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Thread: Venting bath fan out soffit vs. side vs. roof

  1. #1
    DIY Member newowner's Avatar
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    Default Venting bath fan out soffit vs. side vs. roof

    O.K, I posted this question on the shower/bath forum and I have gotten very helpful responses, but since in new home construction it is the HVAC people who handle all the venting, I thought I'd run this by you and see what you thought.

    I live in central Maryland in a house built in 1981. The master bath on the 2nd floor has a window and no exhaust fan. It shares a wall with a full bath that has an exhaust fan that is vented by uninsulated rigid metal ducting to the roof and is attached to some sort of cap that doesn't look anything like what I have seen roof vent caps look like. The metal ducting runs at an angle of probably 45 degrees to reach the back slope of our roof.

    Anyway, I am having an electrician install some overhead lights in the 2nd floor bedrooms and thought I'd have his crew install an exhaust fan in the master bath while they were here but I am confused about the best way to vent the exhaust. The electrician is an expert in electricity, not venting. He said he could run the duct to the side of our house, which would be 22-23 feet of ducting. Going out the front of our house would only be about 2 feet because the master bath is in the front of the house, but who wants to look at a vent cover on the front of our siding!! I haven't asked the electrician if his crew knows how to vent it out the roof using a roof vent cap. My husband was thinking he could maybe try the roof vent if they say they don't have experience with it, but he is not much of a handyman with no experience of this either, so I am worried about that.

    My research indicates people are divided on if venting to the soffit vents under our roof is a good way to vent or not. I got quotes from 2 electricians on my work, and one said soffitt venting is o.k. for central Maryland and the other said he thought some of the moist air would come back in through the soffit intake vents and cracks and gaps in the soffit materials, etc, and cause mold in the attic. The internet has people saying yes it is fine and others saying no, it is not good. Of course it would be the easiest way to vent. I am assuming the electrician means cutting through the soffit perforations and actually putting on a soffit vent, but maybe he means taping the duct to the top of the perforations from inside the attic.

    So what do you all of you think? If it truly is not a good idea to soffit vent, then I am wondering if we should we vent out the side, which would be 22-23 feet or to the roof, which would place a roof vent cap on the front slope of our roof, very near the pipe that is on the front slope of our roof which I think is the pipe that vents out the sewer gas. I don't even know if it is safe to cut a hole near that vent pipe besides the fact we then have two ugly things on our roof to be seen from the front of our house.

    I talked to a Fantech tech rep about an inline fan system before I discovered our hallway bath was vented to the roof and he said running duct 23 feet would be o.k. since each bath is about 8 X 8. A member on the shower forum brought up the point that 23 feet could cause air resistance, leading to condensation, so I am not sure if I should rely on the fan tech guy. Plus the Fantech guy based his answer on if I connected both baths to share a 230 CFM fan, not if I put a separate 100 CFM fan with 23 feet ducting out the side. Please share your thoughts and experience!

  2. #2
    Rancher
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    My HVAC guy, said it's done all the time and the less penetrations thru the roof the better.

    But this is AZ, a little dryer out here.

    Rancher

  3. #3
    DIY Member newowner's Avatar
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    If it makes a difference, I could make the ducting "only" 18 feet by centering it on the bath ceiling vs. 21 feet if centered in front of shower stall. Cool photo, BTW, Rancher.

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    DIY Senior Member molo's Avatar
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    Question waiting

    I am also waiting for the best solution to this problem. I have 2 bathrooms that need vents. I will not install them until I find out where to vent them. Please someone provide a conclusive answer to this problem
    The local hardware owner told me to vent it into the attic!!!

    TIA,
    Molo

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    Hot air likes to rise...venting through the roof is the shortest path so it has the least possibility of condensation, and shorter runs are more efficient. Fantec makes some nice remote fan systems - virtually silent in operation and you only need one fan for the 2 (or more) bathrooms. What you don't want for ducting is something like a dryer vent - accordian style stuff. Way too much restriction. They do make some flexible dual walled, insulated duct that is nearly smooth on the inside what works pretty well. Whatever ductwork you use, you do want to insulate it.
    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 TedL's Avatar
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    Quote Originally Posted by molo
    I am also waiting for the best solution to this problem. I have 2 bathrooms that need vents. I will not install them until I find out where to vent them. Please someone provide a conclusive answer to this problem
    The local hardware owner told me to vent it into the attic!!!

    TIA,
    Molo
    There is no single right answer. The best solution could be side/back wall, roof or through the soffit or fascia, depending on home layout, your tolerance for working in tight spaces, willingness to climb a roob, etc. Notice I said through the soffit, not behind a perfed soffit or into the attic, and not through a front wall.

    Properly sized and insulated vent pipe (or flexible duct) should work over any needed run, but longer runs will tend to decrease vent performance.
    Minimizing the length of the run is a good thing, but not an absolute rule that must always be followed.

  7. #7
    DIY Senior Member Hube's Avatar
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    NEVER vent moist bathroom (or kitchen) exhaust air into any attic area.
    Fr best results, go up and thru the roof .
    A soffit vent installation will do a 90 -95 % job that a going up to the roof installation will do.
    Cut an opening in the soffit to accomodate the special "T" vent grille. This soffit grille looks like an upside down T .The exhausted air is fed into the single entry and is exhausted sideways into each leg of the T.
    Both of these exit legs have a "backdraft flap to avoid any air from re-entering after the fan has shut off.
    Most hardware store have them. A 4" diam. smooth metal pipe and fittings is best from fan housing to this T termination point. This T shape grille, when installed will only protrude approx 1" below the soffit, and it looks good too.

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    DIY Junior Member PTN's Avatar
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    [QUOTE=Hube]NEVER vent moist bathroom (or kitchen) exhaust air into any attic area.
    QUOTE]
    is this really a major problem say if you have a completely empty attic (full length and width of house) with vents at either end?

  9. #9
    DIY Senior Member TedL's Avatar
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    Define "major"......

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    DIY Junior Member PTN's Avatar
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    i termed it major because Hube labeled this action as "NEVER". Implies very serious problems to me.....

    just looking for clarification for the specific example i listed.

  11. #11
    Retired Defense Industry Engineer jadnashua's Avatar
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    In the winter, you can end up with frost when the moisture condenses. In the summer, you can end up with feeding mold, especially if it is directing it on or near the insulation - it could end up saturating it, which also ends up decreasing its effectivness. You really want that concentrated moisture laden air out of the house, that's why you put in the fan, not to relocate it.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  12. #12
    DIY Senior Member Hube's Avatar
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    jadnashua has given some very good reasons why you should NEVER dump this moist(and oderous) air into the attic area
    Do it right, either take the best way and thru the roof, or take the next best route and vent it outdoors via the sidewall or soffet.

  13. #13
    Computer Programmer Bill Arden's Avatar
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    I'm still thinking of venting the bathroom vent into the furnace intake using insulated duct's.

    The bathroom fan would not be used and the switch would instead turn on the furnace fan or possibly a heat exchanger fan.

    Then again I am in a cold area.

    Everyone worrys about the vent and does not ask where the air comes from?
    Important note Ė I donít know man made laws, just laws of physics
    Disclaimer: I'm a big fan of Darwin awards.

  14. #14
    Homeowner geniescience's Avatar
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    Default more facts

    H2O in vapor form is the lightest molecule in air, so it rises. Heated air also rises. Warm air rises. Moisture, or warmth, will rise in air.

    To respond to the original post: Humidity causes dry rot in your climate. You are not in Tucson.

    If you add more humidity into a relatively closed space, you are "playing with fire". Let us create a new expression: playing with moisture. It can make the wood in your structure smell bad and rot. Here is the bigger problem: Anything made with wood pieces and glue (plywood, OSB, lauan, particleboard, and the hundreds of other trade names) can rot through and through, since each cut reveals cell walls (cut cell walls) where the bacteria that are known as "rot" begin their feeding and reproducing. Solid wood holds up fairly well structurally, so that is not the big problem.

    For Bill: consider whole-house Air Exchangers with Heat Recovery. I agree with your line of thinking but I cannot say much about the idea you described.


    David

  15. #15
    Homeowner geniescience's Avatar
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    Default more precise definition

    when i said "cut" i meant to refer to each individual piece of wood chip in a board, or each individual piece of wood speck in boards made of small small specks of wood glued and compressed together. I was not referring to the cutting of the board with a saw cut.

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