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Thread: Can I vent my exhaust fan into a joist bay?? No moisture...

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  1. #1
    DIY Senior Member lithnights's Avatar
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    Default Can I vent my bathroom exhaust fan into a joist bay?? No moisture...

    I am remodeling the 1st floor of my 2 story home.. moving a bathroom 15 ft over. Supply, vent, and waste are not a problem. The exhaust fan may be.

    I want to install an exhaust fan to 1) eliminate odors and 2) provide a slight noise to muffle any bathroom sounds. But the bathroom does not border an outside wall and thus I can't run my exhaust fan to the roof or exterior wall or outside soffit.

    Since there is no shower and thus no major moisture can't I just vent the exhaust fan into the joist bay above the bath? I'm guessing the textbook answer is no, but what is the major downside and ill effects of doing this?

    Thanks in advance!
    Last edited by lithnights; 02-10-2006 at 04:59 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Depending on which way the joists go, whether there is any blocking, and some who knows what, you might be able to run the duct through the ceiling to the outside wall. I have a bathroom in the middle of the house and that is how they did it - a duct through the ceiling to the outside wall.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Senior Member lithnights's Avatar
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    Quote Originally Posted by jadnashua
    Depending on which way the joists go, whether there is any blocking, and some who knows what, you might be able to run the duct through the ceiling to the outside wall. I have a bathroom in the middle of the house and that is how they did it - a duct through the ceiling to the outside wall.
    Good point. The joists run front to back of the house, but it would have to run 8 feet to the front and then come out the soffit there. Not exactly what I wanted. Also, there is a plumbing waste line cutting through the joists perpendicular so I'd have to get the flex tube around that (which probably means crushing it down a bit in that area.

    I guess my real question is... IS ALL THAT NEEDED IF THERE IS NO MOISTURE? CAN'T I JUST ATTACH A 2 FT PIECE OF DUCT TO THE FAN AND JUST LEAVE THAT HANGING BETWEEN THE JOISTS?

    Thanks!

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    Master Plumber master plumber mark's Avatar
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    Talking duct fan

    its probably best to get it somewhere where it can
    ventilate in cas their is moisture


    use the dlexible poly pipe and take it over to the
    soffit vent if you can... I have seen electircians
    hang those bath vents and attach -- nail them up to the
    roof vents too....

    seems to work ok

  5. #5
    DIY Senior Member lithnights's Avatar
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    I agree that it is probably best to get it somewhere but in my situation that would be very difficult.

    Seeing that this is a downstairs bath with no shower or bath and minimal water use, where would moisture come from anyway?

    I checked my father's house (built in 1970) and he has a downstairs vent exhausting right into his joists. Like me, no shower/bath and thus minimal moisture. He has never had any mold/moisture issues.

    So would it really b e so bad to vent it into a 4 foot flex duct that just rests on the joists? I'm sure it's not code, but is it really that crucial for a bath with minimal moisture to begin with?

    Thanks,

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    Tradesman Plumber Kristi's Avatar
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    it sounds like you know what you want your answer to be... no one is going to tell you "yes, go ahead and vent it straight into the open attic" because they all know that it's not a job they would want to stamp their name to (pro OR homeowner). It's not the right way to do it. If you are a handy dandy homeowner who has rationalized the lack of moisture portion of your exhaust dilemma, then you should continue forging ahead with your plan and see what happens. It will not have a disastrious outcome, it will turn into another one of the millions of homes out there when a professional sticks his head up there and says "who the effing eff did this effing job!? They should effing lose their license!"... imho
    Last edited by Kristi; 03-15-2006 at 11:07 AM. Reason: additional comment

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    Let me preface this by saying I am not a professional like many of these guys. My knowledge and experience comes from renovating/remodeling 80+ yr old homes -- our own home(s) as well as friend's and family's homes. I've never been paid for this kind of work, but I'm not a fool and learn how to do things before jumping in.

    As you already know, the preferred method (rather the 'required' method by code) is to vent to the outside. Several have already pointed out the risks of moisture entering into the cavity between your joists. Based on your initial post I assume that you already recognized those concerns. Only you can be the judge of how much moisture may be vented into the cavity and how often it might occur. Based on your comments, I would not view moisture as a likely potential problem -- possible? Yes. Likely? Probably not assuming the facts that you present. Incidental humidity created from one's breathing or a running sink faucet are highly unlikely to be problematic for you unless the 1/2 bath is EXTREMELY small. I typically-sized half bath is going to have sufficient air volume that the humidity created by normal handwashing, flushing, breathing will be negligible, IMO and experience. Now, if someone runs the faucet with steaming hot water for a healthy length of time, then, yes, excess moisture could (probably would) be created in an amount sufficient to cause a problem. However, if you or someone else accidentally vented that moist air into the cavity you could simply leave the fan on for an hour or so and the moving air within the cavity should be sufficient to lower the humidity of the initial moist, vented air and prevent any problems, particularly if the cavity between the joists is of fairly good size. (It should probably be noted that I am assuming that there is no insulation inside the joist cavity -- if there is insulation I would not vent into it no matter what as it is just too easy for problems to develop.)

    The only real concern I would have, if I were in your shoes, is nelie's comments about fire safety. Although it's not likely that your house would ever be involved in a fire, it is possible that the vent could contribute to smoke/fire damage if such occured. I would not be concerned about the possibility of denied coverage though -- failure to follow code by the homeowner is not a valid basis for denial of coverage, at least it's not under Missouri law where I live. Nelie's point is well taken though.

    Ultimately, it is your house and you have to assess the risk/reward of venting or not venting. I can tell you that I have seen a fair number of incorrectly vented fans in these old houses and have yet to see a problem as a result. I wouldn't foresee a problem based on your description, but there are no guarantees.

    If you do decide to vent into the cavity, I would just be diligent about running the fan longer than normal -- the increased and sustained air flow within the cavity will help prevent any problems with regard to moisture.

    Hope that helps -- take it FWIW -- which isn't much considering the cost for the advice. :>)

    good luck, let us know what you decide to do.

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