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Pros and Cons of a Wall-Mounted Bath Fan

Robert Alf | Posted in GBA Pro Help on

Who has experience with installing and assessing wall mount bath fans? It would seem they have some advantages over ceiling mount especially in our cold climate here in Minnesota but I have no experience with them. First, I like that there will be zero ductwork to slow the air flow, allow leakage into an attic space that is hard to reach (yes, those insulated plastic-flex-ducts and the taped connections have the risk of leaking warm/moist air into our frigid attics), and there is one less penetration on the roof (arguably the most crucial membrane on the house for minimizing holes). That said, there are many fewer models of wall mount fans to choose from plus I’m sure there are some downsides I just don’t know about. Any input?

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Replies

  1. Riversong | | #1

    Wall mount fans that exhaust directly are also the most prone to backdrafting, and they are less effective at evacuating the warm, moist air that will rise to the ceiling.

    I always use ceiling fans (super quiet and efficient Panasonic) with rigid aluminum ducting with foil-taped joints, sloped ΒΌ" per foot to the outside for drainage. I never vent through a roof and never duct into an insulated ceiling. I build a dropped soffit or sub-ceiling below the air-tight thermal envelope boundary for fan and duct installation. And, since my walls are double-framed and 12" thick, I drop my ducting 3' vertically before exiting the sidewall to minimize backdrafting (reverse thermosiphoning) of cold air. With single walls, the vertical leg can be inside a chase along the wall that can be cleverly disguised as the side of a shelving unit or hidden in a linen closet.

  2. Robert Alf | | #2

    Robert, thanks for the thoughtful reply. Cool idea on the vertical drop of venting. I'll no doubt be trying this method in the near future. My current client just won't go for a dropped ceiling or soffit. Very important for them to know about the downsides that you mentioned though. Can your refer me to any written resources that you used to determine the backdrafting issue? Or is the backdrafting issue known primarily from word-of-mouth? On this current retrofit (2x4 walls), we have specified a Panasonic ultraquiet wall fan (I like how Panasonic fan housings have so few holes!). It does have a backdraft damper in the integral exhaust hood but maybe they don't work as well as the dampers in the stand-alone exhaust hoods? These clients are scientists by training so written info is very powerful in getting them to consider better options. Thanks again and hope you are getting as early and warm of a Spring as we are. Bob

  3. Riversong | | #3

    Sorry, but I can offer no documentation. This is the way I've built for 20 of the 30 years I've been involved with super-insulated construction. It's simple physics, so these scientists should be able to appreciate it. A short, direct link to the outdoors is going to be a heat loss path if not an infiltration source.

  4. GBA Editor
    Martin Holladay | | #4

    Robert,
    I have installed wall-mounted bath fans that worked fine and did not result in complaints about backdrafting. Panasonic is the best brand. Mount it close to the ceiling.

  5. Jesse Thompson | | #5

    Another thing to keep in mind, the Panasonic wall mount fans are noisier than their ceiling mount fans for some reason. It's another reason to try and make the ceiling mount work, if you can. The ceiling mounts can go as low as 0.3 sones, but the wall mount puts out 1.1 sones, which is quite a bit louder.

    Jesse Thompson
    Kaplan Thompson Architects

  6. John Brooks | | #6

    I realize that a quiet fan is usually thought to be more desireable.

    Sometimes a lttle noise can be a good thing.
    A little awareness that the fan is running.....can be good
    And also along the thought of a "courtesy flush" ;-)

  7. Riversong | | #7

    A "courtesy flush" is for jail cells. In homes, people tend not to use fans that are noisy.

    Not only do ceiling fans perform better in every way than a wall fan, but they can be incorporated with the area lighting (and Panasonic units also include a night light).

  8. Elizabeth Newman | | #8

    Old house question:
    If it is difficult to get a fan into the ceiling (the exterior wall has an eave soffit 6" below the ceiling height, can't touch the metal roof, and the ceiling cannot be lowered in a current project of mine), would a backdraft damper like a Tamarack be helpful in a wall mount? (The only option with a ceiling fan would be to run a duct horizontally over the ceiling and make a sharp 90 degree turn down, to dump out a narrow horizontal eave soffit.) Or would you still go for the ceiling fan? (Thin old walls, no room for a vertical drop inside the room)

  9. Daniel Allen | | #9

    I have both wall & ceiling Panasonic fans, each works well. I less fond of the wall fan because I can hear the damper flap when the air pressure changes, when the fan isn't running. I can also see light through the short length of duct, when I turn it on and the damper opens.

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