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  1. #1
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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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    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 10:03 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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    Default Re: Venting a bathroom ehaust fan

    Unfasten and turn/rotate your fan unit in the ceiling so that you can connect using only one elbow (right at the fan unit)......if at all possible. Each 90 degree elbow is equal to 10' of straight run. You want as few elbows as possible to maintain good air flow.

    If attaching the elbow directly to the fan unit means that a problem is created because the exhaust piping will line up with (or too close to) a roof rafter or truss, then add a short length of straight pipe to the fan unit before mounting the elbow....so you end up between the rafters or trusses.

    Locate the exact position of the vent-hood in the roof VERY carefully or your pipe won't line up right. (If that happens, your project will get complicated and probably very frustrating.) Measure and then assemble (or cut) the longest piece of the vent pipe that will fit between the elbow and the roof deck and use that to locate the right spot to cut the vent-hood hole. (The solid blue part of the pic is an integral part of the roof hood. Your pipe must align nicely with that when all is said and done)

    If you can't make the rigid pipe work, then buy some aluminum flex pipe and use that. (But don't buy the plastic vent tubing)

    Use aluminum foil tape on all the joints/seams.

    Don't forget to well-insulate whatever piping you use.

    Make certain you know how to properly mount the vent-hood as regards the shingles (or whatever material comprises your roofing) or you'll have a leak. Black goop and/or caulk is not a long-term viable substitute for proper installation.

    Run the fan unit for the entire time you are showering or bathing and for 10 minutes minimum after you shut off the shower/drain the tub/leave the bathroom. Install a timer-switch in place of the toggle switch if returning to shut off the fan is too much hassle.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by goldhiller; 09-05-2007 at 11:59 AM.

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

    Thanks for your input, and sorry for the confusion---I don't have software for drawing diagrams. I planned to position the fan vent toward the interior, and run the duct from it 4' horizontally, then 90 degrees vertically up through the roof. In other words, the duct would be a simple right angle 'L' shape. On your diagram view, the 'L' would attach to the left side of the fan.

    I read a fan installation guide that emphasized the need to go at least 4' horizontally before making any bends. It didn't give a rationale, but I thought I should follow that advice. But I gather you don't think that's necessary??

    If I understand it, DwarfWytch's concern with straight vertical duct is that it exits at an acute angle to the roof plane and may cause condensation problems. Couldn't I solve this by bending the top of my 'L' rightward near the roof, so that it exits at a 90 degree angle to the roof plane?

    I have considered a remote inline fan, but haven't found any as quiet as the new Panasonic model (.6 sones measured at the ceiling vent).

  9. #9
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    Default Re: Venting a bathroom ehaust fan

    jray,

    I'd say go ahead and toss in that 4' horizontal run if you like. I've never had a problem resulting from installing without it though. So long as the pipe is well-insulated and you run the fan for 10 minutes or so "after the fact", there really shouldn't be any condensation issues......or at least I've never had any with my installs. I have seen many problems from condensation dripping back down the line and even large lumps of ice in the line when no insulation was used in cold climates.

    Yes, if I understand correctly....you can toss in another ell up at the top of the pipe to get your alignment and connection to the roof hood. As a matter of fact, you're gonna need it to get connected if you've got a straight up/plumb pipe as it approaches the roof deck. Remember these ells are very adjustable and can be twisted to any angle from 90 to 0 degrees....so you should be able to acquire whatever angle you need. Can get frustrating sometimes though as it's kinda like playing with Rubik's Cube. <G> Trial fit all of the pipe run including that last ell at the top before you determine where to cut the hole in the roof deck.

    PS- If you're running a Windows-based OS, you don't need any special software to make simple drawings. MS Word or Paint will do that. I drew that with Word in about 60 seconds and then used Wisdom-soft Screenhunter (free download) to lasso the part of the page that contained the drawing. (Use the rectangle choice which allows you to lasso a specific area of the screen) Screen hunter will automatically save that file to whatever folder you create/desire for caching. Then you just direct the attachment facility here to that file to upload it. If you're looking for a piece of software to do more complex drawings.....try Google Sketchup. It's also free and is richly featured.
    Last edited by goldhiller; 09-08-2007 at 12:23 AM.

  10. #10
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    Default Re: Venting a bathroom exhaust fan

    jray... I had just installed a NuTone IFL120 in-line fan which I found somewhat easier to install than conventional ones. It's pretty quiet even with the fan only three feet away from the vent opening in the bathroom , a slight dull drone. The further the fan is away from the interior vent opening generally the quieter they will be.


    notice the gentle bend instead of a sharp 90 degree.








    That's the only thing that's seen and because everything is plaster this made installation a breeze with minimul damage to the plaster to fix.




    Not to beat this topic to death ... but here are some links to some information :

    http://www.nutone.com/product-detail...roductID=10330

    http://http://joneakes.com/cgi-bin/getdetailscals.cgi?id=1979

    http://joneakes.com/cgi-bin/getdetailscals.cgi?id=1756

    http://joneakes.com/cgi-bin/getdetailscals.cgi?id=1733

    http://joneakes.com/cgi-bin/getdetailscals.cgi?id=1307
    Last edited by canuk; 11-23-2007 at 10:43 PM.

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