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Community and Q&A

Best Practices: Bathroom Venting

lekawa | Posted in General Questions on

My HVAC guy was just out here today to install mini-split line sets in the walls and to install dryer vent and bathroom fan vent.  I had to be at work and because he is so busy I had to practically twist his arm to get him here, I just left everything he’d need and a key and discussed via text yesterday any previously undecided details.  I didn’t think to question the bathroom fan venting…assuming he would vent it straight up through the roof along side the fan.

When I returned home I noticed they had run the vent…which was previously installed by electrician, very close to the ridge beam (high point) in the bathroom (cathedral ceiling)….with the vent….which happens to be very large….down the entire length, and taking up most of the width… of the rafter bay (appx. 12 feet in length) with the roof vent attached to the underside of soffit, aimed toward the next rafter bay over.

“Just a girl” here…but this is not what I expected and I’m wondering if I should ask him to change it so it vents out the roof instead.  My concerns are:

1.  Will warm damp air be able to easily escape “down hill”?
2.  There’s little space left in that entire rafter bay for insulation
3.  Wouldn’t any damp air that is able to escape “down” that 12 foot length get sucked right back up into the  next rafter bay over once it leaves the exterior vent?

I won’t exactly be “thrilled”  if this ends up costing me money to have it changed, but it’s money I will gladly pay if necessary.  This just intuitively looks “wrong” to me, but I have no experience in “what does and doesn’t work in practice“.   If there’s any chance that this could be a problem waiting to happen, I’d rather pay now and avoid the mess.

I did text him just to ask him why they did it that way…thinking the vent size may have been an issue (a whopping 6″ vent in a 10” rafter bay) but he told me they didn’t want to cut through the new roof and wanted to avoid potential leaks down the road (this sounds odd to me….Aren’t roof vents standard practice for bath fans?)  I have not yet asked him to change it.

Would truly appreciate any advice/comments!

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  1. Expert Member


    I'm a bit confused. If there is a new ro0f already in, there is no way an HVAC guy is going to cut a hole in it. The duct termination should have gone in at the same time the roof did. If the roof isn't on yet, why are you doing all that work inside?

    Many builders go to great lengths to limit roof penetrations, resorting to venting that way 0nly when other options are difficult. It sounds like your situation may have been one where going up made sense.

    But if you want work done a certain way, either you or someone you trust, needs to be there to make the decisions for the trades. There is a completely unjustified undertone in your post that blames the HVAC guy for making a call you aren't happy with, when you put him in the situation of having to make it.

  2. lekawa | | #2


    Thanks for reading. If you are reading a "tone" in my question it's likely my concern that the set up could be problematic.

    This is my only concern.

    Would truly appreciate some good advice!

    Side note: We did meet on site prior to this, at which point he asked me everything he felt he needed to know...and I gave him all of the information on locations of vents, mini split drain lines...line set route...dryer vent...etc... The bath vent was not discussed because it seemed like a no-brainer to me. If he wasn't going to be able to vent through the roof and needed a hole drilled prior to work, that sure would have been good information to have. People retrofit bath fans and drill holes through the roofs all of the time. This is not something I would have guessed would be a problem...and would happily have accommodated in order to get a venting system that works.

    He did have my phone number along with encouragement to use it if he had any questions. He had also scheduled and cancelled so many which I was here for him... that I couldn't reschedule my own work for him again. I say this only to illustrate the actual situation.

    I just need a vent that's not going to cause problems....Venting "wisdom" would really be appreciated!

  3. Jon_R | | #3

    > Will warm damp air be able to easily escape “down hill”?

    It will with the fan pushing it that way. And no worries about condensation dripping back into the bathroom.

    > sucked right back up into the next rafter bay

    Some soffit vents are designed to direct the moist air outwards. On the other hand, look into code - it may have wording about exhausts being within 3' of any kind of intake. This might require blocking some soffit vents.

    1. charlie_sullivan | | #5

      From the looks of it--not a great picture--the installed vent is one that is probably made for a different application and is directing the air right at the neighboring soffit vent.

      It's a good point that a better termination directing the air up and out might be a much easier solution than the suggestions I had.

  4. charlie_sullivan | | #4

    Running the vent downhill is no problem--it's actually good practice to allow any condensation to drain downward. The fan will blow plenty hard enough to move the hot air downhill against its inclination to go upward--that extra effort is very minor compared to the work of moving the air through the length of duct.

    Running the vent out the soffit is bad practice, for exactly the reason you said--you don't want that humid air going right into the next soffit vent.

    Unfortunately, the alternatives aren't great. I'm not sure I know the full configuration to know what your options are. The best would be to go across to a flat gable end wall and out the wall. That might require a lowered ceiling (another kind of soffit) in the peak of the roof to go over to that wall--if that's an exterior wall--for example. Or to go through the roof, which does require a lot more work to get it properly flashed and sealed and is something I'd prefer to have a trusted roofer for rather than an HVAC contractor, unless I'd gained confidence the HVAC crew was going to do it carefully and properly.

    I found nothing objectionable in your tone. It's sad but true that one needs to confirm lots of details with contractors to ensure things are done right. One would like to trust that they know more than a homeowner does, but there are plenty who don't know or care as much as they should, or who assume your priorities are different from what they actually are. Clarifying that you are willing to pay more to get things installed with best practice is a good conversation to have and it's good that you understand that that might be needed.

  5. user-5946022 | | #6

    Can't comment on whether or not the vent going downhill is ok (seems like it would help expel moisture as if anything condensates it will run downhill to the soffit vent...).

    I will second the input you received above - if your intent was to vent the bath fan out the roof, the bath fan vent duct needed to be there before the roofer started, and the roofer would flash around that. An HVAC sub may cut a hole in a plywood roof deck, but no way is any HVAC sub going to cut a hole through a finished roof with shingles. They are HVAC not roofers, and they don't want to take responsibility if it leaks.

    Sounds like you may be acting as your own GC, and this is one of those learning experiences that those in the trades intuitively know. As much as you think it was somehow "obvious" you wanted to vent through the roof, it was 10x more "obvious" to the HVAC sub that you did NOT intend to vent through the roof, because if that was the intent there would be a roof penetration already prepared for that.

    The way this is often done these days is house, including roof, is sheathed with Zip and taped, keeping the interior of the house somewhat dry. Then before any finish exterior materials (roof shingles or metal roof, siding, soffit, etc) the plumbing, electrician and HVAC subs do their rough ins. At that point the electrician, plumber and HVAC sub WILL cut through the exterior sheathing. Once they are all done, then the exterior finishes go on. Your HVAC sub had extra work to do because it appears from your photos he/she had to cut through a finished soffit.

    When people retrofit baths and drill holes through roofs, the HVAC sub marks the penetration location and leaves the penetration and termination material. Then the roofer comes out and drills the hole, installs the penetration (this part they hate, so they often want the HVAC or plumbing sub there at the same time), then flashes the entire thing. If a GC has a good "jack/jill of all trades" the GC might do the penetration and roof patching. Then the HVAC sub comes back and finishes.

    The ONLY defense to your position that should have caused a knowledgeable HVAC sub to ask a question is that the brown soffit termination you show in your photo looks to me like a roof termination....I suppose it will work on a soffit, but it is unusual.

  6. Expert Member
    Michael Maines | | #7

    I agree with the others, that creating a hole in the roof is an option I save for last. I don't like venting into soffits. I also don't like using a rafter bay for venting when it should have insulation. I don't like cathedral ceilings partly for this reason. When I do a cathedral or sloped ceiling, I try to include a small level area for the fan, with space to run the vent out a sidewall, where they work best.

  7. Expert Member
    Akos | | #8

    It is a bit too late the change much there, but if your cathedral ceiling will be vented assembly insulated with batts, it would be worth it to spray foam the bay with bath vent. This would prevent the inevitable air leak causing issues there and add back some of the missing R value. You want to get the SPF under the duct, don't embed it in spray foam.

    The way I've done this before is run rigid pipe just above the ceiling height, not insulated flex. This leaves more space for the SPF under the deck. Some of the bath fans can connect up to an oval 4" duct (take a regular 4" duct and squish it), which gives you an extra 1" of space as well.

  8. lekawa | | #9

    Thanks to all for the suggestions! Yes I'm attempting to be my own GC... So far so good...and yes, occasionally I miss something due to relying only on common sense and good communication...which has served me well so far...but doesn't completely make up for the lack of experience and, yes, if I have to take responsibility for having not known something in advance, I can deal with that. Still feel if someone is feeling they need to install something in a less- than-ideal way, a little communication or heads up is in order, but again, this isn't about putting my HVAC guy on trial

    It sounds like there's no "great" way to do this and that I'm going to have to choose which "risk" I prefer to take...and I'm still feeling the roof vent is the " best" option in this scenario for the following reasons:

    I welcome any questioning of this logic

    1. As is, There will be little space in that entire rafter bay for insulation...creating higher likelihood of moisture condensation around the fan than if most of that rafter bay was well insulated. I understand the additional condensation could drain downhill as is, but better to avoid condensation in the first place(???)

    2. Little insulation in that rafter bay could also lead to other issues (in humid KS Summers and cold KS Winters)

    3. These vented soffits will be pulling air in as heat rises toward ridge vent. They are not designed in any special way to avoid that.

    3. Each bay needs it's own air intake because it's a cathedral blocking that neighboring vent doesn't "seem" wise to me. If there's a way to direct air out and up ...away from soffit I'm guessing it would look quite odd and combined with these other downsides, it's probably worth getting the roofer and hvac guy to coordinate...on getting the vent installed safely through the roof.... then insulating it as well as possible.

    4. If a condensation issue ever does arise, it would at least be easy to notice/identify whereas a moisture/mold/rot issue inside neighboring rafter bay could go undetected until there is significant damage (or seems to me)

    Oh, and one last piece of info. I have been air sealing with great stuff pro... and could use that in any of these scenarios if air sealing around fan or in rafter bay makes sense.

    Votes anyone?

    Challenges to my logic?

    Reality checks to my plan of getting roofer and hvac guy to coordinate?

    In the event this plan seems very highly risky, Is a wall fan an option?...

  9. Jon_R | | #10

    This doesn't look too bad:

    Or as code specifies, block 3' of soffit vents on each side of the exhaust. It is probably based on some data showing that this prevents most short-circuiting. Perhaps your soffit is different, but mine is all interconnected (ie, I don't need a vent per bay).

    Removing the foil insulation and spray foaming makes sense to me.

    Hot moist air rises, so I prefer bathroom exhausts at a high point (vs wall).

    1. lekawa | | #11

      Jon, Thanks for the idea...Wondering though...What if spray foam is unsufficient...which doesn't seem unlikely to me given how little space is available. Wouldn't it be hell fixing a moisture/mold problem if spray foam is filling the entire cavity?

      ...and I can't block 3' of soffit vents on either side because they each need their own intake (cathedral ceiling)

      Wall vent could be placed high as well...just a couple of feet from where it is now....

  10. lekawa | | #12

    just spoke with roofer who tells me it's not unusual to have bath fans installed when roof is already in place. He offered to cut the shingles and the hole and even place the roof vent if HVAC guy is not comfortable doing long as HVAC guy marks clearly where it is to go.

    So I guess at this point, I'm wondering...

    Does anyone think that venting through the roof is a "really bad idea", functionally speaking... compared to the other alternatives (leaving cost to change it out of the equation)?

    Though it seems there are multiple ways this "could be handled" none of which are "ideal"...and in looking at the entire picture of what's existing, and the least complicated way involving the fewest people toward "good enough" ....this also seems to support venting through the roof....

    Votes welcome! I don't entirely trust my judgment due to aforementioned lack of experience!

  11. CMObuilds | | #13

    You've got quite a mess, zero ability to insulate that cavity effectively, no vent chutes underneath the roof deck to vent the cavity and it terminates into an upside down roof vent which will probably be a miniature pond of water at the low point of that vent “bottom” that’s really the top, and Im sure the damper spring will get stuck open in no time seeing is it’s fighting gravity.
    If you decide to go out the roof and you keep the fan like it is you'll create a water trap when your vent pipe turns up to go out the roof. If you flip the fan around make sure it is detailed well so you don't get condensation running out of the fan housing.

    If it were mine, among other things visible, I would have looked at having a flat ceiling in the room with the venthood coming through the wall and vented the bath fan out of the gable wall if possible, which looking at your pictures it is, or as a second I’d go out the roof.

    1. lekawa | | #14

      Exactly. No space for vent chute in the rafter bay containing bathfan vent.

      O.k.... I've got two more pictures here showing two scenarios. Would either these work better?

      In the first one, fan would be mounted in the wall, not far from where it is now and vented just below it....or directly behind it "if" that's the way it's built (would buy a different fan...probably a panasonic low profile one)

      In the second picture, ceiling could be lowered.... and vent could exit at gable end.

      If either of these would work similarly well, I'd probably opt for the wall vent just because it's easier and I'd rather not flatten the ceiling, and then I'd spray foam the remaining space of that cavity....But if there's a significant advantage I could create a flat portion as shown in the second picture.

      1. ssnellings | | #21

        The second picture (flattening the ceiling to create space for the fan and the venting without taking up room in the roof structure) is the best option available. It means the fan will be installed in the manufacturer's recommended orientation, the ducting distance is minimized, and provides the largest amount of space for insulation in the roof assembly.

  12. user-5946022 | | #15

    Either of the two options you show are fine and better than what you have now. I would do option 1 - in the wall - with hard duct, maybe even pvc for duct.
    - You might need a different fan; if you can find one that just goes into the wall and exhausts out the other side that would be ideal. If it is thinner than the wall, see if a sheet of rigid insulation will fit between the exterior and the fan.
    - If you use the fan you have now, check to be sure the mfg allows it to be mounted vertical instead of horizontal. If they do, I'd also use hard duct. The duct will come out of the fan, then you will need to figure out how to do a 90 and get it turned to go out the wall - If your wall is 2x4 you might not be able to fit a 90 in that wall, so a fan that does not need any ducting would be a better bet. Picture 2 looks like some of your studs are deeper, so perhaps you will be furring out that entire wall in which case you can make the fan you have now work. That is better as it costs less, and wastes less.

    1. lekawa | | #16

      I don't believe the fan I have now will fit. It's HUGE. I bought in a rush because I wanted low Sones and it's the first one I found...Had good reviews...Had no idea it was so huge and that the vent size was 6". Electrician told me HVAC guy could make it work, so he went ahead and installed it. I could try and sell it at 1/2 price on Craigslist. Honestly, I just want something I won't have to be constantly worrying about.

      Panasonic makes this low profile fan (only 3 3/8" deep) that says it can be mounted in ceiling, wall or shower

      And Panasonic also makes specifically "wall" fans that exhaust straight out the back but they only come in 70 cfm....Which I suppose is fine... This sounds like the easiest to vent. I'll just have to check to see how deep it is. It comes with wall sleeve and hood as well.
      Will probably contact Panasonic Sales this time just to get their approval for the application this time!

      Thank you so much for providing a little confidence that there may actually be a workable solution!

  13. lekawa | | #17

    Thanks to everyone for the ideas and advice! I appreciate it so very much!

  14. CMObuilds | | #18

    Fans are sized to the volume of the room. Having a cathedral ceiling means you may need the extra power to clear out a larger space in a timely fashion.

    Having a fan in the wall is a bit of a climate zone thing, I think you said you were central midwest or south midwest so it probably doesn't matter, I wouldn't attempt in zone 6 where I am as it would frost up in a standard wall.

    You might be able to get an inline fan, Fantech I think still makes them, and then you maybe would have way more flexibility in this situation.

  15. lekawa | | #19

    I always get thrown by the zones.I'm more familiar with planting zones which are different. I'm in N/E KS which I believe is zone 4 for building. Still think the wall fan is o.k.?

    This is a bathroom in a small kitchen/bath addition which is all pretty open.. It's an unusual configuration and the air flows freely from main room to kitchen (where there will also be a cooktop vent) to bathroom. so I can't really go by the standard CFM formula. Bath and kitchen combined are about200 square feet...and there will be a kitchen exhaust fan as well.

    I think I'm going to chance the wall fan (linked above) with the vent and hood that runs straight out the wall.

  16. Expert Member
    Akos | | #20

    Looks like your ridge board is bellow your rafters, so unless you are looking to fur things out, you'll need a small flat portion of ceiling to go over it anyways. In that case, I would just install the bath fan there as per your option 2 and run it out the gable end. Cleanest install with no issues.

  17. lekawa | | #22

    I was planning on leaving that exposed.

    I just ordered a wall fan. When all is said and done this will be my $500 bathroom fan. ...appx $110 for first fan, $240 for new wall fan, and another $85 to pull new wire and remove old fan and install new one. and probably another $60 to repair soffit.... :(

    1. charlie_sullivan | | #25

      If you hang around here for a while, you'll hear plenty of stories of much more expensive mistakes...I've made quite a few myself. And if you hadn't caught this when you did, it could have been a much more expensive mistake. So I think you are doing quite well.

  18. Expert Member
    Akos | | #23

    Exposed beams like that are not the best idea. It is almost impossible to air seal around it as you have it constructed. With a cathedral ceiling, this can cause a lot of condensation up there and a wet ridge beam.

    If you do intend to keep it like that, I would rip a 2x4 to the angle of your rafters and glue it up against the ridge board between the rafters. This will give something solid for the drywall to sit against and a good surface for air sealing. When you install the drywall, make sure to glue the drywall against this. Installing backer rod and caulking the joint between the drywall and the ridge beam before mudding also wouldn't hurt. You want this joint solid and air tight.

    1. lekawa | | #24

      Thanks for the heads-up! That does make sense.

      I had a drywall guy ask me whether I wanted the beam exposed, and I told him it didn't matter too much to me and asked him whether leaving it exposed would pose a problem in the joint between drywall and beam and was told it wouldn't be. He didn't say anything about needing something solid between the rafters. I may change my mind as a result.... Just called my framer who will either do what you've suggested, or prep it to go ahead and drywall over the ridge board (creating the flat area)

  19. lekawa | | #26

    Here's what my framer added. He didn't want to rip an angle..(don't think he wanted to lug the necessary tools over) Here's what he did. It's basically a combination of both options.

    First photo: There's a storage area over the kitchen in which I didn't want to lose valuable space by flattening the ceiling. I will try to air seal the gap above these blocks with spray foam (the gap left by the angle at which they were placed). Did I mention there is a ridge vent?

    Second Photo: In the bathroom where the fan was located (now removed) we flattened a portion of the ceiling. I will have a wall fan installed at the gable end in one of those stud bays...Haven't decided which one yet.

    1. Expert Member
      Akos | | #27

      Better. Make sure to spray foam and caulk those pieces of wood. Better would have been to glue them to the LVL ridge board. 8' times even a 1/16" gap is a big hole.

      Air barrier continuity with cathedral ceilings is very hard, this is why you often see here recommendation of flat ceilings.

      You fixed one issue, but you still have a transition at the interior partition wall. As it sits, that is a giant hole from the house into your roof space. Typically, the way I handle these is by spray foaming the top of the interior wall cavity where it jogs up from the mini attic. A better option would have been to frame this in after the ceiling is up but that is hard to do with standard build workflow.

  20. lekawa | | #28

    Are you talking about where the red dot is in this picture?..or the blue dot?

    If the blue dot, what do think about a well fitting piece of foam board filling the lower part of rafter bay (one on each side of ridge beam) ....sealed around the perimeter with spray foam or glued in with something strong (and as much rockwool batt above foam board as I can fit without eliminating air channel for venting)?

    1. Expert Member
      Akos | | #29

      Well both. Your air barrier, in this case drywall over the mini dropped ceiling, needs to go up along the partition wall, than across the studs and finally connect up to the drywall ceiling in the bathroom. You are also missing some blocking there as well for the drywall to sit on and for you to tape against.

      Rigid insulation+canned foam is a good start, I would also tape the joints with a quality tape (Zip or 3m 3067).

      If you are going to clear and stain the ridge beam, I would do it now before the drywall goes on. Also cover it to protect from drywall/mud.

      1. lekawa | | #30

        Akos, are you speaking from a "This is the best way to do it for energy-savings" standpoint? ...or is this advice to avoid some major problem with vapor/mold/etc...?

        The reason I ask is because my framer who has been the one I"ve been relying on to do "what's necessary" (in his late 60's and doing this most of his adult life) is the type that really resists doing anything that's "not" necessary. It's like pulling teeth with him. (He would sooner argue for 30 minutes about the non-necessity of something he feels strongly about than just do the thing in the same amount of time) I'm willing to go in there and do everything I personally can to save energy, but "everything I personally can" is limited by my own knowledge/experience and money to pay someone else to do what framer insists isn't necessary...If I was even able to find such a person...

        What is worse case outcome if I don't get this area sealed up well enough?
        (keeping in mind that I'm using R-30 mineral wool batt insulation and there will be 2" above it for ventilation....continuous soffit vent and ridge vent)

        1. Expert Member
          Akos | | #31

          I've mostly had to fix this type of detail on older balloon framed houses. There it usually means big air leak path and cold walls. When you are doing a blower test, you can hear the air flowing through the walls.

          The number one issue that comes up on this site with cathedral ceilings is interior water damage from condensation and roof rot, so not something you want to take lightly. Lot of times you can get away with ugly details but sometime they come and bite you in the ass. Easy to fix up front, hard to fix down the road.

          Cathedral ceilings are nice, I'm a huge fan. It is almost universal around me. Just have to make sure that your warm side air barrier is solid and you don't do silly things like shotgun it with pot lights.

          As I said earlier, the easiest way to deal with that section is to fill the top of the partition wall cavity with spray foam.

          1. lekawa | | #32

            ...and that wouldn't cause a ventilation issue in that cavity? I'm imagining warm damp air rising...(the shower head is just on the other side of that partition) and getting in there and not being able to dry out. Is the idea that it would be sealed up enough with spray foam that it wouldn't be able to draw that damp air into that cavity in the first place?

            I'm really not wanting go and buy a "froth pack" and do it myself...and I already called around to see if spray foam companies would do "spot spraying" and it seems it doesn't pay for them to come and do small sections..... So it might be the easiest way for someone who does this regularly. I think I'm going to have to find a "creative" way that's actually easy for me. :)

            Thanks for the tips... Truly appreciate them!

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