
Musings of an Energy Nerd

Bathroom Exhaust Fans


Manufacturers offer a wide range of bathroom exhaust fans, from simple models with few bells and whistles to energy-efficient models with sophisticated controls. This model is made by Panasonic.
Image Credit: Image #1: Panasonic




A bath exhaust fan can perform several functions:




Some of the makeup air is probably coming into the bathroom from other rooms in the house, via the crack between the bottom of the door and the flooring. Of course, if the bathroom fan is exhausting 50 cfm, then 50 cfm must be simultaneously entering the building. If some of the makeup air is entering the bathroom through the crack under the bathroom door, an equivalent volume of exterior air must be entering other rooms of the house through a variety of random cracks in the home’s envelope.

Unfortunately, many exhaust fans pull some of their makeup air through a…

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  1. Rjp : | | #1

    Zoned Ventilation?
    Humm... A central zoned ventilation system with automatic dampers, passive intakes and single ECM motor (actually available). Where does an actual ERV (energy recovery ventilator) or similar system fit when it comes to bath ventilation requirements?

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to RJP
    Q. "Where does an actual ERV (energy recovery ventilator) or similar system fit when it comes to bath ventilation requirements?"

    A. Your question is rather vague. But I have a hunch that this article will answer your question: Does a Home with an HRV Also Need Bath Fans?

  3. Alexander Bell | | #3

    the tissue test
    While the tissue test may be a venerable home inspector classic, it really needs to be retired from the toolbag. Unless you place tissue over the all of the intake vents, you are only evaluating airflow at the location of the tissue. I have flow tested a fan that passed the tissue test, but the damper was still taped closed from the factory. No air was leaving the bathroom, just pulled in one side and pushed out the other.

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Alexander Bell
    Thanks for your comments. I agree that it's a pretty crude test. But it has allowed me to identify a few fans that weren't pulling any air at all.

    The main advantage of the toilet-paper test is that it doesn't require any equipment.

  5. Malcolm Taylor | | #5

    Martin, The usual method of installation has a number of shortcomings. Relying on the supplied housing is problematic both because of the number of holes they contain and the small lip for sealing to the drywall. Using the housing also means that the most problematic joint, at the damper, is usually inaccessible once drywall is installed - as is the damper itself.
    To get around this, on the last two houses I built I mounted an air-sealed plywood box in the ceiling large enough to contain all of the common fan housings. This puts the damper and connections inside the box allowing easy access for connections, removes all problems of air sealing and also means that the whole fan, including housing, can be changed out without any disruption of the building envelope. The only downside is that you have to buy a larger vent cover to use instead of the one that comes with the fan, but in most cases this is a plus as the ones they supply are usually fairly ugly.
    If this seems like too much work I'd still recommend doing as the BC Building Envelope Guide For Houses recommends and mounting a piece of plywood flush with the ceiling framing to provide a larger and more secure surface to seal both the fan housing and drywall to.

  6. Roy Goodwin | | #6

    Location of terminations?
    You recommend against roof terminations ( I understand that, I think poking holes in your roof is against nature) but also state "In no case should a duct be terminated in an attic or at a soffit.". I guess that means if you have a hip roof, you're stuck with a roof termination ?

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Malcolm Taylor (Comment #5)
    I salute your attention to detail. It sounds like you are paying attention to airtightness and anticipating future repairs. Good work.

    Of course, most production builders aren't going to go to the trouble of installing a bath fan the way you suggest. But your approach is admirable.

  8. User avater GBA Editor
    Martin Holladay | | #8

    Response to Roy Goodwin (Comment #6)
    You're right. If your house has a hip roof, you'll be venting your bath fan through the roof.

  9. Kevin Dickson, MSME | | #9

    Mold vs. Farts
    It's worth mentioning that high humidity in the bathroom is really only a problem when it condenses on a surface. Well insulated bathrooms with no thermal bridges really reduce the condensation problem.

    Also, I believe that the amount of air needed to dry out a bathroom is many times more than what's needed to dilute smells. Has anyone seen data on that?

    Smells can also be removed without dilution, using activated charcoal filters. This may become a less expensive and more energy efficient way to handle the little toilet rooms and 1/2 baths/powder rooms. Half baths never have humidity problems, so why run unnecessary ductwork?

  10. User avater GBA Editor
    Martin Holladay | | #10

    Response to Kevin Dickson

    You're right that it's worth mentioning -- which is why I mentioned it. I wrote, "In cold climates, bathrooms with poorly insulated surfaces are more at risk than bathrooms with well-insulated surfaces. ... In many cases, the main factor leading to mold growth is missing insulation above the ceiling. During the winter, cold drywall stays damp. That’s why ceiling mold often occurs near exterior walls, where insulation tends to be thin. Repairing insulation defects helps prevent mold: the insulation keeps the drywall warm, reducing opportunities for condensation or moisture absorption."

  11. Jin Kazama | | #11

    also agree that a bathroom
    also agree that a bathroom without bath or showers ( what is the english term for such a "pipi room" ??) should not have an exhaust fan in any energy conscius design.

    Kevin : do you have experience with quality filters ?
    hard to find locally up here ..everybody still install exhaust fans on every room with a toilet fitted

    An HRV with variable speeds activated by a control panel that monitors humidity and runs on fixed low speed to match required CFM .

  12. Nick Welch | | #12

    Roof vs. wall termination
    I'm particularly interested in the arguments for terminating the duct at the roof vs. the wall, as I still need to do this for both my bath fan and range fan. Martin, I was surprised you so swiftly dismissed roof vents, though not too surprised, given your roofing background.

    Here's my thinking: The roof is completely out of sight (on the rear of the house), while a wall vent would be visible. The roof is also a shorter duct run with fewer bends, because neither of my fans are very close to a gable wall. However, it seems like rain might be slightly more likely to find its way into a roof vent, and there is no way to direct condensation out the vent -- it will drip down into the house. I'm not in a very cold climate though, so I don't think condensation is a big concern.

  13. Jin Kazama | | #13

    what is the number 1 building
    what is the number 1 building related problem ( #1 as in the most serious )
    = water entry related dommage

    so why put vents through a perfect protection surface ?
    Roofs should be completely free of any entry points

    why not shoot it straight out to the wall ?? who cares about how it looks
    just invest the money saved on the tubes and roof sealing toward a better looking grille
    and voila! :)

  14. Nick Welch | | #14

    Roof venting
    Jin, roofs have plenty of penetrations. Roof vents (for attic ventilation), plumbing stacks, etc. The plumbing stacks leak fairly frequently because they usually rely on a neoprene gasket that deteriorates, but roof vents cover themselves with sheet metal in a way that seems to be pretty reliable.

    Also, I figure a roof leak will be noticed and fixed ASAP. A wall leak can go unnoticed and cause a lot more rot before it's found.

    And who cares about looks? Most people.

  15. Jin Kazama | | #15

    i know everybody cares about looks,
    it was sarcasm ..

    not because regular old type pushes "easy" roof penetrations we need to still go with this method

  16. User avater GBA Editor
    Martin Holladay | | #16

    Response to Nick Welch
    Arguing about which is better, a roof termination or a wall termination, is a little bit like arguing over which is better, exterior basement insulation or interior basement insulation.

    Either approach can work, as long as it is detailed properly. And folks with hip roofs don't have a choice anyway.

    There are a lot of variables. Roof terminations are a particularly poor choice in snowy climates, and are much more difficult to install if you have concrete tile roofing than if you have asphalt shingles. But as long as you know what you are doing, a roof termination can work fine.

    [Note to GBA readers: I'll be on vacation next week, but I'll be back at my desk on Monday August 18. If anyone wonders why I'm not answering questions during that period -- now you know.]

  17. John Brooks | | #17

    An Ocean of Air
    Hi Nick,
    Nothing wrong with "Arguing"....
    I think "thru the roof" at least deserves some consideration/discussion.
    Considering that the exhaust fan may not be operating 24/7....
    What's wrong with having a "Flue" or "Chimney" that "connects" to the outdoors at a higher altitude rather than lower?

    "We live at the bottom of an Ocean of Air"
    In a tight house there are not-so-many openings that "connect" to the outdoors (Ocean of Air).
    The termination of the exhaust fan may actually be one of the few "significant" openings.
    The size and altitude of the significant openings are what determines the altitude of the Neutral Pressure Plane(NPP).

    Doesn't a "chimney" thru the roof usually work better than a chimney thru a sidewall?
    (It's possible to run a stove chimney out a sidewall ... but perhaps not ideal)

    And even if the exhaust fan is running 24/7 .... why not work WITH the atmosphere?

    Of course ...If the vent passes thru a cold attic ... the "conduit" should be insulated.
    and in "Snow Country" the termination should be insulated and extended.

  18. Richard Patterman | | #18

    Why not a soffit vent?
    It is not in the roof, it is not highly visible and it is protected from rain & snow.

  19. Malcolm Taylor | | #19

    Reply to Richard
    The soffits often have vents to provide air into the roof space. You don't want them to draw the moist air that was just exhausted from the kitchen into the structure.
    In an unvented roof the danger is that vents that terminate on the soffits can cause rot to the surrounding soffit material - although I've never seen it.

  20. Mark Heizer | | #20

    Small baths with and without showers
    Thanks Martin for metioning that small baths have greater risk: rate of moisture production per room volume overwhelms small baths, even if there are no exterior wall or ceiling exposures: 103F water vapor is going to create problems. And an "oops" I noticed in my bathrooms: the high aeration rate of many low flow shower heads put more water vapor into the air stream than the old 2.5 head.

    Unfortunately for many homes, multple kids (or entire families in apartments) share the 8x5 bath, while the parents share the mega-volume master bath. Guess which bath has water dripping from the walls and ceiling after back-to-back showers? 50 CFM (delivered) exhaust is questionable for a small volume, high-use bath unless you have the run-time switch or humidity sensor (FYI: one of these control options and 80 CFM is required by Oregon code. This was the result of a construction claims task force report).

    The HVI guide has good information. But in my opinion, once the room gets above 200 SF, a bigger fan is a waste.

    As for the 1/2-bath/powder room, these should be allowed 25-30 CFM (*hello 62.2 and I-codes*). Energy use isn't going to break the bank since the fan isn't used on every use. I designed projects with the "chemical" filters (true fart fans) in baths for a national chain. they're not effective for anything other than air mixing. An effective filter would use more fan energy than the combination of a 30 CFM intermittent exhuast and its infiltration/OSA energy penalty. Add the environmental impacts for filter manufacturing and disposal and filtration is laughable as a green option for masking natural bodily functions. Americans aren't ready to accept the powder room is actually used for something other than powdering your nose; so I suspect that elimation of the fan requirement isn't going to happen. Final note to builders: I've met homeowners that don't want low sones in the guest/half-bath and might want the timer switch. Many guests appreciate a little background noise...just in case; and ability to run the fan after you leave can be a real face-saver ("You may want to wait a few minutes, Bob.)

  21. User avater GBA Editor
    Mike Guertin | | #21

    Who installs your bathroom fans?
    We need a new subtrade - Bathroom fan installers. Ideally the HVAC (maybe better referred to as the heating/air conditioning) sub who is used to dealing with ducts should install them but many don't deal with whole-house ventilation never mind measly bathroom fans. Around here bath fans are installed by electricians - go figure.
    Like many house components bath fan installation involves a few subtrades - electrician, duct installer, sider or roofer, interior carpenter (to undercut the door) and there may be others.
    And then there is the brand / type of fan - who selects it and are his/her criteria based on performance or aesthetics? Since there may be a few tradespeople involved, and a client, the contractor needs to be on top of things otherwise it's just another detail that falls through the cracks.

    I checked an exhaust fan for a friend of a friend. The premium fan was rated at 180 cfm and the flow hood read 38. It had all of the install problems Martin outlines and then some. I asked who installed the fan - "We had an electrician install it." Soon as the weather cools a bit we'll spend an hour in the attic and an hour on the roof correcting the problems.

  22. Malcolm Taylor | | #22

    Here the builder selects and installs the fan. The electrician connects it, like any other appliance, as they are the only trade licensed to do so.

  23. A. Bradford | | #23

    Wall mount fans
    Is there any efficacy to the use of direct wall mounted fans in situations where the duct would be hard to properly route? Particularly in cold climates, would it be a recipe for infiltration/condensation?

  24. User avater GBA Editor
    Martin Holladay | | #24

    Response to Asa Bradford
    It often makes sense to install a wall-mounted fan, especially in a house where ducting a ceiling-mounted fan would be difficult. The main advantage of a wall-mounted fan is that there is very little static pressure (a major problem with systems that have long, convoluted ducts).

    A wall-mounted exhaust fan should be installed as close to the ceiling as the wall framing allows. It should be installed near the shower (but not in the shower stall).

    Backdraft dampers that come with bathroom exhaust fans are not airtight, and there are technical reasons why it's difficult to make them airtight, so some air leakage is inevitable. However, the air leakage problems associated with bath exhaust fans exist regardless of where the fan is mounted.

  25. Jin Kazama | | #25

    Asa and Martin ...
    As Martin pointed out, leak from damper is inevitable, however,
    if you use a combination of products such as :

    and a mechanical damper that usually comes with the bathroom fans,
    you might be able to reduce unwated leaks to a minimum.

    The drape damper excels where mechanical dampers fail, wich is at very low pressure , where it should be sitting most of the time on a bathroom exhaust fan.

    I've been wondering about using a double setup as passive inlet for make-up air for quite some time, but i have yet to try it out.

    You can even use those on range hoods ( again best is to use a spring loaded quality damper near exterior output and a drape dapmer further in at inner wall line approx ... )

  26. Paul Barraza | | #26

    How much undercut
    Is there a rule of thumb for how much under cut is needed for a 50cfm fan?

  27. User avater GBA Editor
    Martin Holladay | | #27

    Response to Paul Barraza
    Paul Raymer calculated that a 1 inch gap under a 30-inch wide door is good for 47 cfm. If the bathroom has other cracks, and most do, then you can get away with a smaller undercut.

  28. Hari Kamboji | | #28

    importance of the fan
    I have a new small bathroom with shower, no window, no fan, in a finished attic. I expect that the shower will only be used about a dozen times a year. Since the exterior "wall" is the roof, I think the only option for a fan terminus is straight through the side of the roof. I'm in the planning stage of redoing the roof with exterior rigid insulation, so it might be the right time to do the bathroom fan. But I'm not convinced that a fan is really a good idea in this case, where the bathroom will only be used occasionally. (I don't like the idea of piercing the roof and the resulting heat loss- I live in a place that gets significant snowfall.)

  29. User avater GBA Editor
    Martin Holladay | | #29

    Response to Hari Kamboji
    The decision is yours. It sounds like you understand the pros and cons of each approach.

    Keep in mind that building codes require an exhaust fan in this location. If you ever decide to sell your house, this code violation could be a problem.

  30. Benjamin Root | | #30

    Through-the-wall energy efficiency.
    Hi Martin,
    So would you say that a wall mounted (through-the-wall) vent fan is not more-inherently inefficient than a ceiling mounted unit? IE that the back-draft dampers are an equally weak-link in both styles?

    In my case, a wall-mounted fan would be very easy to install, but I'll go through the extra work to install a ceiling-mounted one, ducted, if there is a reasonable efficiency increase.

    The rumors at the big box improvement store is that the through-the-wall units are"leakier," (though I don't often trust those guys when it comes to efficiency). You make it sound like they're not significantly different (if at all).

    There is an Energy-Star-rated through-the-wall unit, but I get the impression that the rating is based on motor efficiency (w/cfm) and sons level (<2), and not on air-tightness/insulation/etc. And it's several times more expensive than the run-off-the-mill unit.

    Adding the cape style damper someone mentioned is interesting, but I'd be installing in a 2x4 wall, so not enough distance for that extra component. But, are there better-quality exterior dampers that I could replace the stock plastic flapper with?


  31. User avater GBA Editor
    Martin Holladay | | #31

    Response to Benjamin Root
    I have never seen any reputable data showing that wall-mounted exhaust fans are leakier or less leaky than ceiling-mounted fans.

    I really have no advice for you, other than to say that you should choose the approach that you prefer. Either way should work fine.

  32. Adrienne Burt | | #32

    Bath fan in the shower?
    We have a bit of a debate going on between a client and the electrician about where to install the bath fan. Client wants it in the shower enclosure (it's a Panasonic and is rated for this type of installation provided it's on a GFCI branch), electrician says it will be overtaxed and it should be outside the shower. In either case it will be vented through a gable end, not the roof.


  33. User avater GBA Editor
    Martin Holladay | | #33

    Response to Adrienne Burt
    As long as the exhaust fan is rated for installation in a shower, and as long as it is on a GFIC circuit, there is no reason not to install it where the customer wants it. It would be a good idea to control the fan with a time-delay switch, to make sure that the bathroom air is dry after a shower.

  34. Moses Draper | | #34

    Lunos eGo
    I'm curious about how the Lunos eGo fits into this, as it's exhaust size is only cfm, but is sold as a good fit for the bathroom.
    See -

    Is the 50 cfm code requirement a good number to shoot for (aside from whether it meets code, as in my case there isn't any)?


  35. User avater GBA Editor
    Martin Holladay | | #35

    Response to Moses Draper
    The Lunos eGo is rated at 3 to 12 cfm in heat-recovery mode, and "up to" 27 cfm in exhaust mode. That's not much.

    Clearly, the airflow rate of this fan is insufficient to meet the minimum code requirement for a bathroom exhaust fan unless it is operated continuously in exhaust mode. And if you have to operate it in exhaust mode, you don't get any benefit from the fan's heat-recovery capabilities -- so why buy a Lunos fan for this purpose?

    Whether or not the Lunos eGo exhaust fan will satisfy the homeowner depends entirely on the homeowner's expectations. For more information on the wide range of homeowner expectations when it comes to bathroom exhaust fan performance, see Does a Home with an HRV Also Need Bath Fans?

  36. Greg Hahnel | | #36

    roof or wall termination?
    To be honest I am far more confident installing a gooseneck through the roof , If installed correctly should never have a leakage issue. If the roof is clay tile or shakes gable ends may be a better option . Sofit venting is a terrible idea , especially if you have vented softing dont ! I know I can have a perfectly water tight roof termination , but with so many different siding materials in use I realize it may be difficult to create a perfectly water tight sidewall / gable penetration. Plus side gable venting may increase the length of ducting significantly. Another big problem I see all the time {I work with new homes} the lack of attention to the amount of flex ducting connected to the bath fans. I see ones where 5 feet of flex would suffice , however I see 12 or so feet of flex ducting coiled up like a mess. When I quiz the hvac installer why??? They tell me it is to trap water in case it condenses during cold weather. This is a common urban legend among tinbashers in western Canada , yes it gets very cold here in the winter . If the fan ducting is installed correctly , there should be no issues with cold weather condensation. The problem is when you have that much extra flex duct kinked and folded, the flow and velocity drop so low that the ducting and roof jack never warm up , even if the fan has been running for say 10 min or so. Now imagine sending very humid air very slowly through this duct work which is still well below the dew point temperature , it will condense wont it? then when the fan is shut off the duct despite is outer coating of glass wool insulation will cool thus turning the condensed water into frost or ice. Then the next time the fan is used , or if the weather warms up for a few days the frost/ice within the duct melts and drips down through the fan grill cover. This could have all been avoided if the duct had only been 5 feet long. the duct would warm up to room temperature within 2 min of operation , no condensation no ice no frost. Most of all , no water dripping out of the ceiling grill. I do several bath fan replacements every year , and I usually avoid using insulated flex unless I will need less than 4-5 feet. I always start with ridgid galvanized duct. first two pipe diameters say an 8 inch length from the fans discharge. then a 45 degree elbow , 3 inch length then another 45 elbow which at this point I may run more metal duct strait up or transition to a short length of insulated flex . All galvanized duct gets wrapped with foil backed R5.5 fiber glass duct insulation. Using this method with the two 45 degree elbows results in only a 20% flow loss . I do own a digital airflow meter which confirms this. When I install a 110 cfm fan ducted through the roof as described, my meter usually reads 91-94 cfm . So as long as the occupants turn the fan on for a minute or so prior to showering , then let it run up to 30 minutes after there will be no condensation issues. The residential hvac trade in our region was de regulated many years ago, so the installers dont need to know any thing at all about what they are installing and how these systems are supposed to work. I find it is very easy to have an undersized bath fan, having one over sized is really just a bonus. If some one has some unusually foul smelling business to do in the bathroom , the quicker the odors can be removed the better. Most homes have forced air heating in our region and have a few intakes that connect from the outdoors to the return plenum of the furnace. So when exhaust fans are used the majority of the make up air enters the home through these intakes, the more frequently the fans are used the better the air quality and really just the general smell of the home tends to be.

  37. Kevin McGuire | | #37

    Pvc thin walled ok, but what about ABS? And what schedule? 40, or 20, does it mater on thickness?

    1. User avater GBA Editor
      Martin Holladay | | #38

      If we're talking about an exhaust air duct connected to a bathroom exhaust fan, ABS pipe is fine -- and the thickness doesn't matter.

      I wouldn't use PVC or ABS pipe for the supply side of a ventilation system, however.

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