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Thread: Confused!! Should we vent bath fan through roof, soffit, gable end?

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    DIY Junior Member staceyneilcollins's Avatar
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    Question Confused!! Should we vent bath fan through roof, soffit, gable end?

    Our house is built of thick masonry block. NOT fun to drill through! It's a one-story ranch in Maine, so we have cold snowy winters and humid summers.

    We need to replace a bathroom exhaust fan before we have blown-in insulation installed in the attic this winter. Here are our choices... given our location and parameters below, which do you guys think is best?

    1) Through the roof. It would be about a 10' run of pipe and would have to cant at about a 45 degree angle (ie not straight up) to place the roof cap on the back side of the ridge where it wont be horribly obvious. We already have a veritable forest of roof protrusions back there: range hood fan, two plumbing vents, wood stove chimney, and solatube dome.

    2) Through the soffit. Exterior wall is only 3' away from fan location and eaves are more than 12" wide. But I have heard this is not a great solution re: moisture re-entering attic.

    3) Through the gable end wall of the attic (which is wood, not masonry). This however is a ~19' run of pipe.

    If you can also advise what type ducting to use, that would be great. If it's # 2 or #3 and is buried under a foot of blown-in insulation, do we need to use insulated pipe? Where does on buy insulated pipe anyway?

    Thank you so much!!!

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    Retired Defense Industry Engineer jadnashua's Avatar
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    First thing to check is the specs for the fan you want to use...it may have a limitation on how long the ductwork can be. Any good hvac place will have insulated round ductwork. If you can use Grainger, you can buy it from them as well. I use my vent during the summer, and not all that often in the winter, since I normally run the humdifier, so just opening the door means less the humidifer has to deal with. But, given it is on the roof, if there is a lot of snow, depending on the type of roof cap you use, it might be underneath, especially in Maine. It might be easier to run it to the gable end wall, but the length might be a problem. You don't want it to condense and then drip back into the bathroom. Hot, humid air will probably rise enough if you went through the roof to minimize that. If you had the run to the gable end rise all along the way, that would help the airflow as well. Dumping it into the soffit might work, but assuming you have vents there for airflow into the attic, you don't really want to dump all that moisture there.

    See what other thoughts you get.
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    Senior Robin Hood Guy Ian Gills's Avatar
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    I'd go through the gable end. My neighbor did that (although he had a shorter run). Cutting though wood and then siding appeared to be very easy, which is an important consideration.

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    Jack of all trades frenchie's Avatar
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    Venting out the soffit's a good way to rot out your roof.

    Either of the other 2 work, although that does seem an awfully long run to the gable.

    I'd go up & out the roof.
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    DIY Junior Member staceyneilcollins's Avatar
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    Thanks folks!

    So it seems like we're down to either the gable end with a long run, or through the roof.

    Roof is only a 5:12 pitch, but we could put the cap near the ridge.. which hopefully would minimize snow, right? Seems snow's the only detraction from that plan. And I guess unlike a dryer vent or range hood exhaust, the air temp from a bath fan is not necessarily hot enough to melt snow.... hmmmmm.....

    Anyone else want to chime in?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    There are all sorts of roof jacks for that exhaust. You could run it up a ways so snow shouldn't be a problem. It is imperative to have a good damper on it, though.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Senior Member Runs with bison's Avatar
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    I would go the soffit route myself since it is closest (assuming you mean through the soffit to external, not into the soffit.) The exhaust fan is used rarely/short enough with really humid air that it is unlikely to have any impact on the moisture level pulled inside. Heck, two of mine vent into the attic itself, and my attic ventilation is not optimal (does not have ridge vents as I would like, and the ventilator count is low for the roof area.) Yet there is no sign of humidity problems in the attic space from this. If this were a dryer, then it would be another matter. A dryer seems to put out an order of magnitude more humid air, and for an order of magnitude longer duration each time.

    Assuming that the attic itself is properly ventilated I doubt this will matter. Now, if possible I would choose a soffit that did not have vent holes so that the air would actually be expelled and not sucked back in immediately.

    But I'm a relative novice with attic ventilation.

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    Computer Programmer Bill Arden's Avatar
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    The soffit route is not a good idea due to the way the moisture can flow back up into the attic.

    Personally I used insulated flexible pipe from a local big box store (see heating isle) and routed it back down into the utility room. From there I could send it out a drier vent or into the room depending on the season.

    Eventually (in cold climates) both cloth drier vents and bathroom vents will be sent thew heat recovery heat exchangers.
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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by Bill Arden View Post
    The soffit route is not a good idea due to the way the moisture can flow back up into the attic.
    I seriously doubt that for several reasons:

    1. If the duct exits through a soffit without vent perforations there is unlikely to be substantial draw back into the attic anyway.
    2. Showering will only produce so much humid air. (Much less than when it rains for example.) If there is 30 minutes of shower use/venting each day the ratio of fresh to wet circulation is 47:1.
    3. The ratio of the relative mix volumes of fresh outside air to vented bathroom air is likely to be several fold.
    4. Sloppy builder work left the vent fans in my present house covered by insulation, with no ductwork, inside the attic. They had been this way for 14 years. I expected to find obvious water stains on the back side of the sheetrock they were sitting on. Instead, nada. There was dust blown into the insulation though. Ave 24 hour temp here in winter is 30 F for three months. Summer is relatively humid, averaging about 75% humidity.

    I might end up venting my other bath through the soffit because it is trapped between floors at present...and vents into the utility room.

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    DIY Senior Member Hube's Avatar
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    Run the discharge pipe to the SOFFIT and make sure you have the proper back-draft damper installed at this termination point. They sell these back draft dampers at most good hardware/bldg stores. It looks like an inverted "T" with flappers on both sides that open when the fan is on and close whenever the fan is off. it installs neatly to the soffit and takes a 4" pipe connection. pricd at approx $ 8.
    There no chance of any moist air ever coming back into the attic space.
    Use 4" aluminum pipe and fittings (do not use flex, the ribs catch lint)
    Tape every joint .The only part of the pipe that needs to be insulated is the exposed parts. If the pipes are within the insulation they do not need to be insulated.
    Been there, done that
    Good luck.

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    What is shortest distance to the nearest exterior wall. I just did two - one through the roof and one though the side of the house.

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