, my thought was to pipe bathroom exhaust vertically to a point where the duct can slope gently through the raised heel of the trusses (which is also my air and insulation layer), then extend out a foot or two and take a 90º turn to exit down through the eave soffit.

I very much appreciate your help and observations, as always!

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  1. GBA Editor
    Martin Holladay | | #1

    There are two main objections to terminating a bath exhaust vent at the soffit:

    1. This approach discharges humid air at the location where air enters the attic. You don't want humid air from the bathroom to be pulled into the attic. (This objection is understandable, but it is probably not too compelling.)

    2. In cold climates, these soffit terminations develop icicles. If the grille is made of steel, it will eventually rust. The result is unsightly.

    If these objections don't bother you, you can go ahead with your plan. I'd rather see the duct terminate at the gable or go through the roof, but it's your house.

  2. dsmcn | | #2

    Thanks Martin. I feel relieved.

    I have a hip roof, so exiting through the gable is not an option. The attic space is conditioned, and I will seal the raised heels with foam on the interior, so no possibility of humidity being pulled into the attic. And I live in 4A, and have yet to see an icicle except in the nearby Smoky Mountains.

    Photo attached shows why mother told me not to play with icicles.

  3. dankolbert | | #3

    Can you go out through a 2nd floor wall instead? I try to avoid roofs where I can because, at least in northern New England, they can get covered in snow.

  4. charlie_sullivan | | #4

    Photo attached? I don't see it so you might try again.

  5. dsmcn | | #5

    Here's the photo:

  6. GBA Editor
    Martin Holladay | | #6

    Here's a photo of the type of soffit termination you want to avoid. (This builder used a wall-style termination for the soffit, and ended up spraying his soffit and wall with moisture.)


  7. Chaubenee | | #7

    So yu have a nine foot ceiling in that bathroom and if you do, could you drop your ceiling a foot and have a mechanical chase up above it and run your fan out the though the ceiling of the bathroom and elbow over through the wall? Make sure to air seal the upper ceiling where it meets the attic completely, and thus allows the chase to be in the envelope of the house. It then also makes a great place to put a recessed light or two without sacrificing air ceiling or attic insulation.

  8. Expert Member

    Don't forget that if you add bulkheads to a room, beyond just air-sealing, most codes require that they be fire blocked as they are a concealed space connecting the walls to the attic or roof.

  9. dsmcn | | #9

    Am I missing something? If the insulation layer and sealed envelope include the raised heel of the trusses and the roof deck, why would I need to create a dropped ceiling? Putting my question another way, is not the raised heel essentially the outside wall, and even though the duct turns down 90ยบ to exit the eave soffit, is the proposed method not like running the duct through the exterior wall as Joe suggests?

    And since the exit point would be above the top plates, no fire blocking would be necessary.

  10. GBA Editor
    Martin Holladay | | #10

    When a bathroom exhaust duct terminates in a soffit, it drips. In cold weather, it forms icicles. That's why wall terminations are preferable to soffit terminations.

    And the closer the termination is to the soffit vents that supply ventilation air to the attic, the more likely that moisture from the bathroom will be drawn into the attic. Again, that's a point in favor of wall terminations -- they are a bit farther away from the soffit vent intake holes.

  11. dsmcn | | #11

    Martin, why would a soffit vent (that has traveled through conditioned space most of the way) drip more than a wall vent?

    I have a hip roof with not much of a ridge, so it seems ventilating it is not an option. I am choosing instead to apply foam directly to the underside of the roof sheathing. Sorry I did not make this point clear.

  12. Expert Member
    MALCOLM TAYLOR | | #12

    The comments Joe and I made were dealing with a generic situation, not yours. Sorry if they lead to confusion.
    I think your plan is fine.

  13. GBA Editor
    Martin Holladay | | #13

    Q. "Why would a soffit vent (that has traveled through conditioned space most of the way) drip more than a wall vent?"

    A. Good question, and I don't have a ready answer. Perhaps a GBA reader will suggest a good answer. I know that these soffit terminations end up looking damp -- because I've inspected houses with these problems. The answer may have to do with the fact that cold air has an easier time entering these ducts when the fans aren't on (because the soffit terminations are less likely to have dampers), or it may have to do with the way the humid air lingers near the termination (because the soffit interferes with fast dispersion of the damp air).

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