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Thread: Bathroom Fan Vent in Washington State - Soffit Vs Roof

  1. #1

    Default Bathroom Fan Vent in Washington State - Soffit Vs Roof

    Hi. As part of a bathroom remodel, I installed new Panasonic vent fans (2) in the bathoom. When tested with NO ductwork attached, they were quiet and sucked a lot of air. That's the good news...

    The bad news is that the original builder used 3" ducts in the original fans. After attaching the new 4" duct work to the old 3" soffit vents, the air flow went way down, and the fans vibrate and rattle.

    I'm pretty sure that the 3" vents are badly constricting airflow and creating high static pressure. I want to install 4" vents - either in the same location as the old soffit vents or new vent caps in the roof. But there are "issues"...

    Soffit Vents - Our house has open eaves. See pic below showing the eave outside our master bedroom. The "soffit" (correct term?) is 90 degrees from the roof line and canted away from the wall of the house. The current 3" vents are installed in the 2X6 "soffit" in a similar location to the three attic vent holes shown in the pic.

    Issues: My biggest concern is drilling a 4" hole in the 2X6"soffit". Is that safe? Any problems?

    Roof Vents - New vent caps installed in the roof 4ft above the fans. But the roof is cedar shakes and 25 ft up over a steeply sloped driveway. I.e., I'm uncomfortable doing this. Having roof caps installed by a roofing company could be expensive - several hundred dollars to install some $20 vent caps.

    With little snow in western Washington State, snow shouldn't be an issue, but...

    Issues: What are advantages and disadvantages of roof vent caps? Would it be worth the money?

    Thanks for the feedback,

    Dan.
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  2. #2
    Plumber jimbo's Avatar
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    First, the soffit is completely decorative, non-structural, so I would not worry about a larger hole. I would worry about dumping all that humidity under the eave. If your old fan did this, was there any sign of dry rot or mildwew either in the eave, or in the adjacent roof area above the soffit?

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    How far is it to a gable end wall? You could run it out there. I've got mine out the roof (NH gets a bunch of snow on occasion!). Not had problems, but good flashing and sealing around wood shakes takes care to make it look good and not leak.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4

    Default Thanks for the feedback

    Jimbo and Jim,

    Thanks. It turns out that there is no gable end wall. After thinking about this some more, I'm going to have roof vent caps installed. Besides providing better ventilation, it will make any maintenance somewhat easier. (The working space where the roof and joists meet is about 5". Even a couple feet back, it's only about 12-14".)

    Now, my biggest issue is trying to find someone to install the vent caps for less than the cost of the house! It's tough around here.

    Thanks for the feedback.

    Regards,

    Dan.

  5. #5
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    I would cut new holes from the outside (use a hole saw and a guide so you don't chew up the soffit unnecessarily), then go into the attic and run 4" PVC from an accessible point down to the holes. Then attach the flexible hose from the fans to the PVC. Caulk around the PVC on the outside, maybe even put some hardware cloth screen on it (there's a neat way to do that), paint, and kick back with a cold one.
    Last edited by Terry; 05-18-2010 at 02:29 PM.

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