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  1. #1
    Join Date
    Aug 2007
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    Question Venting a bathroom ehaust fan

    In his ****** TOH article on installing a bathroom exhaust fan, Tom Silva describes how to vent the fan outside under the soffit. But Tim Carter at askthebuilder.com strongly recommends venting up through the roof, and warns against potential moisture problems from venting under the soffit in any climate (moist exhaust can seep back into the attic and condense).

    In my situation here in Minnesota, it would be easier to vent under the soffit. Should I be concerned about creating a moisture problem with this approach, or not?

  2. #2
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    Default Re: Venting a bathroom ehaust fan

    Vent through a side wall not the roof. That warm moist air will freeze and the vent won't open or will remain open. In Minnesota It will probably be covered by snow half the winter.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  3. #3
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    Default Re: Venting a bathroom ehaust fan

    jray,

    I would personally recommend venting thru the roof.......which is what we do here in NW Illinois, whenever possible. (If venting a basement bathroom, this won't work, of course. Then we go out the house wall)

    We get our fair share of winter snow accumulation on the roof and have never had a problem here at home or with any of the installs done for clients over the years. Reason being that the exhausted warm air quickly melts a path thru any accumulated snow/ice and everything works as it should. I suspect it would function the same in Minnesota.

    Important points concerning any installation would be to use rigid vent pipe (as opposed to cheapy plastic flex duct), to t ape the seams with aluminum foil duct tape (not duct tape)and to insulate that rigid pipe if/where it passes thru a cold air space (attic or similar) If you can't find pre-insulated rigid duct, then wrap the metal duct in a layer of FG batt.... held in place with wire, zip-ties or similar. Also, keep your duct run as short as is practical and elbows to a minumum.

    (If you decide to go the roof-vent route.....make certain that the vent-hood is properly installed as regards your particular roofing material. Simply flopping it down and gooing the perimeter is incorrect and will be constantly problematic.)

  4. #4
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    Aug 2007
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    Default Re: Venting a bathroom ehaust fan

    Thanks to DwarfWytch, JLMcDaniel and ****hiller for your advice.

    I've heard of two advantages of flexible duct over rigid duct: 1)it's quieter and 2)it permits more gradual bends and thus freer air flow.

    In light of this, why should I use rigid duct?

  5. #5
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    Default Re: Venting a bathroom exhaust fan

    ****hiller.... I totally agree with using the rigid metal ducting and metal foil duct tape then covered with insulated pipe wrap.

    I ran across a little trivia on the origin of the cloth based duck tape that so many people are confused with. It was developed during the second world war for the military to meet the requirement to tape shut the ammunition boxes. It had to be durable and hold up under all sorts of environments including being wet. It was very successful that is was dubbed duck tape for it's ability to still work in wet conditions.

    The venting of a bathroom fan through the roof really depends. For example in this region it's rare they are vented through the roof and the ones that are have problems with freezing the damper doors shut. When they are buried under deep snow and very cold temperatures for up to 5 months they aren't as effective similarly the square roof vents where they don't allow the attic ventilation because the air flow circuit is blocked off by the snow. The bath vents will also melt the snow from underneath and create the same effect as an ice dam and actually backup under the shingles and I've seen where the aggregate on asphalt shingles being worn off because of this.
    In most cases we tend to have them mounted on a vertical surface like a wall or the gable end of the attic. The one thing that we do is have the horizontal run of the ducting slope down towards the exhaust vent for the moisture to run out to the exhaust vent.

    In new home construction it's common to have a centrally located remote exhaust fan located in the basement with the exhaust ducting going out through the basement wall. This is for not piercing the building envelope going through the ceiling or a wall.

    In light to moderate snow accumulation on the roof this may not be a problem but heavy snow amounts it may not be a good idea. Just another perspective.
    Last edited by canuk; 09-01-2007 at 09:03 AM.

  6. #6
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    Default Re: Venting a bathroom ehaust fan

    canuk,

    Am all ears and have received your words of wisdom/first-hand experience. Duly noted and.....I stand corrected. <G> Roof vents up where you are....not so advisable.

    That being the case, I guess I'd then also advise the OP to go the gable end wall route as you mentioned...if there's a gable wall (or similar) available.

    Have witnessed too many problems from under-soffit mounting to recommend or advise that in a cold climate.

    On a side note......My BIL installed his clothes dryer vent in the soffit of their ranch house....against my advice. There does seem to be just enough angle to the mounting and enough exhaust velocity from the CD blower to prevent the moisture from being sucked back up the soffit venting, but........a puddle of ice develops on the sidewalk beside the house whenever temps fall below approx. zero F. Not so good. That's what I feared would happen because of the sheer volume of moisture being ejected from the machine.

  7. #7
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    Aug 2007
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    Default Re: Venting a bathroom ehaust fan

    Thanks again everyone. You've persuaded me rigid duct is the best way to go. Interestingly, several fans, including the new "ultra quiet" Panasonics, come with insulated flex duct. They look like pretty good fans, so it's too bad they throw in flex duct that I won't use.

    I don't have a gable or convenient wall for venting, so I'll take my chances with ice dams and go through the roof. Panasonic's backdraft damper is located near the fan (not in the roof hood), so I shouldn't have a problem with it freezing up.

  8. #8
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    Default Re: Venting a bathroom ehaust fan

    Quote Originally Posted by ****hiller View Post
    canuk,

    Duly noted and.....I stand corrected.
    No need ... we all still learn things. Sometimes when we are thinking outside the box we forget how large the box can be.

    jray ... I just installed a Nutone in-line fan unit that also comes with the damper located at the inside vent. The one thing I noticed was there is daylight visible through the hinge mechanism and the flaps . This obviously indicates this would not be air tight so I made sure to have the exhaust hood with the damper just to help with reducing as much air infiltration as possible. Food for thought.

  9. #9
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    Jun 2007
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    near St. Paul, MN
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    Default Re: Venting a bathroom ehaust fan

    When you install the bath vents make sure that the moist air can not get into the roof (or wall) sheathing

    I reroofed my house in 2003 and found a problem with how the original bath venting was done. The vent hose has a square metal plate which attaches to the back side of the roof sheathing. The moist air was able to get into the roof sheathing. You can see in the attached picture that the sheathing is blackened and rotten. I replaced the rotten pieces of OSB. Then I put a section of ice and water shield (a very sticky waterproof membrane) on the back side of the roof sheathing between it and the metal plate on the vent pipe. After I had attached the metal plate to the underside of the roof sheathing I cut strips of the ice and water shield and fit them down the “throat” of the vent pipe from the top. This totally prevents any moist air from coming in contact with the roof sheathing or escaping into the attic. (I also had ice and water shield on the top of the roof sheathing.)
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  10. #10

    Default Re: Venting a bathroom ehaust fan

    Quote Originally Posted by DwarfWytch View Post
    Sounds like a good plan. Remember you want to go out and away as you go up (i.e. not just straight up but slightly out towards the perimeter) as you direct your venting.
    Why not straight up?

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