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Bathroom vent through an unvented soffit

Birdie L | Posted in General Questions on

I lost the battle. I have been arguing with my husband for months on this. New construction, Climate Zone 5. Bathroom Vents. I thought I had him talked in to exiting out the gable wall. Roof was not an option, as neither of us want to put any holes through the roof, few as possible.

He is planning on exiting the bathroom vents to the soffit. His plan is to use unvented soffits for a certain amount of sq ft, to avoid re-entry into the attic.

I am going to make sure he insulates the ductwork. I have read other posts on the duct tape/duct insulation, and want to make sure he is doing that.

I believe he purchased hard metal ducts, not flex duct, which is a plus.

What other things can we do to help this situation? Is there anything we can do to make sure the moist air being existed doesn’t “hang out” under the eave?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    There are at least three potential problems with terminating a bathroom exhaust fan duct at a soffit. You mentioned the first of these.

    1. Moist exhaust air can be pulled into the attic through soffit vents.

    2. During cold weather, icicles can form at the termination.

    3. Most duct terminations aren't designed for horizontal mounting, so you end up with a termination with a non-functional flapper (back-draft damper).

    As long as you understand these drawbacks, and are willing to live with them, your house may be OK. This type of termination isn't great, but some people have them.

    The only termination fitting I'm aware of that is made for soffits is manufactured by Duraflo. You want to buy Duraflo part #646015, "Bathroom Soffit Exhaust Vent."

    -- Martin Holladay

  2. Anon3 | | #2

    Hmm, there is a ton of Soffit Exhaust Vent at homedepot with positive review.

  3. Birdie L | | #3

    I am hoping using an unvented soffit in that location, and for a few surrounding feet, we can avoid re-entry into the attic. I am less than thrilled. The gable end, however, is a long distance from the bathrooms. I will try to work with this, but not sure how comfortable I am going to be.

  4. Birdie L | | #4

    Martin, a question regarding the duct insulation. We are putting blown in fiberglass insulation throughout the vented attic. The duct work is fastened to the attic floor. Should I still wrap in duct insulation, or will the fiberglass insulation on top be sufficient?

  5. GBA Editor
    Martin Holladay | | #5

    Bridget,
    For a bathroom exhaust duct, the attic insulation will work, as long as it is installed deeply -- 12 or 14 inches is good -- above the duct. Of course, the duct seams should be sealed.

    Cellulose insulation will perform better than blown-in fiberglass insulation or fiberglass batts for this purpose. (Fiberglass insulation is more likely than cellulose to allow cold air to contact the duct during the winter, or warm humid air to contact the duct during the summer.)

    -- Martin Holladay

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