I had a fan added in my bathroom. Surprisingly there wasn't one at all. When the fan was first installed they had it venting into the ceiling. It was a .5 sone fan and you could hardly tell that it was even on. You had to listen closely. After I noticed it was only venting in the ceiling I had them come back out and add the vent out to the outside. Now all of a sudden this thing is pretty loud.

Is there anything I can do to quiet it down? Is there something I should be looking for? It's amazing how loud it got by adding the vent.

  • 2
    Can you describe the duct run? (Size, number of elbows, length.) Is it steady noise or a rattle? Aug 5, 2015 at 13:44
  • It's a steady noise. It's about 2 feet long with no elbows and just vents straight outside over a window.
    – JoshFink
    Aug 5, 2015 at 18:30

2 Answers 2


There more restricted the airflow is, the more noise there will be. All contributing factors to noise:

  • Diameter of duct (larger is better)
  • Overall length (shorter is better)
  • Number of turns/bends (fewer is better)
  • Radius of turns (larger is better)
  • Size reducers used (no reducers is better)
  • Type of duct (smooth, rigid is better than flexible)
  • Type of exterior vent hood (low resistance opening is better for noise)

This is why the low noise fans often have very large (eg. 5") duct connections. Larger duct is more expensive and harder to install.

Note that adding ducting will ALWAYS make the fan louder than no duct, but you just have to accept that because duct is absolutely necessary. Starting with a low-noise fan is a very good move of course (and 0.5 sones is great).

If you had a contractor install a vent that vented it into the attic, all their credibility is immediately gone, because not only is that not to code, but it's an incredibly bad thing to do: in the worst case, it will lead to mold and rot in the attic space, and ice dams in the winter that will destroy your singles (and also lead to rot and leaks).

So if that contractor came back and installed vent, I would not be surprised at all if they did it the cheapest, easiest possible way, which would likely be using a 3" (maybe 4") flexible hose, and not taking care to keep it as straight as possible.

First step is to figure out what was done and size/length, etc, and then likely upgrade it to something better.

The best possible thing to do is have a straight, rigid large-diameter duct directly out the wall to a decent vent hood.

Pay a few extra bucks to get a decent vent hood that is weather- and insect-proof, but has low-resistance to opening with airflow, especially if it's in a place that sees driving rain or lots of wind. The cheap plastic louvered ones can work if they're sheltered, but they are bare minimum.

Couple other tips:

  • Don't allow any dips in the pipe where condensation could pool
  • It's generally preferable to have the duct sloping towards the fan, especially in areas where temperature goes below freezing
  • If you have to have turns, then large radius turns are better, which you can make with a couple adjustable duct elbows. You can also mix rigid and flexible duct where it makes sense to.
  • Adding a bit of flexible duct will actually help absorb sound and reduce vibrations transmitted from any rigid pipe

The goal is to have smooth even airflow, with minimal resistance and turbulence.

  • 4
    That answer is good but be aware that your suggestion/illustration about downward slope goes against both what you should do as a credible installer (all venting must have positive slope) and what the MFG recommends in fan manuals (positive slope). The reason is that hot air rises and turning a vent downwards will make the tiny fans less efficient which will actually allow more condensation to collect due to lower output CFM. Also, in order to reduce condensation through spots that get cold (like attic or balloon frames) you should wrap the duct with insulation and tape it well at the seams.
    – dhaupin
    Aug 5, 2015 at 15:51
  • 2
    The slope of the duct may not always be towards the outside. In areas where freezing is common, sloping back towards the fan is more common. Also if you're venting through the roof, you'd obviously not want horizontal duct lengths to slope away from the fan (toward a puddle). As well as the points made by the previous comment. Couldn't find any code on the slope, but it may be listed in the manufacturer's installation instructions.
    – Tester101
    Aug 5, 2015 at 15:53
  • Thanks, I updated my answer to mention the slopes and removed the misleading picture
    – gregmac
    Aug 5, 2015 at 17:01
  • Thanks for the reply. I need to go check the size of the duct. Honestly though it's about a 2 foot run that goes straight to the outside. It's not very big at all. When I said he was venting it, I meant just into the ceiling cavity originally. It's on the first floor with another floor above it. Would it make sense to take out the rigid and just put in flexible to the outside? It is a straight, rigid, no curves or bends duct right to the outside.
    – JoshFink
    Aug 5, 2015 at 19:08

To go along with gregmac's very good answer I would offer the follow tips.

  1. Make sure your rigid duct is ultra secure, especially near the fan. If it wiggles when you grab it then it will make sounds with the fan going. I strap these things in really tight.
  2. I rockwool the first 3-4 feet really tight. I smash it in around the duct. This absorbs much of the sound/vibration of the fan and duct. Not only will it be more quiet in the bathroom but you won't hear it in the rest of the house.
  • Good idea. It's only 2 feet long, maybe I can pull the fan out and stuff some rock wool around it to reduce vibration.
    – JoshFink
    Aug 5, 2015 at 19:12
  • any suggestions on where to get a small quantity of rock wool?
    – JoshFink
    Aug 6, 2015 at 22:41
  • 1
    @JoshFink lowes, HD, menards all sell rolls at like $10.
    – DMoore
    Aug 6, 2015 at 23:25
  • 1
    @DMoore No, rockwool should not be used in such case. It will keep vibrating and the micro fibers will move all over the place and get inhaled. Rock wool is made to reside in a stable place and not to keep moving.
    – Basil A
    Mar 14, 2018 at 5:48

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