My bathroom fan seems to work OK but every few months we get black specks of mold forming on the ceiling. Both bathrooms in this house have 'popcorn' plaster ceilings, which seems crazy but is how it is. The texture makes it difficult to clean, every few months we've had to dab/wipe/brush it with bleach and seem to be wiping off plaster as we do so. The fan is old but sized appropriately, but the ducting is done poorly. The bathroom is small, 5x7ft and ceiling just under 8ft, so 70 CFM should be plenty and that's what the current fan is rated.

Here are some photos of this 70CFM fan (link is to a newer version of the same model), which has ducting that ends by simply laying down on the soffit screen (seems poorly vented). In the photos you can see the ducting for a 2nd bathroom fan recently installed, which goes out the gable.

duct leaving fan

duct from fan to soffit

duct laying on soffit

The fan itself has seen better days. Just about a year ago when we first had mold appear on the ceiling, I tried opening it and cleaning it out. Already it is pretty dusty and very cobwebbed, now running louder (hopefully a good sign) after I dusted and vacuumed it out a bit. Some before-cleaning photos to show the fan and mold problem:

last bit of mold to clean off ceiling, around edge of fan cover

fan with lightbulb cover off, before cleaning

I might repaint the ceiling but first want to make sure ventilation is better than adequate. The ductwork is the most glaring problem to me, but the fan also seems weak.

For the ductwork, I figure I would mimic the new ducting a quality electrician installed in the 2nd bathroom here. They ran it about 20ft to the gable, rather than out the soffit just 5ft away. I could locate a new gable vent 3 or 4ft from the first one, but it is just 4-5ft away from a grate on the gable to passively vent the attic. See photo below; the tape measurer at center shows where I'd run the new ductwork, note the attic vent in top-right. It is 18ft from this problem fan to the gable, similar run to the newer ductwork already venting at this gable wall.

bathroom exhaust locations

Is new ductwork to the gable likely to help, and is the location of that gable exhaust okay even though it's 4ft from another gable exhaust and from a passive gable attic vent? I am thinking to replace the fan and given the 20ft duct run I'm thinking a 100CFM fan.

  • Toilet paper test. With grid installed and fan running will it hold a pice of toilet paper tp the grid. Also with bathroom door nearly closed (just a slot), will it hold toilet paper (suction test)
    – Traveler
    Commented May 6 at 2:14
  • @Traveler never heard of that, thanks. Not even close, the fan's draw of air upward is very weak. Makes it clearer the fan itself needs to be replaced. Question then is if the ductwork should also be moved to the gable (4ft from other bathroom vent and from attic vent).
    – cr0
    Commented May 6 at 2:47

2 Answers 2


Venting out your gable end will work well. Placement where you indicated (by tape measure) is ok.

IRC says a bath vent must be at least 3 feet away from any other openings. So you are good there.

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4 inch duct is definitely large enough. The room is a total of 280 cubic feet. A fan that moves 70CFM serves the room well. Just be sure that the fan is indeed sucking the air in. (the tissue test is excellent.)

Finally consider removing the popcorn ceiling and painting with a good latex ceiling paint.

  • 1
    Thanks for all this, and good to know about duct diameter. What about distance - given it's a 20ft run from fan to gable, does that change CFM requirements?
    – cr0
    Commented May 6 at 17:25
  • And I am considering removing the popcorn ceiling, but that's a different question...so far experts I spoke with said "paint over" is much more reasonable than "remove", but it would still have a mold-conducive texture (not smooth) if it was painted over as-is.
    – cr0
    Commented May 6 at 17:26
  • 1
    The 20 ft run should be no issue. 70 CFM will change the air in the room every 4 minutes..
    – RMDman
    Commented May 7 at 0:07

Perform the suction test.

Toilet paper test. With grid installed and fan running will it hold a pice of toilet paper to the grid.

Also with bathroom door nearly closed (just a slot), will it hold toilet paper.

More testing. If it is the fan or duct, disconnect the duct from the fan and rung the toilet paper test again, while blowing in the attic.

Needless to say your ducting as it is will choke the air flow.

  • This is helpful but doesn't tell me much more than the repeat mold cleanups tells me! The fan does seem weak but it's unclear if replacing just the fan, or the ductwork, will fix this. To be safe I'm planning both so I rephrased the question to focus on ductwork update.
    – cr0
    Commented May 6 at 3:16
  • @cr0 about your ducts. You have minimal ducts size. If you want to stay with tha type of duct and not use solid type, then at least upgrade to 6 inch or even 8 inch ducts. The mold is a result of humidity. Avoid any turns (elbows) if you can.
    – Traveler
    Commented May 6 at 3:25
  • Also avoid flex duct if possible.
    – Huesmann
    Commented May 6 at 11:31
  • I figured I'd mimic the R6 class 1 form 1 duct that a contractor put in for the newer bathroom vent. Or ideally use rigid duct and as short an elbow as possible right off the fan.
    – cr0
    Commented May 6 at 12:07
  • Is it simply the bigger the duct diameter the better? If I had an 8in inner diameter duct and a 20ft run to the gable, how does that impact fan performance / CFM requirements for this small bathroom?
    – cr0
    Commented May 6 at 12:08

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