Pin It

Getting the details right: Bathroom Exhaust Venting

IRC-2009For numerous years the codes have required that all bathrooms either had a window OR a light & mechanical exhaust system be installed (see legal section below). The thought process was the homeowner could either open the window to remove certain obnoxious gasses &/or the excessive moisture or turn at least turn on a fan if that option wasn’t available.

legal2009 IRC – R303.3 Bathrooms. Bathrooms, water closet compartments and other similar rooms shall be provided with aggregate glazing area in windows of not less than 3 square feet, one-half of which must be openable.Exception: The glazed areas shall not be required where artificial light and a mechanical ventilation system are provided…Quick reminder – R308.4.2.5 Hazardous locations (requiring tempered glass, etc…): The following shall be considered specific hazardous locations for the purposes of glazing: Glazing in enclosures for or walls facing hot tubs, whirlpools, saunas, steam rooms, bathtubs and showers where the bottom exposed edge of the glazing is less than 60 inches (5’) measured vertically above any standing or walking surface. (exception granted for those 5’ or more away from said item)

Generally we have to dig through insulation to spot this issue inside the attic, in this case it was quite easy to spot…

For those that were not located where a window was possible, (or for builders tired of callbacks with rotted trim) one might assume that most contractors would follow all the manufacturer’s directions & vent the fixture outside…

Well, they would be sadly mistaken as the industry standard in most areas was to simply let it dump into the attic or even between the first & second floors. Part of the issue was that while the electrician had to install/wire up the fan, that was the extent of his involvement & the HVAC contractor was generally only hired to handle the Heating & Air Conditioning portion.

Tips, tricks, and things to rememberWhile the codes allow for either windows or mechanical ventilation in a bathroom, with today’s tighter buildings, an exhaust fan should be installed on a timer in all bathrooms. There are not too many people that will open up a window when it is freezing outside leading to a ton of moisture being dumped into the house. The other issue is for those that do, is remembering to shut it later or shutting it too early.

The codes to the rescue:

Just like the dryer vents, duct leakage testing, etc… where the codes used to be silent on this issue; they now mandate that the Builder / Mechanical Contractor actually ventilate said exhaust outside.

  • R303 (Exception cont.) Ventilation air from the space shall be exhausted directly to the outside.
  • M1501.1 Outdoor discharge. The air removed by every mechanical exhaust system shall be discharged to the outdoors.
  • M1507.1 General. Where toilet rooms and bathrooms are mechanically ventilated, the ventilation equipment shall be installed in accordance with this section.
  • M1507.2 Recirculation of air. Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or to another dwelling unit and shall be exhausted directly to the outdoors.

Along those lines & seeing common sense isn’t so common they also state that;

  • R303.4.2 Exhaust openings. Exhaust air shall not be directed onto walkways.
  • M1501.1 Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.
  • M1507.2 Exhaust air from bathrooms and toilet rooms shall not discharge into an attic, crawl space or other areas inside the building.
Tips, tricks, and things to rememberSizing a bathroom fan can be somewhat tricky as the information out there is all over the place. Some are quite simply based on Square Footage, some account for volume, while others simply give you a set number for each item in your bath. Per the codes the minimum size must be at least 50 CFM for those controlled by a switch or timer.With that said, that only accounts for what the fan is rated for & does not account for the resistance in moving the air through & out of the exhaust duct. Along those lines most also don’t take into account jetted tubs or steam showers which release a lot more moisture in the air which requires more air to not only exhaust it from the bathroom but through the line.So with that I recommend: (Width x Length x Height / .8) / Factor

  • Factor = 8 for most new & existing houses
  • Factor = 6 for steam showers, jetted tubs, etc…

So a 6’ x 8’ bath with 8’ tall ceilings would require a fan with 60 CFM while one with a steam shower should be around 80 CFM. Ahhh but what happens if you have a true Master Bath (let’s just say around 200+ SF) with a steam shower? Do you really need a 300+ CFM fan that can suck the towel off your head? The answer can be both yes & no based on the design & layout.

The Duct:

airflow-issuesUnlike the “dryer vent” section the code is pretty quiet on what can be used & how it should be run. This leads to a lot of the nice pictures you have seen and some interesting “practices” from groups that should know better. The biggest concern is to make sure the water vapor & any possible condensation issues are directed outside & not allowed back into the house.

  • If at all possible the exhaust port on the fan needs to be pointing straight at where you are going to run & terminate the exhaust
  • Eliminate as many bends or fittings as possible – while some like using 90’s to gain height or change direction, 2 – 45’s will do a better job & not add as much resistance.
  • The most optimal location to exhaust the air is out the gable end (remember soffits & ridges are off limits as they can allow the moisture back into the attic)
  • Just like a dryer,  the smoother the pipe the better (I prefer PVC & yes the joints need to be glued)
  • If the gable end is over 16’ to 20’ away, the roof might be a better option (this generally also allows you to have the needed pitch without requiring a you to use elbows or fittings to artificially raise it.
  • Just like other ducts & vent-lines, it must be supported every 4’. (For PVC running directly out, simply grab some ½” foam board & cut in 2” x 6” wide pieces. Place one at the end & then every 4’ in add in additional pieces – this will give you a 1/8” per inch slope in the line from the fan to the outside allowing any condensation that may form in the line to flow outside the building.)
  • No matter where you terminate the line, a louver, screen &/or other damper should be installed which not only prevents the wind from blowing in but will also help prevent insects & other creatures access.
  • If you wish to use insulated flex duct (not recommended) you should use 6” line with an adapter)
  • Any ventilation going through uninsulated / unconditioned spaces should be insulated to help eliminate condensation issues (not only that which may form inside the pipe / but also on the exterior)
  • If you do use metal duct with a seam, the seam should be facing up & taped with approved tape
The PVC pipe visible is the bathroom exhaust without the spacers under it yet

The PVC pipe visible is the bathroom exhaust without the spacers under it yet

Tips, tricks, and things to rememberOne of the big questions is just how long should I let the exhaust fan run. As a general rule it should be running while you take your shower & 15 minutes afterwards. The biggest issue is getting the water vapor out of the air – so if you take cooler showers it may not require 15 minutes, while someone taking a very hot shower might need 20 minutes. This is just something you have to monitor & gauge for yourself. Another option is to buy one of the newer fans which includes a humidistat in it so it can decide when it needs to turn on & off. I must say tough, I am not fully sold on those yet.

How about vent-less or recirculation types?

bad-ventSeeing I popped this picture in this last weeks “Wacky Wednesday” series… the answer is they are not allowed. Per M1507.2 “Recirculation of air. Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or to another dwelling unit and shall be exhausted directly to the outdoors.” Just remember the goal of a bathroom fan is to exhaust obnoxious gasses & moisture resulting in a healthier, safer living environment.

This entry was posted in Common Sense Building and tagged . Bookmark the permalink. Post a comment or leave a trackback: Trackback URL.
  • Erik

    SLS,
    I recently gutted my second floor for renovation. During wiring for the new bathroom, my electrician ran the bathroom exhaust flex tubing to discharge beneath one of the roof vents (see photo). This is not directly outside, but is close. Is this ok? Thanks! Erik

  • Geoff

    I’m venting two bathroom exhaust lines (PVC) through a single opening through brick construction. Any recommendations on appropriate Y connectors would be greatly appreciated. Thanks in advance!

    • http://blog.SLS-Construction.com/ SLS Construction

      A trip to your local supply house with measurements and a diagram would be your best bet but don’t forget you would need dampers to help prevent one from blowing into the other. A better solution might be a system with one fan that pulls from both bathrooms at the same time – they can be a little tricky to install but performance wise they cant really be beat. The only one I have experience with is FanTech but there are numerous options out there

  • Jonathan Eagles

    Hello,
    I need to add a fan to a bathroom with a low slope roof. Directly underneath the bathroom is a crawlspace. Can I put the fan in the wall and run the ducting down to the crawlspace and out underneath the deck?

    • http://blog.SLS-Construction.com/ SLS Construction

      Uh probably not as the deck would be considered a walkway (though as I always say check with your local building official to be sure) – beyond that there is nothing to prevent you from using a crawlspace as a way to run the vent

  • treads

    Hi, I would like to install a bathroom vent fan, but if I run a 4″ pipe the termination it would look really bad on the exterior while a 3″ would be able to fit nicely. The bathroom has a window so installing a fan is optional. I know I would lose some air flow, but does the IMC have anything to say about reducing from 4″ to 3″?
    Thanks,
    Sam

    • http://blog.SLS-Construction.com/ SLS Construction

      Not that I recall but I would have to punt to the local building inspector on this. You may ask if it is possible to run a 4″ pipe for the bulk of the distance and then reduce down.

  • Arden

    I have railroaded bathrooms (one 3/4, one full). The 3/4 bath is internal and full bathroom is on an external wall. I am considering a solar tube for the 3/4 bath with a vent through the roof. Is that practical? Is it more economical to vent both bathrooms with one duct going to the external wall?

    • http://blog.SLS-Construction.com/ SLS Construction

      Good question – first I love solar tubes but I am not sure I would trust or be able to recommend a combo unit like that. As for common venting, I would recommend checking into solutions like the one from fantec that has one motor located near the exit point & pulls air from both bath rooms when the three way switch/timer is on. While you can vent both bath rooms using a common line the potential for issues is high with that type of system and I would recommend avoiding

  • geoffcoco

    Hi, I am adding a new vent + exhaust fan to my home as part of a basement remodel. The vent will service a basement laundry/utility area (dryer vent is separate) about 12 ft. away from the exterior wall. Unfortunately, the best place to exit the 4″ vent out of the house is below a kitchen window (about 4 ft. below), and adjacent to a basement window, about 2 ft. to the side. I know with the Furnace exhaust, this would not be acceptable. But, will this be ok for a simple room vent? Thanks in advance.
    –Geoff in Seattle, WA

    • http://blog.SLS-Construction.com/ SLS Construction

      The best person to ask first is the building inspector – will they accept it or not. Beyond that you know the uses, how often windows are open, etc… Sometimes one is just faced with making the best choice for the situation.
      With that though I am wondering why you do think one is needed – are you line drying clothes or other items in there? The dryer itself will easily pull 200 CFM of air out when running if vented properly. Washing the clothes should not release that much moisture especially if you are using cold water. I know in our area many do this but that is generally because they are using this one for a whole house exhaust system.

  • marilyn

    Hello,
    We need to replace our old bathroom exhaust fans. Two of them are located on the second level; I am not certain, but I am assuming that they currently vent into the attic–as does the microwave/stovetop (Illinois home built in 1999). If I am correct, do we need to make them vent outside now-is that the code for my area?
    Also, our downstairs 1/2 bathroom vents outside just at the front door! Craziness. That would be considered a walkway, yes? Lastly, I am unclear as to how many air changes per minute is required. I have read, 12, 8, or 5. What is recommended?
    Thank you so much for your time.
    ~~~Marilyn

    • http://blog.SLS-Construction.com/ SLS Construction

      My pleasure & yes if you replace them, you must bring the assembly up to code. I would also do it to prevent any issues from cropping up even if you don’t. As for the front door – not the best way to great guests but your building inspector is highly unlikely to have you move it as it is exiting the structure & it is considered existing…
      As for air changes – please see the tip area above for my recommendation. There is no exact requirement on how much is required with the exception of the minimum size required for all bathrooms.

  • chuck

    I have a straight shot outside but it’s about 20 ft. away. Is that ok?

    • http://blog.SLS-Construction.com/ SLS Construction

      Well as mentioned above 16′ to 20′ is generally the maximum I recommend but I have gone up to 25′ with no issues – just watch your slope

  • Mike S.

    I have an aluminum damper for my sidewall termination, any suggestions on how to join a 4″ PVC duct pipe to this? If I bevel the inside of the PVC I should be able to insert the aluminum duct into the PVC pipe, how can I then seal this since I’m pretty sure the PVC cement won’t bond to the aluminum. It’s a very short run, less than 4′ total with 2 45 deg elbows (dormered backwall so I needed to drop the vent down a few inches behind an interior wall and then out. Thanks.

    • http://blog.SLS-Construction.com/ SLS Construction

      I would not bevel the inside of the pipe, but would use a coupler like I did for the main connection. As for the connection, I would go with a GeoCel or some sort of elastomeric caulking with one or two light beads run all the way around the metal before inserting it & probably finish it off with one at the edge

  • Gary Dean Jenkins

    Hello, I have a log cabin with an upstairs bathroom. The vent fan is cut into a plywood side wall. It vents straight up and out of the roof. The run is only a couple of feet. Unfortunately, the old fan runs counter clockwise and exits out of the top right. All replacements, I can find, run clockwise and exit top left. Any suggestions?

    • http://blog.SLS-Construction.com/ SLS Construction

      Panasonic has a ton that vent out the right side & some that vent left… The catch might be the depth though – do you still have the older unit as many times you can simply replace the fan unit itself (with a quitter more efficient one) while leaving the case intact

  • SJ

    I have an old bathroom I’m remodeling (1957), without an exhaust fan. I’m adding one, but it is far from the gables. The rest of the house has already been remodeled and added on to, and seems to be up to CA code. The exhaust fan is about 4′ away from the range hood exhaust ducting. Would there be any problem with tapping into that ducting, which then runs directly out the roof? I’d rather not put another hole in my roof.

    • http://blog.SLS-Construction.com/ SLS Construction

      Good morning & in short – yes there are major issues with that as pointed out by comments from JBH below. Sorry while I hate putting holes in the roof myself, sometimes it is the best option if done right, not a worry at all

  • JohnVoter

    I wish the author had said something about how exactly he made the transition from the 3 7/8″ exhaust port on the fan to the 4″ interior diameter of the PVC pipe.

    • http://blog.SLS-Construction.com/ SLS Construction

      Ahh thanks for letting me use one of my favorite lines from Star Wars – all to easy – I used a pipe connector like you see where I joined two sections of PVC together. You may also wish to check the manufacturers directions &/or attachments included as they may have included or have available a fitting for that. In this case it was plastic to PVC with a little PVC cement

  • tom B

    I have a house built in 1994 with 2 bathrooms upstairs with
    fans vented in the soffits. After years
    of moisture in the attic due to moist air coming back into the attic via the
    soffits (mostly noticeable in the winter time), I had a contractor re-run the 2
    vents (separately) from the fans to the gable end of the house, each
    around 10 to 15 feet away from the gable
    to the vent. Flex was used from the fan
    to the attic floor (very short run) and then transitioned to rigid metal all
    the way to the vents on the gable ends. Runs are pitched down, and the vents have a
    flap on them on the outside of the house.
    Each run is covered with insulation (seems like fiberglass with a plastic
    outer covering). This seems to work, no
    more moisture that I notice in the attic in the winter. But, I have found the end of the insulation
    inside the plastic covering around the conduit at the gable end (where it vents
    outside) are wet. The fiberglass is wet,
    and at one point in the winter, I could feel ice chunks in the
    fiberglass/plastic insulation around the conduit. I do not think moisture is coming back into
    the attic at the gable vent and going into the fiberglass insulation around the
    conduit. I think either the conduit has
    a seem, and moisture is escaping out of the seem into fiberglass, or somehow
    where the vent is near the cold gable side of the hours, moisture in the
    conduit collects between the outside of the conduit between the fiberglass
    insulations. Any thoughts on how to find
    and correct the problem? I hate to take
    the insulation all off unless I can buy new insulation that is split so I can
    re-cover the conduit and tape it back up.

    • http://blog.SLS-Construction.com/ SLS Construction

      You may wish to see how well you’re connection is (maybe add tape – see below0 & seal that appropriately along with any seams (which are hopefully pointed up) by simply pulling back the fiberglass covering. If you refer to our duct sealing article that might help out. http://blog.sls-construction.com/2011/air-sealing-duct-system

      If you need to remove & replace the foil covered insulation, you may wish to check at your local building supplier – whether it is a Home Depot, Lowes, Menards, Ace, Tru Value or lumber yard. As for removing & replacing, it isn’t split, but you would disconnect the connection, slide it on & then reattach.

  • bkc

    I have a new fan installed, need to run it about 3′ to the nearest wall (in an insulated attic space, in New England) – but perpendicular to the exit port of the fan and the direction of the rafters. This means I need to raise the vent pipe above the rafter and then turn it 90 degrees and run 3′-4′. Suggestions? Do I need to put anything in place to ‘catch’ the condensation where it will be rising as soon as it leaves the unit?

    • http://blog.SLS-Construction.com/ SLS Construction

      If you refer to the “Dryer vent” article I mentioned above (http://blog.sls-construction.com/2013/common-sense-building-dryer-venting) we do discuss what happens with very sharp corners & how much effective length each one adds. You may wish to visit your local plumbing/HVAC supply house & see if they have any pipes or vents with a longer sweep. Depending on the exact layout you maybe able to get by with 2 45’s (one angled up & then the other angled to the wall).
      Ahh condensation, this probably won’t be an issue as long as the effective run is short, fan is sized properly & ran for an appropriate amount of time. With that is should also be insulated to help prevent any issue (I prefer making sure mine is buried deep in cellulose & mounded above as needed)

  • Moe

    I have two bathroom, neither of which currently have a bathroom fan. Could I use one hole out of my roof instead of cutting two? The longest run would be under 7ft. Thanks in advance.

    • http://blog.SLS-Construction.com/ SLS Construction

      What you may wish to consider than is what is called a remote or shared fan – Instead of the having a fan in both bathrooms, both simply have the exhaust ducts ran to one fan which would pull air from both baths when turned on. The nice thing about this is you will find it generally more quite in the bathrooms, though with that short of a run, the noise reduction might not be as noticeable
      Another option is going with a dampered lines, but any issues with the damper can redirect the air flow from one bath to the other

  • JBH

    can i vent a bathroom fan into a next-door furnace room? Or, can i share the same ventilation duct that exists in the furnace room to get the bathroom air to the outside, or does the bathroom fan require its own hole?

    • JBH

      Nevermind – found my answer. This is a big huge no no. Didn’t think about the potential of a blocked furnace room vent and what that would do to the poisonous gasses in my scenario (it would send them right back into the bathroom!). I will give the bathroom fan its own separate hole.

      • http://blog.SLS-Construction.com/ SLS Construction

        No problem & glad you found the proper answer

  • Shawn

    i am renovating my bathroom just wondering how far my exhaust fan should be from my heating duct

    • http://blog.SLS-Construction.com/ SLS Construction

      Interesting question & I don’t recall anything in the code books on that & I have seen plenty that are almost side by side. With that you should make sure that both are well sealed (I would hate to be sucking the exhaust fumes back into the furnace / AC) & insulated to help prevent any dew point issues which also happens to be code for runs in attics & other unconditioned spaces

  • alex

    quick question. home built in 07 and the exhaust vents through the soffit. I guess it was code at the time in my area. well now I want to do this the right way through the gable end but the attic trusses are in the way. should i go over the trusses in this instance? any help would be recommended.

    • http://blog.SLS-Construction.com/ SLS Construction

      Good question & on this one I really have to ask where you are located & how far is the run – in Phoenix I see no issues, colder climates you are definitely looking at needing insulation & maybe even that might not be enough in some instances like Minneapolis that had 37 days below 0 in a row. As pointed out above & the dryer exhaust piece, bends add resistance which effectively reduce the length you can run the exhaust. If in doubt I would look at running it out at the roof as close to the fan as possible

  • bill

    Greetings, I am rebuilding an old house and adding exhaust to all the bathrooms. My second story windows overlooking the back of the house look out on about part of the roof for the first floor. The roof extends about 12 feet out from the second story and is about 16′ wide. Questions, is there a required (or recommended distance) the vent should be from the upstairs windows?

    Thanks! Bill

    • http://blog.SLS-Construction.com/ SLS Construction

      Sorry I don’t have my code book handy but 6′ is ringing a bell. One other item to consider is which way does the wind primarily blow – in this case (esp. if you open the windows) you may want to be further away if the wind would be blowing it towards the windows

  • todd

    When running the PVC to the gable I have a screen there at the gable now. Do I need to cut a hole in screen to put the PVC outside of the screened gable opening or can I just end the PVC at the gable screen? Thanks

    • http://blog.SLS-Construction.com/ SLS Construction

      It sounds like you would be running the vent pretty high & as pointed out above you don’t want it running for a long length and should keep down any bends. If you do have it leaving the insulation it should also be insulated.

      It really should vent directly outside & your building inspector may or may not allow what you are considering. With that, compared to many cases where they are never vented this is definitely an improvement & shouldn’t cause any issues in most cases

  • ltlblugrl

    Is a heating element required by 2013 code in bathrooms?

    • http://blog.SLS-Construction.com/ SLS Construction

      I have never heard of that though your local building department might, I highly doubt it, but they are whom I would check with