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Community and Q&A

Bathroom Exhaust Fan and Moisture

alcoprop | Posted in General Questions on

Hey guys quick question. Have a huge remodel that I finished up and the customer has been living there about 2 months now. Climate zone 5a. It has 2 full bathrooms back to back, hall bath and a master bath. Master has 2 shower heads and also has a window in it. I installed the Fantech PB270 that has the 270 cfm fan remote mounted in the attic with a 6″ exhaust and has 2 4″ going to each bath. Hall bath is fine with moisture and is only about 35sqft. They are having bad moisture problems after they shower in the master, it is about 120sqft. Moisture is puddling on the window sill and they said moisture is running down the drywall. I do have the humidity sensor controls in the wall and is set for auto turn on with humidity and when manually turned on it will run for I think 45 mins.  Should I dedicate this fan to just the master and install a smaller fan in the hall? Any other ideas that I might be missing? Thanks guys!

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  1. Expert Member
    BILL WICHERS | | #1

    Check that your duct run is clear. A long run of flex duct can be an issue, as can rigid duct with a zillion bends in it. You want as straight a run of rigid duct as possible for best results. You also want to seal all fittings.

    If the people in the master are taking long, hot showers, you're likely to still have some moisture even with the fan running. It sounds like you have a LOT though, so I'd check the ductwork first. Another possibility is unequal airflow between the two baths due to very unequal duct runs (like one five foot and the other 35 feet). I'm not familiar with your specific vent fan so I can't give any specific advice about that unit. If you do have very unequal duct runs, a damper in the shorter run may be able to compensate but I'd check with the manufacturer of the fan first. You'd basically be intentionally introducing a bit of back pressure into the short run so that the long run gets an equal amount of flow.


    1. alcoprop | | #3

      Thanks Bill. I will double check that nothing has a kink in it. The bathrooms are back to back with the hall bath being closer to the fan then the master but I would say the master is at most 5' longer. It is 4" flex duct that runs up to a 6"x4" y directly into the fan and then 6" hard pipe directly out of the building. I thought about getting a cheap airflow hood like the Easyhood to see if I have a big difference in airflow between the two.

  2. Expert Member


    I'd be interested to know what the ambient humidity in the house is. I find the size of the fan necessary to clear bathrooms varies a lot depending on where you are starting from.

    1. alcoprop | | #4

      Humidity has stayed around 50-55 since they have moved in. In the summer while I was working there with a/c on it would be around 35-40. This was a full remodel and the hvac is 15 years old and way oversized. I pushed and pushed to redo the hvac in the house b/c of humidity and comfort problems they are going to have but they would not let me do it b/c their hvac "professional" said what he installed 15 years ago is perfect for the house. But he is not taking into account the new zip installed, exterior and interior insulation and the attention to air sealing that we did. I had an erv in the budget along with a blower door test and lost the battle on all that. I did buy them an indoor IAQ to monitor c02 b/c of not having any ventilation. Their hvac guy is giving them very bad info that I believe is going to cause them problems. So with all that being said if I do actually have a fairly tight house with no mechanical ventilation could this be the problem? The fan just can't move enough air being so air tight in the house. Like I said above I might get a cheap air flow hood to see how much air the fan is actually moving.

    2. user-723121 | | #7

      Malcolm is right on here. What is the starting point of the humidity level in this house.? If it is high the ventilation will be overwhelmed when the shower is going. If the indoor relative humidity is low before showering there will be a buffer of sorts to the added humidity load. With the bath fan running the replacement air needs to be sufficiently dry to keep condensation from occurring on the window or bathroom mirror for that matter. The relative humidity in my own house (MSP) right now is 27 %, a shower will not steam anything up with the 80 cfm bath fan running. The window U-value is important as well, but I think the winter humidity is high in this house.

      1. alcoprop | | #8

        Doug please see my response to Malcom that is above you.

      2. alcoprop | | #10

        Just asked the home owner what the humidity is in the house its at 56%. I have a feeling higher humidity levels is the problem and the cause of it is the oversized HVAC.

  3. GBA Editor
    Kiley Jacques | | #5

    Hi Shaun,

    In addition to investing the ducting situation and determining relative humidity in the house, I suggest taking a look at the many considerations Martin Holladay lays out in this article: Bathroom Exhaust Fans.

  4. charlie_sullivan | | #6

    What kind of window is it? New or old? Double pane? A better window would have less condensation.

    Also, it can help to have the shower more fully enclosed. You can lower the ceiling there, make the walls and door taller, or there's even a company that sells a "shower lid".

    1. alcoprop | | #9

      All new Marvin casements. The house was just completely redone this year. See my response to Malcolm above. The shower is a wide open walk in shower with no doors or dividers. It is a complete wet room.

  5. Expert Member
    Peter Engle | | #11

    Indoor humidity at 56% in a zone 5a house in winter is WAY too high. Either you succeeded in making the house super tight, or the homeowners have a lot of indoor moisture sources (plants, frequent cooking, aquariums, swimming pools, etc.)

    A HRV would bring in fresh air and more importantly, dryer air. You are trying to use damp air to flush out somewhat damper air. That will take a lot of oomph. And if you're pulling from a very tight house, there might just not be much fresh air to make up what's going out.

    With the very small hall bath closer to the fan, you do have some imbalance there, so dampering the intake from that bath might help. But you really need some dry makeup air in that house to get the humidity down. At 56% RH, I would be worried about condensation in the roof and wall systems.

    1. alcoprop | | #13

      I suspect that I have achieved a very tight house. I am going to blower door test it and see for sure. I don't know if you read my comment above but I had new HVAC and an ERV in the budget and their HVAC "professional" talked them out of it. So I suspect I have a tight house that has no make up air and it can not move the bath humidity out and plus it is dealing with high humidity in the entire house to begin with.

  6. exeric | | #12

    Have you investigated if there is a make-up air problem for the master bathroom? This can be a real problem in tight homes. Steam filled air just doesn't want to move if there isn't a source of air to replace it. If there isn't there then the fan will try to do something it isn't designed for - create an air vacuum. It won't actually do that but instead the fan blades will just cavitate.

    That lack of make-air can be caused by two things:
    1. Absence of a make-up air vent for the entire house. 270 cfm compares to the requirement for a smaller range hood fan that would also require make-up air

    2. A whole house make-up vent exists but doesn't have access to the bathroom. This would be caused by a too small undercutting of a door in the master bathroom. Two shower heads adds a whole new dimension of added requirement for movement of air.

    For your case I think it is very probable that one of those two conditions are likely. If it is the second case I would advise installing a transfer vent into the wall to another room to allow unrestricted movement of air to the whole house makeup vent if you have one. If you don't have that then install one.

    1. alcoprop | | #14

      I don't know if you read my comment above but I believe I achieved a fairly tight house on this remodel. I had a new HVAC and ERV on the budget but their HVAC "professional" talked them out of it. I will do a blower door test next week and see where it is at. I believe since there is no makeup air for the house that the bath fan is not able to exchange the air.

      1. Expert Member
        MALCOLM TAYLOR | | #15


        I think you have it right. We live in an 25 year old house built quite tightly, but with no mechanical ventilation, in a damp climate where it is hard to get the indoor RH anywhere below 50%. I notice a distinct difference between the days it gets up just a few points in how well the bathroom fan performs.

        1. alcoprop | | #16

          Malcolm I am worried that this is going to cause problems for them. They just spent a LOT of money on a complete remodel. Do you think I should be worried about mold? Should I talk them into installing an ERV and would that help. Right now we are experiencing 30 degree weather with 87% humidity outside. I know furnace is WAY oversized so I would like to see them do the complete HVAC with ERV.

          1. exeric | | #17

            That outside relative humidity of 87% is just telling you that the air is just holding 87% of what air can hold at 30 degrees. It doesn't represent high moisture content. As soon as that air comes in the house (say through a makeup vent) it will warm up to house temperature and that volume of air when mixed with existing inside air will actually REDUCE the overall humidity of inside air.

            I understand that it isn't your fault that the occupants didn't take your good advice over the HVAC guy and that you probably feel a little responsible anyway. You shouldn't. Just install the make-up air vent. It won't add humidity. At least it won't in winter. And it will solve their bathroom condensation problems year around. Make sure there is enough airflow to that bathroom with the door closed, however you decide to do that.

            It really isn't your problem that they decided to not go with the better solution you offered. But going the route of make-up air will be an effective solution for the problem they are describing short of an HRV and new HVAC system. It will also be cheap to boot. I very much doubt you would have a call-back on it.

  7. alcoprop | | #18

    Eric so I am guessing you suggest just supplying a make-up vent for that bathroom only? Is there a specific product that you recommend getting?

    I also have a feeling that the reason the hall bath does so well is b/c the door to that bathroom is a barn door so it has access to pull air from the entire house. This is all making sense now! Thanks for the help!

  8. Jon_R | | #19

    I recommend that you use a powered make-up air fan. Passive vents have to be large and require high pressures. Or open the window (and run the fan) while showering - it's large enough.

  9. exeric | | #20

    I think if you install a make-up air vent directly to the outside air in the bathroom you will get complaints. That will bring in blasts of cold air just where you don't want it. It would be better to have a make-up air vent in a location where the air has to move far enough that there is plenty of mixing with the cold air in winter with the warm indoor air. It should preferably do double duty for a range hood fan. So that means it should be close to the range hood. I used a plain old spring loaded butterfly duct valve in my house for both the range and the bathroom. Aldes makes good ones. Make sure it doesn't go to a shared wall to a garage. Make sure it's an actual outside wall.

    You then have to decide how to get the needed airflow to a bathroom with the door closed and having two hot spray outlets. I'd consider putting a large bridge vent in the bathroom wall to another room that nearly always has a clear path to the range hood make-up air vent.

  10. exeric | | #21

    In the American Aldes product literature the vents are described as backdraft dampers. Here's a link:

    I used an 8" diameter one which I felt was a happy medium between the monstrous ones and the too small ones. It works fine for my 100 cfm bath fan located about 30 ft away in another room. It also is big enough that the house isn't depressurized with the range hood fan on. It should work fine for your 270 cfm fan. They are not expensive or complex to install.

    1. alcoprop | | #23

      Eric with these backdraft dampers do you have to install a fan along with it? Or does the spring open the fins up when the pressure changes in the home from the bath fan turning on?

      1. exeric | | #24

        Shaun, yes they don't require a fan and just sense pressure changes to open them. If you want to reassure yourself that that works compare the specs in the Aldes literature of WG required to open each size against the literature for your fan that tells you what WG it produces at typical power levels (the tighter the house the more power required) Then select the vent size that matches the airflow you need.

        This probably all seems excessively complicated and convoluted but it actually makes sense. Even in a tight house that uses an ERV exhaust in the bathroom you would probably still need a jump duct to another room to get adequate movement of air for two shower heads. Also, an ERV in a tight house would not provide the makeup air for a range hood. So, if it makes you feel any better these are all things you would typically need to do even IF you put in an ERV.

        Like Rick said, the owners are lucky to have you!

  11. Expert Member
    RICHARD EVANS | | #22

    Shaun, your clients are lucky to have you as their contractor. You are way more patient than me!

    By installing TWO shower heads in a small room with NO balanced ventilation in the house to reduce indoor RH (thanks HVAC 'pro'!), they are fighting an uphill battle against physics.

    There are complex solutions that will involve new holes, fans, ducts, dampers, wiring, etc. Or, God forbid.... they simply crack the window ever so slightly while taking their shower? :-)

    I bet their HVAC "Professional" still watches all of his movies on VHS.

  12. bennettg | | #25

    Hmm. With both bath doors open, the master should be pulling maybe 3x more than the hall, going by sq ft and double shower head.

    Some Q / thoughts:
    With the master bath door closed, where does the makeup air come from? It would be interesting to see how the humidity / condensation is with the door open. With the hall bath door closed? What's the pressure across the bath door? Maybe a thru-wall transfer would work.

    1. Expert Member
      MALCOLM TAYLOR | | #26


      If the ambient humidity in the whole house is too high, does it matter how open the bathroom is to elsewhere? In some ways it's similar to the problem that rooms without heat sources have. You can try and transfer warm air from the rest of the house, but if the difference is too small, the amount of air movement necessary to be effective is prohibitively large. To me that's what's happening in the house: They are trying to dilute the moist bathroom air with air that is itself too humid to bring the surfaces down below the dew point.

    2. alcoprop | | #27

      Bennett the hall bath has a barn door on it so I believe it is able to pull from the rest of the house and is why this bathroom has no problem.

    3. bennettg | | #28

      Malcolm, I grant you that the replacement air may be too humid.

      Shaun, I think it's possible that the fan isn't pulling (much) air out of the master bath with the door closed, since it's able to pull air from the other bath. This seems like an easy thing to test - have the owners shower with the door open and see what happens. If onsite, you could test all manner of combos. You may need to put a damper in the hall bath duct to get sufficient flow out of the master.

      You note above that the two-headed master shower is open, no enclosure / wet room. Some thought on the relative locations of the shower, fan grille makeup air source and outside wall/window where the condensation is occurring might be helpful too.

      1. Expert Member
        MALCOLM TAYLOR | | #29


        You are right - the problem is probably better characterized as a combination of the two. Humidity and constricted air-flow.

  13. JWolfe | | #30

    I stumbled across this thread and have a similar issue except on steroids from what you described. Did you ever come up with a solution?

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