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Bathroom Exhaust Fan Duct Location

telebiker | Posted in General Questions on

Hello GBA members-

I would like your advice on a new bathroom fan, interior ducting, exterior vent housing and location of this.

I have a hipped roof with eaves and soffits.  Currently an old fan is vented into one of the soffits but I don’t detect any mold issues as best as I can visualize.

I will be replacing the tile roof with a metal one and will insulate the attic floor space, in a cold roof fashion.  BR is approx 75 ft sq/8′ ceiling.

Any advice on a quality exhaust fan?  I have a few links to products that seem to have good reviews.
https://www.deltabreez.com/GBR100H.php
or
https://na.panasonic.com/us/home-and-building-solutions/ventilation-indoor-air-quality/ventilation-fans/whisperfitr-dc-fan-50-80-110-cfm

This company seems to have good products.  I like the durable seal and have read that someone also uses butyl tape for extra seals around the screws:
https://www.deksus.com/dektite-metal-roof

any recommendations on the duct from the bathroom to the exterior?  Smooth aluminum (insulated or not?) ducting or insulated flexible?

I could run the duct horizontally/parallel/between the floor joists and then 90 degrees vertically through the roof.  This would be approx. 4.5′ rise to the interior of the decking.  This penetration would be about 3′ from the hip ridge and about 8-10′ from the lower roof/gutter edge.

Would this vent location be better than venting through the soffit?

Thanks to all.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    telebiker,

    The Panasonic fans are great - and most importantly very quiet. Use rigid duct, and slope it slightly to the exterior. As long as it is adequately covered by the insulation on the attic floor, it shouldn't need additional protection. If you buy a soffit vent termination that directs the air-flow away from the house, soffit exhausts aren't a problem. I prefer to go that route, rather than a roof penetration, which also necessitates a vertical leg of ducting where moisture can accumulate.

    https://www.greenbuildingadvisor.com/article/bathroom-exhaust-fans

  2. telebiker | | #2

    Thank you, Malcom.

    I've read many forum posts from various online sources which present concerns of warm, humid exhausted air returning into the attic space and promoting mold growth.

    If the rafter channel which creates the soffit "box" does not have any perforations in the eave panels, and only houses the exhaust vent grille, how would air return into the soffit?

    Additionally, to create a slope away from the exhaust fan toward the vent, I would actually have to fabricate an elevated exhaust fan mount that would place the exhaust port from the fan above the joists. This would reduce the number of bends in the run. Otherwise, there would be 2 sharp 90 degree bends in the middle of the run. Any thoughts?

    Thanks for your time-

  3. Expert Member
    MALCOLM TAYLOR | | #3

    telebiker,

    In some climates - like our wet PNW one - even un-vented soffits can get moisture damage from exhaust air. But if the termination is close to the fascia and directed outwards, I've never had a problem.

    The easiest way to gain a bit of height for slope is to curve the short piece of flexible duct attached to the fan box quite sharply upwards, then let the remaining rigid duct slope slightly for the rest of the run. If you do decide to go above the attic joists, you will want to insulate the ducts.

  4. rockies63 | | #4

    In looking at videos from Corbett Lunsford on Youtube (he runs the Home Performance channel) he mentions that bath fans should be placed over the tub or shower, not the toilet or in the middle of the room. This is because the fans are primarily meant to remove moisture, not odors.

    Bath Fans

    https://www.youtube.com/watch?v=YAi9rJ31wqU

    Channel

    https://www.youtube.com/c/HomePerformance

    1. Patrick_OSullivan | | #5

      > This is because the fans are primarily meant to remove moisture, not odors.

      While that's true, a well functioning bath fan directly over the shower area can induce an uncomfortable draft during showering.

      A bath fan is not like a kitchen hood; it's not meant to capture effluent and eject it before it can spread. A correctly sized bath fan can be anywhere in the space and likely accomplish the goal, particularly if it's run post-shower.

  5. canada_deck | | #6

    Where abouts do you live? Do you regularly get snow on the roof?
    I dealt with both situations recently. Two fans. Both were venting into attics.

    For one fan, I tore it out and replaced it with a Panasonic version. I ran 6" smooth ducting and terminated it through the shingle roof. I took great care to heavily insulate the duct. You want to be sure that the warm shower air does not condense in the duct and then drip back towards the fan. One advantage of going through the roof is there were a number of good options for vents and no risk of the moist air getting sucked back up into the soffit. It was also easier for me to avoid having to run ducting to the very edge of the attic given some tight spaces. The disadvantage is that you are messing with your roof. Very happy with the fan. I can't remember if I got the 110 cfm or 150 cfm version but it's very powerful.
    Note that in the user manuals, they may advise 2-3 feet of straight ducting before an elbow. Not sure how much that matters. (see fig 18) https://ftp.panasonic.com/ventilationfan/installation/fv-0811rq1_fv-1315rq1_en_install.pdf
    Also GFCI considerations if you put it directly above the shower - check the manual and local codes as always.

    For the other fan, there was already ducting to the edge of the attic space and the duct was just sitting above the perforated soffit. It was easiest for me to just cut a hole in the soffit from the outside and then pull the flexible duct out and attach a vent like the one that Malcolm showed. In this case, the insulation is not a concern if the duct is embedded in the attic insulation anyway. This worked out quite well and it's quite close to the edge of the soffit so I'm not too worried about moisture getting sucked up. Even if some gets sucked back, it's a massive improvement to the before-state. My only real gripe is that the options for vents were much more limited. I don't think I saw any for 6" ducting and the ones I did see for 4" ducting were a little hokey. I would have happily paid $50 for a quality piece of hardware. I ended up adding some metal mesh to reduce the chances of birds or bats entering the duct work.

    1. telebiker | | #7

      Thank you Malcom, Canada_Deck, Patrick and Scott-

      My vent fan would remain in the center of the bathroom, as the ceiling of the shower is tiled, with a small opening for a light.

      I came across this soffit vent last night during web researching:

      https://moistureflow.com/

      Any familiarity with this vent?

      Somewhat novel, although somewhat unsightly design. I could run the duct to the south end of the house where this vent would not be as noticeable. Unfortunately, my BR faces the front/street side and I don't know if I'd like that vent in the front of the house. Running the duct to either the east or south end wouldn't be more than about 11-12 feet. I would go with a higher output fan with a timer on a switch to have it run for 30 min.

      I was up in my attic again last night and I was not able to see any mold growth with the current vent situation (exhaust duct just terminates into soffit which has perforated "cheesegrater" soffit vent panels.)

      Malcom-thank you for the duct graphic-I will model that.
      Canada_Deck-I appreciate the link in your post.

      I will keep you updated on the project.

      1. Expert Member
        MALCOLM TAYLOR | | #8

        telebiker,

        That would work fine, although I'd have no reservations using the one I linked to. We have two on our house. On damp days I can clearly see the airflow from the termination, and it all goes out well past the soffit and dissipates.

      2. canada_deck | | #9

        That does look like a nice design. Pushes the air beyond the edge of the soffit and the built-in screen should prevent problems with bats and birds. If you are putting in fresh ducting, I would encourage you to go with 6" smooth ducting and to cover it with insulation so that you don't get condensation in the duct. Hard to judge without seeing the internals but potential issue with the mechanism freezing up in certain weather - I guess that is a risk with many vents and can always be thawed out.

      3. Otosan | | #11

        Telebiker, I tried to purchase that vent from the manufacturer but after 5 months of promises while he had my money in an escrow account I finally had to file a dispute with my credit card bank to regain my funds. According to him, the supply chain problems are preventing him from getting the raw materials to manufacture these. He also says that he needs to sell 3500 of them a month to stay in business but he can't even get out the three that I ordered. Strange dynamic.
        Despite it's appearance I think it would make an excellent soffit vent. It kicks the exhaust out well beyond the roof line by carrying the duct to the outside edge of the fascia board. It has a back draft damper, pest screen and weep hole for condensation and is easily mounted from the outside without taking off the soffit vent screens.
        It's too bad he can't solve his problems and ship them. I think they'd be a good solution.

      4. MDub07 | | #12

        Hello, Im familiar with ordering of the MoistureFlow.com vent and Richard Schofel. Its been 2 years but I still havent received the 2 vents I ordered. Hes a scammer and a liar. Ive also found multiple reviews of others who have ordered and never received the products they paid for. Im only posting here to warn others so they are not ripped off by him too.

  6. telebiker | | #10

    One final consideration before I mount the fan:

    My joists run perpendicular to the desired exhaust point. Installing a fan would require a bend up over the joist and another bend perpendicular to that toward the termination. All told, there would be 4 bends-one vertical from the fan above joist, one perpendicular to that running the long duct to the soffit, and 2 partial bends (roughly 135 deg) from the straight run down to the vent termination.
    I could build a box to mount the fan above the joists to essentially eliminate the first two bends leaving a straight shot to the soffit. Is there any meaningful benefit to doing that? Should I just run it and install a larger capacity fan (80-110 vs a 110-130-150)?

    I think I'd get some rectangular ducting and use 2x stock to create the box extension and spray great stuff around the interfaces.

    Thanks again for all of your help.

  7. user-6623302 | | #13

    I have used roof mounted fans with great success. Also, they make combined light/vents.

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